Amebiasis

Definition

Amebiasis is an infectious disease caused by a parasitic one-celled microorganism (protozoan) called Entamoeba histolytica. Persons with amebiasis may experience a wide range of symptoms, including diarrhea, fever, and cramps. The disease may also affect the intestines, liver, or other parts of the body.

Description

Amebiasis, also known as amebic dysentery, is one of the most common parasitic diseases occurring in humans, with an estimated 500 million new cases each year. It occurs most frequently in tropical and subtropical areas where living conditions are crowded, with inadequate sanitation. Although most cases of amebiasis occur in persons who carry the disease but do not exhibit any symptoms (asymptomatic), as many as 100,000 people die of amebiasis each year. In the United States, between 1 and 5% of the general population will develop amebiasis in any given year, while male homosexuals, migrant workers, institutionalized people, and recent immigrants develop amebiasis at a higher rate.

Human beings are the only known host of the amebiasis organism, and all groups of people, regardless of age or sex, can become affected. Amebiasis is primarily spread in food and water that has been contaminated by human feces but is also spread by person-to-person contact. The number of cases is typically limited, but regional outbreaks can occur in areas where human feces are used as fertilizer for crops, or in cities with water supplies contaminated with human feces.

Causes and symptoms

Recently, it has been discovered that persons with symptom-causing amebiasis are infected with Entamoeba histolytica, and those individuals who exhibit no symptoms are actually infected with an almost identical-looking ameba called Entamoeba dispar. During their life cycles, the amebas exist in two very different forms: the infective cyst or capsuled form, which cannot move but can survive outside the human body because of its protective covering, and the disease-producing form, the trophozoite, which although capable of moving, cannot survive once excreted in the feces and, therefore, cannot infect others. The disease is most commonly transmitted when a person eats food or drinks water containing E. histolytica cysts from human feces. In the digestive tract the cysts are transported to the intestine where the walls of the cysts are broken open by digestive secretions, releasing the mobile trophozoites. Once released within the intestine, the trophozoites multiply by feeding on intestinal bacteria or by invading the lining of the large intestine. Within the lining of the large intestine, the trophozoites secrete a substance that destroys intestinal tissue and creates a distinctive bottle-shaped sore (ulcer). The trophozoites may remain inside the intestine, in the intestinal wall, or may break through the intestinal wall and be carried by the blood to the liver, lungs, brain, or other organs. Trophozoites that remain in the intestines eventually form new cysts that are carried through the digestive tract and excreted in the feces. Under favorable temperature and humidity conditions, the cysts can survive in soil or water for weeks to months, ready to begin the cycle again.

Although 90% of cases of amebiasis in the United States are mild, pregnant women, children under two years of age, the elderly, malnourished individuals, and people whose immune systems may be compressed, such as cancer or AIDS patients and those individuals taking prescription medications that suppress the immune system, are at a greater risk for developing a severe infection.

The signs and symptoms of amebiasis vary according to the location and severity of the infection and are classified as follows:

Intestinal amebiasis

Intestinal amebiasis can be subdivided into several categories:

ASYMPTOMATIC INFECTION. Most persons with amebiasis have no noticeable symptoms. Even though these individuals may not feel ill, they are still capable of infecting others by person-to-person contact or by contaminating food or water with cysts that others may ingest, for example, by preparing food with unwashed hands.

CHRONIC NON-DYSENTERIC INFECTION. Individuals may experience symptoms over a long period of time during a chronic amebiasis infection and experience recurrent episodes of diarrhea that last from one to four weeks and recur over a period of years. These patients may also suffer from abdominal cramps, fatigue, and weight loss.

AMEBIC DYSENTERY. In severe cases of intestinal amebiasis, the organism invades the lining of the intestine, producing sores (ulcers), bloody diarrhea, severe abdominal cramps, vomiting, chills, and fevers as high as 104-105°F (40-40.6°C). In addition, a case of acute amebic dysentery may cause complications, including inflammation of the appendix (appendicitis ), a tear in the intestinal wall (perforation), or a sudden, severe inflammation of the colon (fulminating colitis).

