Intestinal Parasites

What Are Intestinal Parasites?

How Are Intestinal Parasites Spread?

What Are Some Common Intestinal Parasites?

How Are Intestinal Parasitic Diseases Diagnosed?

How Are Intestinal Parasitic Diseases Treated?

Can Intestinal Parasitic Diseases Cause Medical Complications?

How Can Infection with Intestinal Parasites Be Prevented?

Resources

Intestinal parasites* are organisms that live in the gastrointestinal* tract of animals, including humans. They can cause diarrhea (dye-uh-REE-uh) and other symptoms.

*parasites

(PAIR-uh-sites) are organisms such as protozoa (one-celled animals), worms, or insects that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. Parasites live at the expense of the host and may cause illness.

*gastrointestinal

(gas-tro-in-TES-tih-nuhl) means having to do with the organs of the digestive system, the system that processes food. It includes the mouth, esophagus, stomach, intestines, colon, and rectum and other organs involved in digestion, including the liver and pancreas.

KEYWORDS

for searching the Internet and other reference sources

Amebas

Amebic dysentery

Hookworms

Nematodes

Pinworms

Protozoa

Roundworms

Tapeworms

What Are Intestinal Parasites?

In humans, three types of intestinal parasites may live in the small and large intestines: tapeworms, roundworms (or nematodes, NEE-muh-todes), and protozoa (pro-tuh-ZOH-uh). Certain types remain in the intestines; others travel outside the intestines to invade other organs. Some are so small they can only be seen under a microscope; others can be many feet long. Most tapeworms and roundworms develop in the human body and lay their eggs there. The eggs then pass out of the body through feces (FEE-seez, or bowel movements) and can infest others.

Intestinal parasites exist throughout the world. The World Health Organization (WHO) estimates that 3.5 billion people worldwide are infested with some type of intestinal parasite, and as many as 450 million of them are sick as a result. Children are most frequently infected with these parasites.

Intestinal parasites spread in areas with poor sanitation and are most common in tropical developing countries on the African, Asian, and South American continents. They are not a large problem in the United States, and Americans are most likely to get intestinal parasites when they travel to remote areas.

How Are Intestinal Parasites Spread?

Intestinal parasites can be acquired in many ways. Some parasites can live in the soil for extended periods. They may penetrate the body through the skin or if contaminated soil is ingested accidentally. Other parasites live in animals, such as pigs and cows. People can become infested with these by eating undercooked meat or drinking unpasteurized milk (milk that has not been processed with heat to kill parasites and bacteria).

Both public water supplies and natural water sources can become contaminated with human or animal waste (mainly from dogs and beavers) harboring the parasite (shown here) that causes giardiasis. The disease causes stomach upset and diarrhea when the parasite attaches itself to the lining of the digestive system, where it interferes with the body’s ability to absorb fats and carbohydrates. Custom Medical Stock Photo, Inc.

The eggs of some intestinal parasites pass through an infested person’s gastrointestinal tract and into feces. The parasite then can spread to other people through unintentional contact with the feces. Depending on the type of parasite, a person may become infested by touching his or her mouth after contact with feces that contain the organism (when changing a diaper or doing laundry, for example) or a contaminated area. Parasites can spread when a person eats contaminated food (such as un-washed raw fruits or vegetables, which can carry parasites from the soil or from people who have handled them) or drinks water contaminated by feces. Swimming in contaminated water also may result in infestation by certain parasites.

Parasitic intestinal infestations* often occur in outbreaks, when several people have symptoms at the same time. This is especially likely if many people come into contact with the same supply of contaminated food or water.

*intestinal

refer to illnesses caused by multi-celled parasitic organisms, such as tapeworms, roundworms, or protozoa.

What Are Some Common Intestinal Parasites?

Ascariasis

Ascariasis (as-kuh-RYE-uh-sis) is caused by Ascaris lum-bricoides, an intestinal roundworm. It is one of the most common intestinal parasites, affecting people in all parts of the world, especially in areas with poor sanitation. In the United States, ascariasis is rare, but it occurs most frequently in the rural parts of the Southeast. The worm also can infest pigs.

The life cycle of Ascaris lumbricoides begins when an adult worm lays its eggs in the intestines of an infected person. The eggs leave the body through the feces and can live in soil for up to 2 years. When people eat raw food containing this contaminated soil, they may swallow the worm’s eggs, which hatch in the stomach as larvae (LAR-vee, or immature worms). The larvae migrate through the blood to the lungs and then to the throat, where they are swallowed. Eventually, they pass into the intestines, where they begin the cycle again. The adult worms, which can grow to be more than 12 inches long, can live 1 to 2 years in the small intestine*. Ascariasis is not contagious, and a person can become infested only by ingesting the worm’s eggs.

*small intestine

is the part of the intestine between the stomach and large intestine.

Ascariasis usually causes no symptoms or only mild stomachaches or bloating. If a person is heavily infested, he or she may experience more severe pain. Some people also may have a cough or breathing problems when the larvae move through their lungs.

People often discover they have ascariasis when a worm passes in their bowel movements, or when they cough up a worm or it crawls out through the nose. This can be frightening, but the ascaris worm usually does not cause permanent damage to the body. Because of the relatively large size of adult ascaris worms, they can partially block the intestinal tract as well as the ducts leading from the biliary tract* and pancreas*. In rare cases, surgery may be needed to remove them.

*biliary

(BIH-lee-ah-ree) tract refers to the organs and ducts, including the liver and gallbladder, that produce, store, and transport bile, a substance which aids in digestion.

*pancreas

(PAN-kree-us) is a gland located behind the stomach that produces enzymes and hormones necessary for digestion and metabolism.

Strongyloidiasis

Strongyloidiasis (stron-juh-loy-DYE-uh-sis) is caused by another roundworm, Strongyloides stercoralis. This common infestation can be especially dangerous in people with weakened immune systems. If a person comes into contact with contaminated soil, the larva of the parasite can burrow through the skin. It travels to the lungs and then, in a manner similar to ascaris, is swallowed and ends up in the intestines, where the worm grows to adulthood and begins laying eggs. What is special about this parasite is that the eggs can hatch inside the intestines and the worms can continue to cycle through many generations (called the auto-infective cycle), causing an infestation that can last for decades.

In people with weakened immune systems, particularly those taking drugs such as corticosteroids*, strongyloidiasis can become overwhelming, and huge numbers of larvae can invade the lungs and other organs. This problem is called the hyperinfection syndrome and, although rare, it can be fatal.

*corticosteroids

(kor-tih-ko-STIR-oyds) are chemical substances made by the adrenal glands that have several functions in the body, including maintaining blood pressure during stress and controlling inflammation. They can also be given to people as medication to treat certain illnesses. People being treated with corticosteroid medication, particularly with high doses, may have a reduced ability to fight certain infections.

Giardiasis

Giardiasis (jee-ar-DYE-uh-sis) is the most common waterborne parasitic infection in the United States. Caused by Giardia intestinalis, a single-cell protozoan (also known as Giardia lamblia ), this infection can lead to diarrhea, cramping, and an upset stomach.

Giardia intestinalis lives in humans and animals. People become infected by drinking or swimming in contaminated water or by touching the feces of an infected person, or a contaminated surface, and then their mouths. People can spread the parasite if they do not wash their hands properly. Giardiasis occurs most frequently in settings where contaminated feces can be spread easily, such as in children in diapers, especially those in daycare centers, and in people who live in institutional settings such as nursing homes. Some people who are infected do not become sick but still can pass the infection on to others.

In people who do develop symptoms, stomach pain and watery diarrhea usually start 1 to 2 weeks after infection. About half the people who are infected also lose weight. The illness lasts 2 to 6 weeks, or longer in people who are sick with another disease.