KEY TERMS

Ameboma— A mass of tissue that can develop on the wall of the colon in response to amebic infection.

Antibody— A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Appendicitis— Condition characterized by the rapid inflammation of the appendix, a part of the intestine.

Asymptomatic— Persons who carry a disease and are usually capable of transmitting the disease but who do not exhibit symptoms of the disease are said to be asymptomatic.

Dysentery— Intestinal infection marked by diarrhea containing blood and mucus.

Fulminating colitis— A potentially fatal complication of amebic dysentery marked by sudden and severe inflammation of the intestinal lining, severe bleeding or hemorrhaging, and massive shedding of dead tissue.

Inflammatory bowel disease (IBD)— Disease in which the lining of the intestine becomes inflamed.

Lumen— The inner cavity or canal of a tube-shaped organ, such as the bowel.

Protozoan— A single-celled, usually microscopic organism that is eukaryotic and, therefore, different from bacteria (prokaryotic).

AMEBOMA. An ameboma is a mass of tissue in the bowel that is formed by the amebiasis organism. It can result from either chronic intestinal infection or acute amebic dysentery. Amebomas may produce symptoms that mimic cancer or other intestinal diseases.

PERIANAL ULCERS. Intestinal amebiasis may produce skin infections in the area around the patient's anus (perianal). These ulcerated areas have a "punched-out" appearance and are painful to the touch.

Extraintestinal amebiasis

Extraintestinal amebiasis accounts for approximately 10% of all reported amebiasis cases and includes all forms of the disease that affect other organs.

The most common form of extraintestinal amebiasis is amebic abscess of the liver. In the United States, amebic liver abscesses occur most frequently in young Hispanic adults. An amebic liver abscess can result from direct infection of the liver by E. histolytica or as a complication of intestinal amebiasis. Patients with an amebic abscess of the liver complain of pain in the chest or abdomen, fever, nausea, and tenderness on the right side directly above the liver.

Other forms of extraintestinal amebiasis, though rare, include infections of the lungs, chest cavity, brain, or genitals. These are extremely serious and have a relatively high mortality rate.

Diagnosis

Diagnosis of amebiasis is complicated, partly because the disease can affect several areas of the body and can range from exhibiting few, if any, symptoms to being severe, or even life-threatening. In most cases, a physician will consider a diagnosis of amebiasis when a patient has a combination of symptoms, in particular, diarrhea and a possible history of recent exposure to amebiasis through travel, contact with infected persons, or anal intercourse.

It is vital to distinguish between amebiasis and another disease, inflammatory bowel disease (IBD) that produces similar symptoms because, if diagnosed incorrectly, drugs that are given to treat IBD can encourage the growth and spread of the amebiasis organism. Because of the serious consequences of misdiagnosis, potential cases of IBD must be confirmed with multiple stool samples and blood tests, and a procedure involving a visual inspection of the intestinal wall using a thin lighted, tubular instrument (sigmoidoscopy ) to rule out amebiasis.

A diagnosis of amebiasis may be confirmed by one or more tests, depending on the location of the disease.

Stool examination

This test involves microscopically examining a stool sample for the presence of cysts and/or trophozoites of E. histolytica and not one of the many other intestinal amebas that are often found but that do not cause disease. A series of three stool tests is approximately 90% accurate in confirming a diagnosis of amebic dysentery. Unfortunately, however, the stool test is not useful in diagnosing amebomas or extraintestinal infections.

Sigmoidoscopy

Sigmoidoscopy is a useful diagnostic procedure in which a thin, flexible, lighted instrument, called a sigmoidoscope, is used to visually examine the lower part of the large intestine for amebic ulcers and take tissue or fluid samples from the intestinal lining.