Hookworm

Hookworms (a type of roundworm) are another common intestinal parasite. The U.S. Centers for Disease Control and Prevention (CDC) estimates that 1 billion people worldwide have hookworm infestations, although improved sanitation has reduced the number of cases in the United States.

Two species, Ancylostoma duodenale and Necator americanus, infest humans. The worms’ eggs hatch into larvae in warm, moist soil. Hookworms can penetrate human skin, so many people become infested when they walk barefoot in or touch contaminated soil. They also can become infested when they eat such soil (on unwashed raw fruit or vegetables, for example). The hookworm larvae travel to the lungs via the bloodstream; the larvae then travel to the throat and are swallowed, in a similar fashion to the ascaris worm. When they reach the small intestine, the larvae latch onto the intestinal walls and suck blood. There they mature and eventually lay eggs, which pass out of the body in feces. Hookworms can live for 1 to 2 years in the body.

A rash or itching at the site where the larvae entered the skin may signal hookworm infestation, followed by mild cramping and diarrhea. Heavily infested people may lose their appetite, lose weight, and have abdominal* pain. Hookworms can cause serious problems, including malnutrition and anemia* from intestinal bleeding. Newborns, young children, pregnant women, and malnourished people are most susceptible to these complications.

*abdominal

(ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

*anemia

(uh-NEE-me-uh) is a blood condition in which there is a decreased amount of oxygen-carrying hemoglobin in the blood and, usually, fewer than normal numbers of red blood cells.

Dogs and cats sometimes carry their own types of hookworms (Ancylostoma ceylanicum and Ancylostoma braziliense ), and these occasionally infest humans who come into contact with soil contaminated with cat or dog feces. In this type of infestation, called cutaneous larva migrans (kyoo-TAY-nee-us LAR-vuh MY-granz) or creeping eruption, the worm larvae burrow into the skin and cause severe itching but do not invade deeper into the body. The condition resolves without treatment after several weeks or months.

Amebiasis

Amebiasis (ah-mih-BYE-uh-sis) is caused by a single-cell parasite called Entamoeba histolytica. It occurs mainly in areas with poor sanitary conditions. Cases in the United States usually are seen in people who have recently arrived from or traveled in remote areas.

Amebiasis spreads when people touch infected feces or contaminated surfaces and then touch their mouths, or when they eat or drink contaminated food or water. It also can spread through certain types of sexual contact. Symptoms such as mild diarrhea and stomach pain may occur 1 to 4 weeks after infection, but only 1 infected person in 10 becomes sick and develops symptoms.

Amebic dysentery (uh-ME-bik DIH-sen-ter-e), a more severe form of the illness, causes bloody diarrhea, extreme stomach pain, and fever. Rarely, the infection spreads to other body organs, particularly the liver*, where the parasite can form large abscesses*. Because of the risk of amebic dysentery, Entamoeba histolytica is one of the most dangerous intestinal parasites, and infection with it can be fatal.

*liver

is a large organ located beneath the ribs on the right side of the body. The liver performs numerous digestive and chemical functions essential for health.

*abscesses

(AB-seh-sez) are localized or walled off accumulations of pus caused by infection that can occur anywhere within the body.

Other forms of amebas, including Entamoeba coli, Entamoeba dispar, and Entamoeba hartmanni, infect humans but cause no illness. These amebas can live in the human body for months or years without causing problems.

Cyclosporiasis and Cryptosporidiosis

Scientists identified cyclosporiasis (sy-klo-spoh-RYE-uh-sis), caused by the protozoan Cyclospora cayetanensis, in 1979. The infection is found worldwide, most

Two species of hookworm, Ancylostoma duodenale (left) and Necator americanus (right), can infect humans. Custom Medical Stock Photo, Inc.

frequently in developing countries, although there have been outbreaks in the United States and Canada.

Because Cyclospora cayetanensis must spend some time outside the human body to become infectious, it is not contagious from person to person. Infection usually results from ingesting contaminated water or soil or fresh produce grown in them.

Symptoms, which appear 1 week after infection and last from 1 to several weeks, may include diarrhea with frequent, watery, and sometimes explosive bowel movements; loss of appetite; stomach cramps; bloating; nausea (NAW-zee-uh); fever; vomiting; and weight loss. The diagnosis can be made by examining a sample of the patient’s bowel movements under a microscope to view the organism. If the illness is not treated, its symptoms may return.

Cryptosporidiosis (krip-toh-spor-id-e-O-sis) is an intestinal infection with symptoms similar to cyclosporiasis caused by the protozoan Cryptosporidium parvum that can live in people and animals. People can pick up the parasite through person-to-person contact or through water contaminated by the feces of infected animals.

Initially, it was thought that only people with weak immune systems, such as those with AIDS, contracted the infection. It is now known that the organism can infect people with normal immune systems and that cryptosporidiosis is one of the most common causes of protozoal diarrhea in the world. The infection goes away on its own in most people, but antibiotics and other treatments may be necessary for people with weak immune systems who contract cryptosporidiosis.

Enterobiasis

Enterobiasis (en-tuh-roh-BY-uh-sis), also known as pin-worm infestation, is caused by a staple-size worm known as Enterobius vermicularis. It is the most common worm infestation in the United States and is found primarily in children. Outbreaks of pinworm often occur in schools and daycare centers. From there, infested children may spread the worms to their family members.

Enterobius vermicularis lives in the rectum, the last part of the large intestine*, and comes out at night to lay eggs on the perineum (per-ih-NEE-um), the area around the anus and genitals. These eggs become contagious in a few hours and can spread to sheets and clothing, where they can remain contagious for about 2 weeks. Infestation occurs when people touch a contaminated area and then their mouths.

*large intestine

is the part of the intestine that contains the colon and rectum.

Itching of the perineum is the most common symptom of pinworm. This can lead to sleeplessness and irritability. Frequently, however, people show no signs of infestation.

Human tapeworm

Human tapeworm infestations usually are caused by eating meat or fish contaminated with worm larvae. Like other intestinal parasites, these worms frequently cause infestations in areas with poor sanitation, where livestock animals are exposed to contaminated soil or fish to contaminated water.

There are three common species of tapeworms: Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), and Diphyllobothrium latum (fish tapeworm). After someone eats contaminated meat or fish, tapeworm larvae travel to the intestines, where they latch onto the lining of the intestines and gradually grow into adults. The largest tapeworms can reach amazing sizes, measuring more than 20 feet long in some cases. The worms shed their eggs into the feces, from which they find their way into soil and water and are ingested by animals or fish. Humans ingest the larvae when they eat the contaminated meat or fish. Symptoms of a tapeworm infestation are often mild or nonexistent but can include abdominal pain, diarrhea, and malnutrition.

Two other diseases in humans can be traced to tapeworms that usually infest animals. In echinococcosis (ih-kye-nih-kah-KO-sis), large cysts* can develop in the liver, lungs, and other organs; in cysticercosis (sis-tuh-sir-KO-sis), the parasites can invade the muscles, brain, and eyes. Both echinococcosis and cysticercosis can occur when people eat food contaminated with the eggs of tapeworms that are found in the droppings of certain animals.

*cysts

(SISTS) are shell-like enclosures that contain small organisms in a resting stage.

Trichinosis

Trichinosis (trih-kih-NO-sis) arises from several varieties of Trichinella roundworms. Although once very common, it is now relatively rare in the United States, with the CDC reporting an average of just 38 cases per year. Trichinosis is more common in developing countries, however.

Trichinella larvae live in cysts in pigs and wild animals. When people eat their meat raw or undercooked, the cysts travel to the stomach, where acid dissolves the walls of the cysts and releases the immature worms. They move to the small intestine, mature, and lay eggs. Once the eggs hatch, the worms travel through the bloodstream to muscles, where they burrow in, forming cysts. This ends the cycle in humans.

The first symptoms of trichinosis, which include stomach pain, extreme tiredness, nausea, diarrhea, vomiting, and fever, appear 1 or 2 days after people eat infested meat. Headaches, chills, swelling of the eyes, cough, muscle aches and pains, and constipation (infrequent bowel movements) may follow. People with severe infestations also may have heart problems or trouble breathing.

How Are Intestinal Parasitic Diseases Diagnosed?

Doctors use samples of feces, sometimes taken a day or two apart, to diagnose intestinal parasitic diseases. The feces are examined for evidence of parasites, such as eggs, larvae, or adults. Blood samples can be taken to check for antibodies* to specific parasites, and doctors may use a medical instrument called an endoscope* to examine the intestines for infection.

*antibodies

(AN-tih-bah-deez) are protein molecules produced by the body’s immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

*endoscope

(EN-doh-skope) is a tool for looking inside parts of the body. It consists of a lighted tube and optical fibers and/or lenses.

To detect pinworms, doctors often request that patients take a “tape test.” For this test, patients briefly apply a piece of transparent tape to the skin around the anus in the early morning, after the worm has laid its eggs. The tape is removed and examined at the doctor’s office for any eggs that might be sticking to it.

How Are Intestinal Parasitic Diseases Treated?

Some cases require little or no treatment, and the parasites eventually disappear on their own. People with diarrhea and other signs of intestinal parasitic disease should talk to a doctor if their symptoms last more than a few days.

Medication used to treat the illnesses varies with the type of infection. Doctors may use antibiotics or antiparasitic medicines. In most cases, patients can remain at home and maintain a normal schedule. Children must stay out of daycare until they have been treated adequately and can no longer spread the infection. While they recover, patients are advised to drink plenty of fluids to avoid dehydration*. Anti-diarrhea medicine is not recommended because it may keep the parasites in the body longer. More severe cases may require treatment in the hospital.

*dehydration

(dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unre-placed loss of body fluids, such as through sweating, vomiting, or diarrhea.

In most cases, patients who have symptoms should feel better within 1 to 2 weeks, although it may be several more weeks before their bowel movements are completely back to normal.

Can Intestinal Parasitic Diseases Cause Medical Complications?

Dehydration is the most common general complication of intestinal parasite infections. Infants and young children are particularly vulnerable to dehydration and nutrition problems when they become infected. In

people with weak immune systems (such as people undergoing chemotherapy), infants, and the elderly, these infections can be fatal.

Some infections cause specific complications: amebiasis can affect the liver, lungs, and brain; parasites migrating through the lungs may cause difficulty breathing; and hookworm infestation can cause anemia and malnutrition, which can affect growth and development in children.

How Can Infection with Intestinal Parasites Be Prevented?

Good hygiene is the best defense against intestinal parasites. This includes frequent and thorough hand washing, especially after changing diapers, after going to the bathroom, and before handling food.

Doctors advise that travelers to undeveloped countries drink and brush their teeth with bottled water and avoid eating raw fruits and vegetables, food from street vendors, and unpasteurized dairy products. In addition, cooking all food until it is steaming hot kills parasites. Always wearing shoes and avoiding swimming in bodies of fresh water such as ponds, rivers, and lakes can minimize the risk of contact with contaminated soil and water.

See also

Intestinal Infections

Pinworm Infestation

Trichinosis

Resources

Organizations

U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. The CDC is the U.S. government authority for information about infectious and other diseases. It has fact sheets for the most common types of intestinal parasite infestations at its website.

Telephone 800-311-3435 http://www.cdc.gov

U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition (CFSAN), 5100 Paint Branch Parkway, College Park, MD 20740. CFSAN has an online “Bad Bug Book” that gives facts and figures on many foodborne parasites and illnesses.

Telephone 888-723-3366 http://www.cfsan.fda.gov

Symptoms of late-stage hookworm infestation are an enlarged abdomen and diarrhea. Worms can live up to 15 years in the human body, and females can lay 10,000 to 25,000 eggs every day. In severe cases the number of parasites may grow so large that the intestines become blocked. Photo Researchers, Inc.

Parasites

Protozoa

Helminths

Nematodes (roundworms)

Cestodes (tapeworms)

Trematodes (flukes)

Arthropods

Insects

Arachnids

Control of parasites

Resources

A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite derives the benefits of this relationship, while the host may suffer or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host’s body surface to feed are called ectoparasites, while those that live inside a host’s body are called endoparasites. Parasitism is a highly successful biological adaptation. There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria.

Parasitology is the study of parasites and their relationships with host organisms. Throughout history people have coped with over 100 types of parasites affecting humans. Parasites have not, however, been systematically studied until the last few centuries. With his invention of the microscope in the late 1600s, Anton von Leeuwenhoek (1632–1723) was perhaps the first to observe microscopic parasites. As Westerners began to travel and work more often in tropical parts of the world, medical researchers had to study and treat a variety of new infections, many of which were caused by parasites. By the early 1900s, parasitology had developed as a specialized field of study.

Typically, a parasitic infection does not directly kill a host, though the drain on the organism’s resources can affect its growth, reproductive capability and survival, leading to premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. Though the majority of parasitic infections occur within tropical regions and among low-income populations, most all regions of the world sustain parasitic species, and all humans are susceptible to infection.

Though many species of viruses, bacteria, and fungi exhibit parasitic behavior and can be transmitted by parasites, scientists usually study them separately as infectious diseases. Types of organisms that are studied by parasitologists include species of protozoa, helminths or worms, and arthropods.

Protozoa

Protozoa are one-celled organisms that are capable of carrying out most of the same physiological functions as multicellular organisms by using highly developed organelles within their cell. Many of the over 45,000 species of known protozoa are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of disease causing organisms.

Intestinal protozoa are common throughout the world and particularly in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst, where they have encased themselves in a protective outer membrane and are released through the digestive tract of a previous host. Once inside the host, they develop into a mature form that feeds and reproduces.

Amebic dysentery is one of the more common diseases that often afflicts travelers who visit tropical and sub-tropical regions. This condition, characterized by diarrhea, vomiting, and weakness, is caused by a protozoan known as Entamoeba histolytica.

Another protozoan that causes severe diarrhea, but is also found in more temperate regions, is Giardia lamblia. Among Leeuwenhoek’s discoveries was G. lamblia, which is a now well-known parasite that can infect hikers who drink untreated water in the back country. The disease it causes has been dubbed “beaver fever” (animals including the beaver can harbor the parasite).

Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector, which carries the parasite before it enters the final host. Often the vector is an invertebrate, such as an insect, that itself feeds on the host and passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are members of the genera Plasmodium, that cause malaria; Trypanosoma, that cause African sleeping sickness; and Leishmania, which leads to a number of debilitating and disfiguring diseases.

Helminths

Helminths are worm like organisms of which several classes of parasites are found including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Leeches, of the phylum Annelid, are also helminths and considered as ectoparasitic, attaching themselves to the outside skin of their hosts.

Nematodes (roundworms)

Nematodes, or roundworms, have an estimated 80,000 species that are known to be parasitic. The general morphology of these worms is consistent with their name; they are usually long and cylindrical in shape.

One of the most infamous nematodes is Trichinella spiralis, a parasite that lives its larval stage encysted in the muscle tissue of animals, including swine, and make their way into the intestinal tissue of humans who happen to digest infected, undercooked pork.

The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow up to 14 in (35 cm) in length within the small intestine of its host.