Blood tests

Although tests designed to detect a specific protein produced in response to amebiasis infection (antibody) are capable of detecting only about 10% of cases of mild amebiasis, these tests are extremely useful in confirming 95% of dysentery diagnoses and 98% of liver abscess diagnoses. Blood serum will usually test positive for antibody within a week of symptom onset. Blood testing, however, cannot always distinguish between a current or past infection since the antibodies may be detectable in the blood for as long as 10 years following initial infection.

Imaging studies

A number of sophisticated imaging techniques, such as computed tomography scans (CT), magnetic resonance imaging (MRI), and ultrasound, can be used to determine whether a liver abscess is present. Once located, a physician may then use a fine needle to withdraw a sample of tissue to determine whether the abscess is indeed caused by an amebic infection.

Treatment

Asymptomatic or mild cases of amebiasis may require no treatment. However, because of the potential for disease spread, amebiasis is generally treated with a medication to kill the disease-causing amebas. More severe cases of amebic dysentery are additionally treated by replacing lost fluid and blood. Patients with an amebic liver abscess will also require hospitalization and bed rest. For those cases of extraintestinal amebiasis, treatment can be complicated because different drugs may be required to eliminate the parasite, based on the location of the infection within the body. Drugs used to treat amebiasis, called amebicides, are divided into two categories:

Luminal amebicides

These drugs get their name because they act on organisms within the inner cavity (lumen) of the bowel. They include diloxanide furoate, iodoquinol, metronidazole, and paromomycin.

Tissue amebicides

Tissue amebicides are used to treat infections in the liver and other body tissues and include emetine, dehydroemetine, metronidazole, and chloroquine. Because these drugs have potentially serious side effects, patients given emetine or dehydroemetine require bed rest and heart monitoring. Chloroquine has been found to be the most useful drug for treating amebic liver abscess. Patients taking metronidazole must avoid alcohol because the drug-alcohol combination causes nausea, vomiting, and headache.

Most patients are given a combination of luminal and tissue amebicides over a treatment period of seven to ten days. Follow-up care includes periodic stool examinations beginning two to four weeks after the end of medication treatment to check the effectiveness of drug therapy.

Prognosis

The prognosis depends on the location of the infection and the patient's general health prior to infection. The prognosis is generally good, although the mortality rate is higher for patients with ameboma, perforation of the bowel, and liver infection. Patients who develop fulminant colitis have the most serious prognosis, with over 50% mortality.

Prevention

There are no immunization procedures or medications that can be taken prior to potential exposure to prevent amebiasis. Moreover, people who have had the disease can become reinfected. Prevention requires effective personal and community hygiene.

Specific safeguards include the following:

• Purification of drinking water. Water can be purified by filtering, boiling, or treatment with iodine.
• Proper food handling. Measures include protecting food from contamination by flies, cooking food properly, washing one's hands after using the bathroom and before cooking or eating, and avoiding foods that cannot be cooked or peeled when traveling in countries with high rates of amebiasis.
• Careful disposal of human feces.
• Monitoring the contacts of amebiasis patients. The stools of family members and sexual partners of infected persons should be tested for the presence of cysts or trophozoites.

Resources

BOOKS

Friedman, Lawrence S. "Liver, Biliary Tract, & Pancreas." In Current Medical Diagnosis and Treatment, 1998, edited by Stephen McPhee, et al., 37th ed. Stamford: Appleton & Lange, 1997.

Amebiasis

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

BIBLIOGRAPHY

Introduction

Amebiasis (am-e-BI-a-sis) is an infection that is caused by a one-celled parasite called Entamoeba histolytica. The infection, which produces an inflammation of the cells lining the intestinal tract, is also referred to as amebic (or amoebic) dysentery.

Amebiasis often results in relatively mild illness, producing diarrhea and abdominal pain. However, the infection can be quite severe, with inflammation being so extensive that the intestinal wall in the colon can become perforated, and damage can occur to both the liver and the brain. As well, diarrhea can be copious and often accompanied by vomiting, which can lead to dehydration if fluids are not replaced.