One roundworm, Enterobius vermicularis, or pin-worm, actually thrives in more temperate climates. This relatively small roundworm is not limited to humans living in relative poverty, but is also found in individuals living under more robust conditions.

Adult worms of Wuchereria bancrofti live in the blood and lymph of the host. Elephantiasis, characterized by extreme enlargement of a host’s extremities, is the rare but dramatic result of the host body’s defensive reaction to the presence of the worms.

Known to dog owners Dirofilaria immitus, or heartworm infection, if left untreated, can kill a dog as the worms infect the heart tissues and eventually weaken the cardiac muscles to the point of failure.

Cestodes (tapeworms)

Cestodes, or tapeworms, are a class of worms characterized by their flat, segmented bodies. The segments are called proglottides and hold both male and female reproductive organs, allowing self-fertilization. Proglottides that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tape-worms typically reside in the intestinal tract of vertebrates, attaching themselves to the mucosal lining with hooks or suckers on their scolex, or head. They do not possess a digestive tract, or alimentary canal, of their own, but absorb nutrients through their tegument, or skin, from partially digested food as it passes through the host.

Common tapeworms that frequent humans are Taenia saginata, Taenia solium, and Diphllobothrium latum. These parasites use intermediate hosts—cattle, swine, and fish respectively. Many parasites infect an intermediate host organism while in a developmental form, but they do not grow to maturity until they have been transmitted to the final or definitive host. In the Taenia species, the eggs are passed into cattle or swine through infected soil. They develop into an intermediary stage, called a cysticercus, which embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite and pass the cysticerci on when consumed by humans. The cysticeri develop into adult tapeworms that attach to the intestinal lining of the host. The cysticerci of T. solium can, themselves, cause medical complications. Instead of developing immediately into adult tapeworms, these cysticerci can migrate to any organ of the body, commonly ending up in the muscles or brain. A serious infection in the brain can lead to mental complications, including seizures and personality changes.

Trematodes (flukes)

Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the tegument, or skin, that allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker on their anterior end, sometimes ringed with hooks, that is used to attach themselves to the host’s tissues.

The life cycle of a typical trematode begins when eggs, that are passed out of a previous host’s digestive tract, find themselves in fresh water. The ciliated larval form, called miracidia, emerge from the eggs and swim until they find the appropriate species of their intermediate host: usually a snail. The miracidia penetrate the snail and change into another form, called sporo-cysts. The sporocysts undergo further changes resulting in yet another form of the parasite called cercariae, which burrow out of the snail and pass into the water again. A cercaria has a flagella like tail that helps it swim through the water in search of its final host, typically a mammal or avian species. The cercariae make contact with the skin of a host and burrow in. Host animals may also become infected with flukes by ingesting meat, usually fish or crustaceans, that are harboring the cercariae in an encysted form, called metacercaria, within their tissues. In either form, once inside the final host the parasite moves through tissues or the blood to the desired organ, often the intestine or liver, where it matures into a reproducing adult, starting the cycle again.

Clonorchis sinensis, a fluke common in the Far East, is a trematode that uses fish as one of its intermediate hosts and fish-eating mammals, including humans, as a final host. The adult C. sinensis flukes eventually make their way through the bile ducts to the liver of the host.

Another fluke that uses both a snail and a second intermediate host is Paragonimus westermani. Freshwater crabs and crayfish can harbor P. westermani metacercaria that, in turn, may be consumed. The adult of this parasite makes its way into the lungs of its host.

Some of the most infamous flukes are species of the genus Schistosoma that cause the often fatal schistosomiasis. The cercariae of these flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the blood stream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.

Arthropods

Arthropods are organisms characterized by exoskeletons and segmented bodies such as crustaceans, insects and arachnids. They are the most diverse and widely distributed animals on the planet. Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.

Insects

Certain insect species are the carriers of some of humanity’s most dreaded diseases, including malaria, typhus, and plague. As consumers of agricultural crops and parasites of our livestock, insects are also humankind’s number one competitor for resources.

Mosquitoes, are the most notorious carriers, or vectors, of disease and parasites. Female mosquitoes rely on warm-blooded hosts to serve as a blood meal to nourish their eggs. During the process of penetrating a host’s skin with their long, sucking mouth parts, saliva from the mosquito is transferred into the bite area. Any viral, protozoan or helminth infection carried in the biting mosquito can be transferred directly into the blood stream of its host. Among these are malaria, yellow fever, W. bancrofti (filariasis and elephantiasis), and D. immitis (heartworm).

Flies also harbor diseases that can be transmitted to humans and other mammals when they bite to obtain a blood meal for themselves. For example, black flies can carry river blindness, sandflies can carry leishmaniasis and kala-azar, and tsetse flies (found mainly in Africa) carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that can infest and feed on the skin, throat, nasal passages, and stomachs of their hosts.

Fleas and lice are two of the most common and irritating parasitic insects of humans and our livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of the epidemic inducing typhus fever. Fleas usually infest birds and mammals, and can feed on humans when they are transferred from pets or livestock. Fleas are known to carry a variety of devastating diseases, including the plague.

Arachnids

Another prominent class of arthropods that contains parasitic species is the arachnids. Though this group is more commonly known for spiders and scorpions, its parasitic members include ticks and mites.

Mites are very small arachnids that infest both plants and animals. One common type is chiggers, which live in grasses and, as larva, grab onto passing animals and attach themselves to the skin, often leading to irritating rashes or bite wounds. Scabies are another mite that causes mange in some mammals by burrowing into the skin and producing severe scabs, lesions, and loss of hair.

Ticks also live their adult lives among grasses and short shrubs. They are typically larger than mites, and

KEY TERMS

Arthropod —A phylum of organisms characterized by exoskeletons and segmented bodies.

Cestodes —A class of worms characterized by flat, segmented bodies, commonly known as tapeworms.

Definitive host —The organism in which a parasite reaches sexual maturity.

Helminths —Various phyla of worm-like animals.

Intermediate host —An organism infected by a parasite while the parasite is in a developmental form, but does not sexually mature.

Nematodes —Characterized by long, cylindrical bodies, commonly known as roundworms.

Protozoa —One-celled organisms.

Trematodes —A class of worms characterized by flat, oval shaped bodies, commonly known as flukes.

Vector —Any agent, living or otherwise, that carries and transmits parasites and diseases.

it is the adult female that attaches itself to an animal host for a blood meal. Tick bites themselves can be painful and irritating. More importantly, ticks can carry a number of diseases that affect humans. The most common of these include Rocky Mountain spotted fever, Colorado tick fever, and Lyme disease.

Control of parasites

Most parasitic infections can be treated by use of medical and surgical procedures. The best manner of controlling infection, though, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier, or prophylaxis, to certain parasites. Other measures of control include improving sanitary conditions of water and food sources, proper cooking techniques, education about personal hygiene, and control of intermediate and vector host organisms.

Resources

BOOKS

Combes, Claude, and Daniel Simberloff. The Art of Being a Parasite. Chicago: University of Chicago Press, 2005.

Fleisher, Paul. Parasites: Latching on to a Free Lunch. Breckenridge, CO: Twenty-First Century Books, 2006.

Gittleman, Ann Louise. Guess What Came to Dinner?: Parasites and Your Health. New York: Avery, 2001.

Jeffrey R. Corney

Parasites

A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection . The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and moves through the environment and one or more hosts. Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation . There are more known parasitic species than nonparasitic ones, and parasites affect just about every form of life, including most all animals, plants, and even bacteria .

The study of parasites

Parasitology is the study of parasites and their relationships with host organisms. Throughout history people have coped with over 100 types of parasites affecting humans. Parasites have not, however, been systematically studied until the last few centuries. With his invention of the microscope in the late 1600s, Anton von Leeuwenhoek was perhaps the first to observe microscopic parasites. As Westerners began to travel and work more often in tropical parts of the world, medical researchers had to study and treat a variety of new infections, many of which were caused by parasites. By the early 1900s, parasitology had developed as a specialized field of study.

Typically, a parasitic infection does not directly kill a host, though the drain on the organism's resources can affect its growth, reproductive capability and survival, leading to premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. Though the majority of parasitic infections occur within tropical regions and among low-income populations, most all regions of the world sustain parasitic species, and all humans are susceptible to infection.

Though many species of viruses, bacteria, and fungi exhibit parasitic behavior and can be transmitted by parasites, scientists usually study them separately as infectious diseases. Types of organisms that are studied by parasitologists include species of protozoa , helminths or worms, and arthropods .

Protozoa

Protozoa are one-celled organisms that are capable of carrying out most of the same physiological functions as multicellular organisms by using highly developed organelles within their cell . Many of the over 45,000 species of known protozoa are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of disease causing organisms.

Intestinal protozoa are common throughout the world and particularly in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst, where they have encased themselves in a protective outer membrane and are released through the digestive tract of a previous host. Once inside the host, they develop into a mature form that feeds and reproduces.

Amebic dysentery is one of the more common diseases that often afflicts travelers who visit tropical and sub-tropical regions. This condition, characterized by diarrhea, vomiting and weakness, is caused by a protozoan known as Entamoeba histolytica.

Another protozoan that causes severe diarrhea, but is also found in more temperate regions, is Giardia lamblia. Among Leeuwenhoek's discoveries was G. lamblia, which is a now well-publicized parasite that can infect hikers who drink untreated water in the back country.

Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector, which carries the parasite before it enters the final host. Often the vector is an invertebrate, such as an insect, that itself feeds on the host and passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are members of the genera Plasmodium, that cause malaria ; Trypanosoma, that cause African sleeping sickness ; and Leishmania, which leads to a number of debilitating and disfiguring diseases.

Helminths

Helminths are worm-like organisms of which several classes of parasites are found including nematodes (roundworms ), cestodes (tapeworms), and trematodes (flukes). Leeches, of the phylum Annelid, are also helminths and considered as ectoparasitic, attaching themselves to the outside skin of their hosts.

Nematodes (roundworms)

Nematodes, or roundworms, have an estimated 80,000 species that are known to be parasitic. The general morphology of these worms is consistent with their name; they are usually long and cylindrical in shape.

One of the most infamous nematodes is Trichinella spiralis, a parasite that lives its larval stage encysted in the muscle tissue of animals, including swine, and make their way into the intestinal tissue of humans who happen to digest infected, undercooked pork.

The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow up to 14 in (35 cm) in length within the small intestine of its host.

One roundworm, Enterobius vermicularis, or pinworm, actually thrives in more temperate climates. This relatively small roundworm is not limited to humans living in relative poverty, but is known to infect the well-off just as easily.

Adult worms of Wuchereria bancrofti live in the blood and lymph of the host. Elephantiasis , characterized by extreme enlargement of a host's extremities, is the rare but dramatic result of the host body's defensive reaction to the presence of the worms.

Known to dog owners Dirofilaria immitus, or heart-worm infection, if left untreated, can kill a dog as the worms infect the heart tissues and eventually weaken the cardiac muscles to the point of failure.

Cestodes (tapeworms)

Cestodes, or tapeworms, are a class of worms characterized by their flat, segmented bodies. The segments are called proglottides and hold both male and female reproductive organs, allowing self-fertilization. Proglottides that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tapeworms typically reside in the intestinal tract of vertebrates , attaching themselves to the mucosal lining with hooks or suckers on their scolex, or head. They do not possess a digestive tract, or alimentary canal , of their own, but absorb nutrients through their tegument, or skin, from partially digested food as it passes through the host.

Common tapeworms that frequent humans are Taenia saginata, Taenia solium, and Diphllobothrium latum. These parasites use intermediate hosts—cattle, swine, and fish respectively. Many parasites infect an intermediate host organism while in a developmental form, but they do not grow to maturity until they have been transmitted to the final or definitive host. In the Taenia species, the eggs are passed into cattle or swine through infected soil . They develop into an intermediary stage, called a cysticercus, that embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite and pass the cysticerci on when consumed by humans. The cysticeri develop into adult tapeworms that attach to the intestinal lining of the host. The cysticerci of T. solium can, themselves, cause medical complications. Instead of developing immediately into adult tapeworms, these cysticerci can migrate to any organ of the body, commonly ending up in the muscles or brain . A serious infection in the brain can lead to mental complications, including seizures and personality changes.

Trematodes (flukes)

Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the tegument, or skin, that allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker on their anterior end, sometimes ringed with hooks, that is used to attach themselves to the host's tissues.

The life cycle of a typical trematode begins when eggs, that are passed out of a previous host's digestive tract, find themselves in fresh water. The ciliated larval form, called miracidia, emerge from the eggs and swim until they find the appropriate species of their intermediate host: usually a snail. The miracidia penetrate the snail and change into another form, called sporocysts. The sporocysts undergo further changes resulting in yet another form of the parasite called cercariae, which burrow out of the snail and pass into the water again. A cercaria has a flagella-like tail that helps it swim through the water in search of its final host, typically a mammal or avian species. The cercariae make contact with the skin of a host and burrow in. Host animals may also become infected with flukes by ingesting meat, usually fish or crustaceans, that are harboring the cercariae in an encysted form, called metacercaria, within their tissues. In either form, once inside the final host the parasite moves through tissues or the blood to the desired organ, often the intestine or liver, where it matures into a reproducing adult, starting the cycle again.

Clonorchis sinensis, a fluke common in the Far East, is a trematode that uses fish as one of its intermediate hosts and fish-eating mammals , including humans, as a final host. The adult C. sinensis flukes eventually make their way through the bile ducts to the liver of the host.

Another fluke that uses both a snail and a second intermediate host is Paragonimus westermani. Freshwater crabs and crayfish can harbor P. westermani metacercaria that, in turn, may be consumed. The adult of this parasite makes its way into the lungs of its host.

Some of the most infamous flukes are species of the genus Schistosoma that cause the often fatal schistosomiasis. The cercariae of these flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the blood stream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.

Arthropods

Arthropods are organisms characterized by exoskeletons and segmented bodies such as crustaceans, insects and arachnids . They are the most diverse and widely distributed animals on the planet . Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.

Insects

Certain insect species are the carriers of some of humanity's most dreaded diseases, including malaria, typhus , and plague. As consumers of agricultural crops and parasites of our livestock , insects are also humankind's number one competitor for resources.

Mosquitoes , are the most notorious carriers, or vectors, of disease and parasites. Female mosquitoes rely on warm-blooded hosts to serve as a blood meal to nourish their eggs. During the process of penetrating a host's skin with their long, sucking mouth parts, saliva from the mosquito is transferred into the bite area. Any viral, protozoan, or helminth infections carried in the biting mosquito can be transferred directly into the blood stream of its host. Among these are malaria, yellow fever , W. bancrofti (filariasis and elephantiasis), and D. immitis (heartworm).

Flies also harbor diseases that can be transmitted to humans and other mammals when they bite to obtain a blood meal for themselves. For example, black flies can carry river blindness, sandflies can carry leishmaniasis and kala-azar, and tsetse flies (found mainly in Africa ), carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that can infest and feed on the skin, throat, nasal passages and stomachs of their hosts.

Fleas and lice are two of the most common and irritating parasitic insects of humans and our livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of the epidemic inducing typhus fever. Fleas usually infest birds and mammals, and can feed on humans when they are transferred from pets or livestock. Fleas are known to carry a variety of devastating diseases, including the plague.