Disease History, Characteristics, and Transmission

E. histolytica can occur in two forms. One form is known as a cyst. This form is very tough, and can survive harsh conditions of temperature and lack of moisture that would kill the other, growing form of the organism called the trophozoite. This hardiness makes a cyst similar to a bacterial spore. The parasite is excreted in feces as a cyst. It can survive for a long time until it finds itself in a more favorable environment, such as the intestinal tract of another person. There, the cyst can resume growth. The trophozoite is the form that causes amebiasis. Some trophozoites will form cysts and can be excreted, beginning another cycle of infection.

The cysts can also invade the walls of the intestine, where they can germinate into the trophozoite forms. Then, ulcers and diarrhea can be produced. Or, much more seriously, the cysts may enter the bloodstream and can be carried all over the body. Damage to tissues such as the brain and liver can result.

When symptoms develop, they tend to begin about 2 to 4 weeks after the parasite has entered the body, although some people develop symptoms in only a few days.

Amebiasis has been known since the early years of the twentieth century. Despite this, the diagnosis of amebiasis has not changed in over a century, still relying on the visual detection of the cyst in feces from the person suspected of having the infection. This can be a tedious and lengthy process, often requiring days of examination. Complicating diagnosis, the cysts of E. histolytica resemble that of other amoeba called Entamoeba coli and Entamoeba dispar, which are normal and harmless residents of the intestinal tract of warm-blooded animals, including humans. Indeed, E. histolytica and E. dipar are virtually identical in appearance. This means that many cases of amebiasis are likely diagnosed incorrectly.

WORDS TO KNOW

DYSENTERY: Dysentery is an infectious disease that has ravaged armies, refugee camps, and prisoner-of-war camps throughout history. The disease still is a major problem in developing countries with primitive sanitary facilities.

TROPHOZOITE: The amoeboid, vegetative stage of the malaria protozoa.

Scope and Distribution

Some people who are infected carry E. histolytica in their intestinal tract without displaying symptoms. Since the parasite can be excreted along with feces, a person can unknowingly pass the parasite to someone else by handling food with unwashed hands after going to the bathroom, by person-to-person contact (including sexual intercourse), or by contaminating drinking water with feces. This route of transmission can persist for years after a person has been exposed to the parasite. The persons who subsequently become infected might become ill.

Amebiasis affects about 50 million people world-wide each year, making it one of the two most common causes of intestinal inflammation; the other is caused by Shigella. Approximately 100,000 people die of the infection each year. Those most often affected are in poorer health; thus, amebiasis tends to be more common in developing countries, where sanitation is inadequate and where people live in crowded conditions, making the spread of the parasite much easier. However, anyone is susceptible; several hundred cases are reported each year in the United States, for example. In developed countries, those who become infected tend to be pregnant women, the young and the elderly, and those whose immunesystems have become compromised due to malnourishment or disease (such as acquired immunodeficiency syndrome [AIDS]).

Treatment and Prevention

Amebiasis is treatable using a combination of drug therapies. Some drugs generically called amebicides kill the organisms that are growing in the intestinal tract, while other drugs can lessen the chance that the infection will spread to tissues such as the liver.

Impacts and Issues

Persons who travel to high-risk countries such as parts of Africa, India, Latin America, and Southeast Asia, where the infection is commonly prevalent in some regions (such an infection is described as being endemic) should take precautions against contracting amebiasis. Precautions include drinking bottled water or boiling drinking water for at least one minute, peeling the skins off fresh fruits and vegetables before eating them, and proper handwashing using soap.

An important issue concerning amebiasis is that the parasite can be excreted in the feces of someone who has no symptoms of the infection. In fact, this is true for the majority of people; estimates are that only one in ten people who are infected actually become sick. While this is a small percent, the fact that millions of people become infected each year still means that a great many people become ill, with many more remaining capable of spreading the infection to others.

Research is ongoing to find more definitive ways of treating amebiasis, and in preventing the infection in the first place. As of 2007, there is no vaccine for the infection. A blood test is available that can detect the presence of the parasite. However, because the test detects the presence of antibodies—molecules produced by the immune system that are targeted against the particular invading organism—the test only reveals if someone has ever had an infection, not necessarily an ongoing infection.