Arachnids

Another prominent class of arthropods that contains parasitic species is the arachnids. Though this group is more commonly known for spiders and scorpions, its parasitic members include ticks and mites .

Mites are very small arachnids that infest both plants and animals. One common type is chiggers, which live in grasses and, as larva, grab onto passing animals and attach themselves to the skin, often leading to irritating rashes or bite wounds. Scabies are another mite that causes mange in some mammals by burrowing into the skin and producing severe scabs, lesions, and loss of hair.

Ticks also live their adult lives among grasses and short shrubs. They are typically larger than mites, and it is the adult female that attaches itself to an animal host for a blood meal. Tick bites themselves can be painful and irritating. More importantly, ticks can carry a number of diseases that affect humans. The most common of these include Rocky Mountain spotted fever, Colorado tick fever, and the latest occurrence of tick-borne infections: Lyme disease .

Control of parasites

Most parasitic infections can be treated by use of medical and surgical procedures. The best manner of controlling infection, though, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier, or prophylaxis, to certain parasites. Other measures of control include improving sanitary conditions of water and food sources, proper cooking techniques, education about personal hygiene, and control of intermediate and vector host organisms.

See also Flatworms; Strepsiptera; Tongue worms.

Resources

books

Brown, Harold, and Franklin Neva. Basic Clinical Parasitology. Norwalk, CT: Appleton-Century-Crofts, 1983.

Noble, Elmer, and Glenn Noble. Parasitology: The Biology of Animal Parasites. Philadelphia: Lea and Febiger, 1989.

Schmidt, Gerald, and Larry Roberts. Foundations of Parasitology. St. Louis: Times Mirror/Mosby College Publishing, 1989.

Warren, Kenneth, and John Bowers, eds. Parasitology: A Global Perspective. New York: Springer-Verlag, 1983.

periodicals

Jaenike, John. "Behind-the-Scenes Role of Parasites." Natural History June 1994: 46-48.

Moore, Janice. "The Behavior of Parasitized Animals." Bio-Science (February 1995): 89-96.

Tilton, Buck. "Don't Drink the Water." Backpacker (February 1994): 50-55.

Jeffrey R. Corney

KEY TERMS

Arthropod

—A phylum of organisms characterized by exoskeletons and segmented bodies.

Cestodes

—A class of worms characterized by flat, segmented bodies, commonly known as tapeworms.

Definitive host

—The organism in which a parasite reaches sexual maturity.

Helminths

—Term to define various phyla of worm-like animals.

Intermediate host

—An organism infected by a parasite while the parasite is in a developmental form, but does not sexually mature.

Nematodes

—Characterized by long, cylindrical bodies, commonly known as roundworms.

Protozoa

—One-celled organisms.

Trematodes

—A class of worms characterized by flat, oval shaped bodies, commonly known as flukes.

Vector

—Any agent, living or otherwise, that carries and transmits parasites and diseases.

Parasites

A parasite is an organism that depends on another organism, known as a host, for food and shelter. As an example, tapeworms live in the digestive system of a large variety of animals. The tapeworms have no digestive system of their own, but absorb nutrients through their skin from partially digested food as it passes through the host.

A parasite usually gains all the benefits of this relationship. In contrast, the host may suffer from various diseases, infections, and discomforts as a result of the parasitic attack. In some cases, however, the host may show no signs at all of infection by the parasite.

The life cycle of a typical parasite commonly includes several developmental stages. During these stages, the parasite may go through two or more changes in body structure as it lives and moves through the environment and one or more hosts.

Words to Know

Arthropod: A phylum of organisms characterized by exoskeletons and segmented bodies.

Definitive host: The organism in which a parasite reaches reproductive maturity.

Helminths: A variety of wormlike animals.

Intermediate host: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.

Nematodes: A type of helminth characterized by long, cylindrical bodies; commonly known as roundworms.

Protozoa: Single-celled animal-like microscopic organisms that must live in the presence of water.

Trematodes: A class of worms characterized by flat, oval-shaped bodies; commonly known as flukes.

Vector: Any agent, living or otherwise, that carries and transmits parasites and diseases.

Parasites that remain on a host's body surface to feed are called ectoparasites, while those that live inside a host's body are called endoparasites. Parasitism is a highly successful biological adaptation. More parasitic species are known than nonparasitic ones. Parasites affect just about every form of life, including nearly all animals, plants, and even bacteria.

The study of parasites

Parasitology is the study of parasites and their relationships with host organisms. Throughout history, people have coped with over 100 types of parasites affecting humans. Parasites have not, however, been systematically studied until the last few centuries. With his invention of the microscope in the late 1600s, the Dutch scientist Anton von Leeuwenhoek (1632–1723) was perhaps the first person to observe microscopic parasites. As Westerners began to travel and work more often in tropical parts of the world, medical researchers had to study and treat a variety of new infections, many of which were caused by parasites. By the early 1900s, parasitology had developed as a specialized field of study.

Typically, a parasitic infection does not directly kill a host. The stress placed on the organism's resources can affect its growth, ability to reproduce, and survival. This stress can sometimes lead to the host's premature death. Parasites, and the diseases they cause and transmit, have been responsible for tremendous human suffering and loss of life throughout history. The majority of parasitic infections occur within tropical regions and among low-income populations. However, almost all regions of the world sustain parasitic species, and all humans are susceptible to infection.

Infectious diseases

An infectious disease, or infection, is a condition that results when a parasitic organism attacks a host and begins to multiply. As the parasite multiplies, it interferes with the normal life functions of the host more and more. The host begins to feel ill as a symptom of the parasite's invasion and activities. In many cases, the host's immune system (which fights foreign bodies in the body) may be able to respond to the parasite and destroy it. In many other cases, however, the parasitic infection may over-whelm the immune system, resulting in serious disease and even death.

Until a century ago, infections were the primary means of human "population control" worldwide, often killing enormous numbers of people in epidemics of diseases such as bubonic plague and typhoid fever. Even today, infections actually cause more deaths during war and famine than do actual injuries and starvation. Fortunately, many infectious diseases can now be treated by means of antibiotics and other drugs and by a variety of preventative methods.

Almost all infections contracted by humans pass from other humans or animals. Some infections originate from outside the body, among them a cold from kissing someone with a cold; rabies from a dog bite; hepatitis B from a contaminated needle entering the bloodstream; hepatitis A from germs transferred from fingers to mouth after touching a dirty toilet seat; measles, mumps, and the flu from tiny moisture particles that exit the mouth and nose when a person sneezes, coughs, or talks; syphilis from an infected sex partner; tetanus from a soil-contaminated wound; salmonella from ingesting undercooked eggs, meat, and poultry; and many diseases ranging from the relatively innocent to the fatal—such as gastroenteritis, cholera, and dysentery—from drinking or bathing in contaminated water.

Endogenous (caused by factors within the organism) infections occur when the host's resistance is lowered, perhaps by malnutrition, illness, trauma, or immune depression. Weakening of the host's immune system may permit normally harmless organisms already present in or on the host or in the environment to cause illness.

Types of parasites

The major types of organisms that cause parasitic infections include species of protozoa, helminths or worms, and arthropods.

Protozoa. Protozoa are single-celled organisms that carry out most of the same physiological functions as more complex organisms. More than 45,000 species of protozoa are known, many of which are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of diseasecausing organisms.

Intestinal protozoa occur throughout the world. They are especially common in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst. A cyst consists of a protozoan encased in a protective outer membrane. The membrane protects the organism as it travels through the digestive tract of a previous host. Once inside a new host, the parasite develops into a mature form that feeds and reproduces.