The World Health Organization (WHO) recommends that if the presence of amoeba in the feces is confirmed microscopically but the person is not experiencing any symptoms, then it should not be assumed that the person has amebiasis.

On a larger scale, the WHO is building an international network, now totaling over 100 organizations, that together aim to reduce worldwide deaths from diseases such as amebiasis. The group, called the International Network to Promote Household Water Treatment and Safe Storage, plans to implement sustainable and affordable methods of purifying drinking water supplies in communities without access to sanitation or treated water, or with water that is improved but from unsafe sources. Although large waterborne outbreaks of amebiasis are uncommon, water treatment and sanitation measures are complimentary and are developed together when possible.

In the era of molecular biology, procedures have been developed that can detect the genetic material of E. histolytica in feces. However, the test is relatively expensive and requires specialized equipment and training that may not be part of a clinic, especially in an underdeveloped region.

AVOIDING INFECTION WITH E. HISTOLYTICA

To avoid infection with E. histolytica, The Centers for Disease Control and Prevention (CDC) recommends that a person traveling to a country that has poor sanitary conditions should observe the following with regard to eating and drinking:

• Drink only bottled or boiled (for 1 minute) water or carbonated (bubbly) drinks in cans or bottles. Do not drink fountain drinks or any drinks with ice cubes. Another way to make water safe is by filtering it through an “absolute 1 micron or less” filter and dissolving iodine tablets in the filtered water. “Absolute 1 micron” filters can be found in camping/outdoor supply stores.
• Do not eat fresh fruit or vegetables that you did not peel yourself.
• Do not eat or drink milk, cheese, or dairy products that may not have been pasteurized.
• Do not eat or drink anything sold by street vendors.

SOURCE: Centers for Disease Control and Prevention (CDC)

IN CONTEXT: IMPROVED WATER ACCESS

The list below reflects data from the World Health Organization indicating countries recently reporting access to improved water sources for less than 50% of the population (with the year of the report indicated):

• Afghanistan 13% of the population (year reported: 2002)
• Somalia 29% (2002)
• Cambodia 34% (2002)
• Chad 34% (2002)
• Papua New Guinea 39% (2002)
• Mozambique 42% (2002)
• Lao People's Democratic Republic 43% (2002)
• Equatorial Guinea 44% (2002)
• Madagascar 45% (2002)
• Congo 46% (2002)
• Democratic Republic of the Congo 46% (2002)
• Niger 46% (2002)
• Mali 48% (2002)

SOURCE: World Health Organization (WHO)

See AlsoGiardiasis; Parasitic Diseases; Sanitation.

BIBLIOGRAPHY

Books

Guerrant, Richard I., David H. Walker, and Peter F. Weller. Tropical Infectious Diseases: Principles, Pathogens & Practice. Oxford: Churchill Livingstone, 2005.

Web Sites

Centers for Disease Control and Prevention. “Amebiasis.” <http://www.cdc.gov/ncidod/dpd/parasites/amebiasis/factsht_amebiasis.htm> (accessed March 15, 2007).

Brian Hoyle

Amebiasis

What Are the Signs and Symptoms of Amebiasis?

How Is Amebiasis Diagnosed and Treated?

How Is Amebiasis Prevented?

Resources

Amebiasis (am-e-BY-a-sis) is an infection of the large intestine by the single-cell parasite Entamoeba histolytica (ent-a-ME-ba his-to-LIT-i-ka). It frequently causes diarrhea or dysentery.

KEYWORDS

for searching the Internet and other reference sources

Digestive system

Dysentery

Infection

Parasites

Amebiasis is a disease caused by an ameba, or microscopic parasite*, called Entamoeba histolytica. At one stage in the ameba’s life cycle, it is enclosed in a protective wall called a cyst. Infection begins when a person swallows cysts in contaminated food or water. Amebiasis is found worldwide, including the United States, but it is most common in tropical areas where sanitation is poor.