Amebic dysentery is one of the most common parasitic diseases. It often afflicts travelers who visit tropical and subtropical regions. The condition is characterized by diarrhea, vomiting and weakness. It is caused by a protozoan known as Entamoeba histolytica.

Another protozoan that causes severe diarrhea is Giardia lamblia. This organism was originally discovered by Leeuwenhoek and has been well-publicized as a parasite that can infect hikers who drink untreated water.

Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector. A vector is an organism that carries a parasite from one host to another host. In many cases, the vector is an invertebrate, such as an insect that itself feeds on a host and then passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are the ones that cause malaria and African sleeping sickness.

Helminths. Helminths are wormlike organisms including nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Leeches are also helminths and are considered ectoparasitic, since they attach themselves to the outside skin of their hosts.

One of the most infamous nematodes is Trichinella spiralis. At one stage of its life cycle, this nematode lives in the muscle tissue of animals, including swine. Eventually, these organisms make their way into the intestinal tissue of humans who happen to ingest infected, undercooked pork.

The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow to a length of 35 centimeters (15 inches) within the small intestine of its host.

A parasitic roundworm that affects dogs is Dirofilaria immitus, or heartworm. This worm infects the heart tissues and eventually weakens

the cardiac (heart) muscles to the point of failure. If left untreated, heart-worm can kill a dog.

Tapeworms are a class of worms characterized by their flat, segmented bodies. The segments hold both male and female reproductive organs, allowing self-fertilization. Segments that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tapeworms typically reside in the intestinal tract of vertebrates, attaching themselves to the stomach lining with hooks or suckers on their head.

Common tapeworms that attack humans are Taenia saginata, Taenia solium, and Diphyllobothrium latum. These parasites use intermediate hosts, such as cattle, swine, and fish respectively, before entering the human body. Parasites such as these infect an intermediate host organism while in a early developmental form. But they do not grow to maturity until they have been transmitted to the final host.

In the case of Taenia species, for example, tapeworm eggs are passed into cattle or swine through infected soil. They develop into an intermediary

stage that embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite, which are passed on when consumed by humans. The tapeworms develop into adults that attach to the intestinal lining of the host.

Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the skin. These muscles allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker, sometimes ringed with hooks. They use the sucker to attach themselves to the host's tissues.

Some of the most infamous flukes are species of the genus Schistosoma that cause the often-fatal disease known as schistosomiasis. These flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the bloodstream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.

Arthropods. Arthropods are organisms characterized by exterior skeletons and segmented bodies. Examples include the crustaceans, insects, and arachnids. The arthropods are the most diverse and widely distributed animals on the planet. Many arthropod species serve as carriers of bacterial and viral diseases, as intermediate hosts for protozoan and helminth parasites, and as parasites themselves.

Certain insect species are the carriers of some of humanity's most dreaded diseases, including malaria, typhus, and plague. As consumers of agricultural crops and parasites of our livestock, insects are also humankind's number-one competitor for resources.

Mosquitoes are the most notorious carriers of disease and parasites. Female mosquitoes rely on warm-blooded hosts to serve as a blood meal to nourish their eggs. During the process of penetrating a host's skin with their long, sucking mouth parts, saliva from the mosquito is transferred into the bite area. Any viral, protozoan, or helminth infections carried in the biting mosquito can be transferred directly into the blood stream of its host. Among these diseases are malaria, yellow fever, filariasis, elephantiasis, and heartworm.

Flies also harbor diseases that can be transmitted to humans and other mammals when they bite to obtain a blood meal for themselves. For example, black flies can carry Onchocerciasis (which causes river blindness), sandflies can carry leishmaniasis and kala-azar, and tsetse flies can carry the trypanosomes that cause sleeping sickness. Livestock, such as horses and cattle, can be infected with a variety of botflies and warbles that infest and feed on the skin, throat, nasal passages, and stomachs of their hosts.

Fleas and lice are two of the most common and irritating parasitic insects of humans and livestock. Lice commonly live among the hairs of their hosts, feeding on blood. Some species are carriers of typhus fever. Fleas usually infest birds and mammals, and can feed on humans when they are transferred from pets or livestock. Fleas are known to carry a variety of devastating diseases, including the plague.

Another prominent class of arthropods that contains parasitic species is the arachnids. Included in this group are spiders, scorpions, ticks, and mites.

Mites are very small arachnids that infest both plants and animals. One common type of mite is the chigger, which lives in grasses. As larvae, they may grab onto passing animals and attach themselves to the skin, often leading to irritating rashes or bite wounds. Scabies are another

mite that causes mange in some mammals by burrowing into the skin and producing severe scabs, lesions, and loss of hair.

Ticks also live their adult lives among grasses and short shrubs. They are typically larger than mites. The adult female tick attaches itself to an animal host for a blood meal. Tick bites themselves can be painful and irritating. More importantly, ticks can carry a number of diseases that affect humans. The most common of these diseases include Rocky Mountain spotted fever, Colorado tick fever, and Lyme disease.

Control of parasites

Many parasitic infections can be treated by a variety of medical procedures, such as the use of antibiotics. The best way of controlling infection, however, is prevention. Scientists have developed and continue to test a number of drugs that can be taken as a barrier to certain parasites. Other measures of control include improving sanitary conditions of water and food sources, proper cooking techniques, education about personal hygiene, and control of intermediate and vector host organisms.

[See also Arachnids; Arthropods; Plague; Protozoa ]

parasites Like many words in the English language, ‘parasite’ has its origins in ancient Greek, where its full meaning was ‘one who eats at the table of another’. With the growth of knowledge of infectious agents, worm infestations, and general microbiology in the eighteenth and nineteenth centuries, the term became eventually identified with organisms, whether of plant or animal origins, living in or upon others (‘host’ organisms), from which they derive nutriment. Bacteria and viruses qualify for inclusion in this definition; but once bacteriology and, later, virology, acquired their own academic disciplines and terminologies from the late nineteenth century onwards, ‘parasite’ in common usage came to refer, more often than not, to the larger and more or less visible organisms preying on human and animal bodies either externally (mites and ticks, fleas and lice) or internally (the large, multicellular helminths — i.e. worms or ‘flukes’ — and the unicellular protozoa).

External parasites as disease vectors

Mites, ticks, and lice have a long history as true parasites, as well as carriers of infections, notably of typhus (the body louse identified as carrier by Charles Nicolle in 1909), of African relapsing fever (carried by a spirochete, Dutton and others in 1904), and of Rocky Mountain spotted fever (carried by the wood tick, as shown by H. T. Ricketts in 1907). The demonstration by Theobald Smith of transmission of the protozoon, Pyrosoma bigeminum, of Texas fever of cattle by the cattle tick Boöphilus bovis, in 1893, was a milestone in the history of the study of disease transmission by ticks and insects. Insects rarely parasitize mammals, having originated in the Palaeozoic with a long period of adaptive evolution long before the arrival of mammals. Far more important is their role as vectors, transmitting some of the most serious tropical diseases such as yellow fever.

Although a nuisance rather than a serious threat to general health, scabies is the best known of the diseases in man which are caused directly by mites. The itch mite acarus (Sarcoptes scabiei var. hominis, a.k.a. Acarus scabiei) is ubiquitous, but outbreaks of scabies vary in frequency, both seasonally and geographically. Historically, it may or may not have been described in Biblical texts, but Hildegard of Bingen certainly referred to itch mites (suren) in her Physika, published in the twelth century. Five centuries later, Cestoni of Leghorn and Giovanni Bonomo famously provided in 1687 the first complete evidence for the causal role played by the mite in scabies in man. Noted by Francesco Redi in the same year, the acarus mite and its biology and pathology were used from then on as model systems for smaller organisms — visible only with the help of microscopes — as responsible for development and spread of ‘contagious’ diseases. First introduced by C. F. Cogrossi in his Pensieri in 1713, this use of scabies and its pathogenesis as a favourite paradigm for the cause and spread of infections was to last well into the nineteenth century.

Anthony van Leeuwenhoek, using one of his own early microscopes, had found the ciliated protozoon Giardia lamblia in his own stools during an attack of diarrhoea in 1681. Today giardiasis is an increasing, if relatively innocuous threat to local populations and travellers in the tropics and the Western world.

Helminths

Worms are more easily recognized, even with the naked eye, in bodily fluids and waste products, and have been associated with disease in man and his domestic animals since the seventeenth century. ‘Worms’ and ‘insects’ were used as convenient synonyms for what were then unknown agents of ‘contagion’ of any kind, even when no such outward signs were observed. Scrutiny of the Ebers papyrus (1550 bc), and evidence from tissue samples of Egyptian mummies, have suggested that infections caused by the common roundworm (Ascaris lumbricoides), tapeworms, the guinea worm (Dracunculus medinensis), and the African schistosome (S. haematobium) were present in Egypt long before. More recent results also point to the presence of Trichinella spiralis, the nematode worm of global distribution whose pathology may have been responsible for the Jewish kosher taboo on the eating of pork products. In our own century, trichinosis has been prevalent in Germany and Eastern Europe — and in settlers from those countries on the North American continent — because of habits of using undercooked meat in sausages etc.

Another nematode worm, the humble hookworm, played a major role in the formation of the public health policies of disease control and prevention, which began in Europe and the US at the turn of the century, and were to grow into campaigns with worldwide concerns and consequences throughout the twentieth century. Hookworm disease in man (Ancylostoma duodenale and Necator americanus) had long been considered characteristic of tropical latitudes without any understanding of its epidemiology, until an outbreak among workers on the StGotthardt Tunnel in 1880 forced the Italian medical authorities to review the situation. In spite of work by G. B. Grassi and E. Perroncito, little progress was made until Arthur Looss's discovery, at the turn of the century, of the ability of infective hookworm larvae to penetrate intact skin. This opened the way to introduction of control measures, at first tested in the early years of the twentieth century in mines in Belgium: the use of sanitary buckets, regular testing and treatment of infected miners, and instruction in personal prophylaxis. By then it had also been noted by Perroncito that salt was toxic to hookworm larvae, and that miners working in areas with a high salt content, in Poland, France, and Cornwall, showed resistance to infection. Scattering of salt around mines was recommended, but implementation was slow, and only became standard practice in South African gold mines more than twenty years later. Meanwhile, John D. Rockefeller had been persuaded to launch his philanthropic work by giving financial support to the Sanitary Commission's Hookworm Eradication Campaign, envisaged by C. W. Stiles, in the southern US, where the debilitating effects of hookworm disease had long been a threat to poor working populations. Although full eradication was never achieved, a satisfactory measure of control was reached after four years.

After that, the Rockefeller Foundation set its sights on wider targets: it was the beginning of the Foundation's concerns with the improvement of public health on a global scale. As a by-product, it also paved the way for Rockefeller support for expansion of Patrick Manson's London School of Tropical Medicine into a full-scale school of public health concerned with temperate as well as tropical localities.

Among helminths classed as trematodes, the schistosomes occupy an important position as agents of the disease formerly called bilharzia (after Theodor Bilharz), now professionally referred to as schistosomiasis. The worms (vesical blood flukes) of the disease develop through stages in intermediate hosts via life-cycles as complex as those of many protozoa. Different species of schistosomes use different species of water snails as intermediate hosts. The cercariae emerge from their snail host into the water (river, rice field, etc.), and from there penetrate the skin of persons unlucky enough to swim or wade in contaminated water. If the disease remains undiagnosed and untreated, what initially appears in patients as merely vague feelings of being ‘below par’, leads at best to years of reasonably well tolerated infection with occasional acute episodes of decreased working capacity, at worst to liver failure or involvement of the central nervous system.

Parasitic zoonoses

Other parasitic zoonoses (parasites involving animals as primary or intermediate hosts of disease in man), rare but closer to home, include infestation with tapeworms of dogs and cats transmitted by the human flea Pulex irritans, and with Toxocara canis, sometimes transmitted via dog faeces to children, either playing in public parks or otherwise in contact with puppies. Unhappily this parasite can cause serious disease, including impairment — at worst, total loss — of sight. Such tragic consequences of local nematode infections are rare in the West; but in parts of Africa, Onchocerca volvulus, transmitted by the blackfly Simulium damnosum, continues to cause thousands of cases a year of ‘river blindness’ in Kenya's ‘Valley of the Blind’.

Among parasites causing diseases regarded as largely tropical, three have been subjects of intense research from the beginning of the twentieth century: the plasmodia of malaria; the trypanosomes of African sleeping sickness (Trypanosoma gambiense and rhodesiense); and that of Chagas' disease in South America, T. cruzi. Yet they were not described, let alone linked to the serious diseases they cause, until the late nineteenth and early twentieth centuries.

Malaria (Italian: mal aria, ‘bad air’, named at a time when miasma theories of contagion prevailed) has a long and colourful history, which, in spite of real advances in understanding of its aetiology, and past and present hopes of the therapies, and lately of effective vaccines, still shows no firm promise of nearing an imminent conclusion. Malaria is caused by infection with one of four species of Plasmodium (P. falciparum, P. vivax, P. malariae, P. ovale) and transmitted by female anopheline mosquitoes, again of different species; the life cycle of malaria parasites is complex and the disease caused by P. falciparum is the most severe — in recent years increasingly so. It is now eradicated in Europe: it disappeared from northern latitudes early in this century, and a determined campaign saw it finally eradicated in Italy after World War II. There were high hopes of eradication in Africa, India, and the Far East following the discovery of DDT and the synthesis of new anti-malarial compounds. These hopes were dashed when the mosquitoes developed resistance to DDT, and the Plasmodium parasites to new synthetic drugs, in step with their development. Additional problems include serious side-effects caused by synthetic quinine substitutes. Malaria ranks second only to the diarrhoeal and respiratory diseases in terms of global morbidity and mortality.

African sleeping sickness, like a number of other tropical diseases caused by protozoa, is almost invariably fatal unless promptly treated with drugs, which were introduced shortly after the trypanosomes were discovered in the first decade of the twentieth century. Transmitted by tsetse flies, in which the trypanosomes of African sleeping sickness develop, they enter their human host by the bite of the fly. The American form of trypanosomiasis, (Chagas' disease), is transmitted through the faeces of insects of the family Reduviidae (‘assassin bugs’) often infesting walls in dwellings in poor and remote areas of South America, making attempts at eradication difficult.

Humans and parasites have coexisted uneasily for centuries; only in the latter half of the twentieth century, following the end of hostilities after World War II, did hope burgeon for well-organized scientific control. Supported by the WHO and various private foundations, this might at last win the battle with parasites in tropical and temperate zones worldwide. The best hope lies in vaccine development. Advances in this area, especially against malaria and schistosomiasis, are not yet conclusive, but give cause for more optimism than before.

Lise Wilkinson

Bibliography

Donaldson, R. J. (ed.), (1979). Parasites and Western man. MTP Press Ltd. Lancaster.
Warren, K. S. and Bowers, J. Z. (ed.), (1983). Parasitology. A global perspective. Springer-Verlag, New York etc.
Englund, P. T. and Sher, A. (ed.), (1998). The biology of parasitism. A molecular and immunological approach. Alan R. Liss, Inc., New York.