* parasites

are creatures that live in and feed on the bodies of other organisms. The animal or plant harboring the parasite is called its host.

When swallowed, the cysts resist destruction by stomach acids and travel to the intestine. In the intestine, the amebae (plural) emerge from their cysts and multiply, usually without causing any symptoms. In some people, however, for unknown reasons, the amebae invade the walls of the large intestine, where they cause abdominal pain, bloody diarrhea (dysentery), and sometimes fever. At this stage, there is a danger that the amebae will invade other body organs.

During the infection, the amebae produce cysts that pass out of the intestines in the stools. Outside the body, the cysts can survive for days or weeks. In areas with poor sanitation, drinking water contaminated with human waste can quickly spread amebiasis and the cycle begins again.

What Are the Signs and Symptoms of Amebiasis?

Most ameba infections are asymptomatic, which means there are no symptoms. Even without symptoms, however, cysts are still produced, and the person is considered a carrier or cyst passer.

Amebic colitis

Symptoms occur primarily when the amebae attack the wall of the large intestine. This is known as amebic colitis. The most common symptom is abdominal pain that begins gradually. Additional symptoms may include diarrhea that contains blood or mucus, frequent bowel movements, and a constant nagging feeling of needing to move the bowels. In about one third of cases, fever is present.

Amebic dysentery

In rare cases, the symptoms of amebic colitis worsen to fever, chills, and severe diarrhea with blood and mucus. This condition is known as amebic dysentery (DIS-en-ter-y) and often leads to severe dehydration (excessive loss of body water).

The U.S. and the World

• Nearly 500 million people worldwide may be carrying the Entamoeba histolytica parasite, but only about 50 million people develop symptoms of amebiasis.
• Although the Entamoeba histolytica parasite is found in the United States, the disease is a much larger problem in developing nations in the tropical areas of Africa, Latin America, and Asia. Poor sanitation, inadequate water treatment, and the use of human waste as fertilizer contribute to the problem.
• Worldwide, amebiasis causes up to 110,000 deaths each year, but fewer than a handful of those deaths occur in the United States.

Hepatic amebiasis

If the amebae move through the bloodstream to other parts of the body, pockets of infection and pus can form in other organs. In about 1 percent of cases, the amebae infect the liver and cause a condition known as hepatic (he-PAT-ik) amebiasis. The symptoms of hepatic amebiasis include fever, a distended (swollen) abdomen, and pain and tenderness in the area of the liver just below the right ribs.

How Is Amebiasis Diagnosed and Treated?

The most common method for diagnosing amebiasis is examining stained stool smears under a microscope. Entamoeba histolytica also can be identified from samples of tissue obtained during visual examination of the colon with a flexible instrument called a colonoscope (ko-LON-o-scope) or during surgery.

If doctors find Entamoeba histolytica, they will prescribe medication for asymptomatic carriers (who can spread amebiasis if the amebae are not killed) and for people with active infections. Treatment usually lasts for about three weeks.

How Is Amebiasis Prevented?

There is no vaccine or prophylactic (disease-preventing) drug for amebiasis. Prevention of amebiasis depends on maintaining clean drinking water supplies, disposing of human waste properly, and using appropriate hygiene measures, such as thoroughly washing hands after going to the bathroom and before eating.

Municipal water supplies approved by local health departments in the United States are usually considered safe. When camping or traveling in other countries, however, it is important to use water only from safe sources and to avoid sources such as mountain streams.

See also

Diarrhea

Giardiasis

Resources

The U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition posts a Bad Bug Book at its website that includes a fact sheet about Entamoeba histolytica. http://vm.cfsan.fda.gov/-mow/chap23.html

The U.S. Centers for Disease Control and Prevention (CDC) has a Division of Parasitic Diseases that posts a fact sheet about amebiasis infection at its website. http://www.cdc.gov/ncidod/dpd/amebias.htm