SECERNENTEANS: Secernentea

PHYSICAL CHARACTERISTICS

Secernenteans (seck-uhr-NEHN-shuns) are land-dwelling parasites of plants and animals. Parasites (PAIR-uh-sites) are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Secernenteans can be microscopic to several feet long. The body of secernenteans consists of a flexible cylinder with a pointed tail and a blunt head. A flexible but tough covering with a system of crosswise grooves from head to tail protects the insides of secernenteans. Lengthwise ridges run along most of the body. Most secernenteans have a hard, sharp spear on the head. They use muscles to move the spear in and out to puncture their host's cells and empty the contents.

GEOGRAPHIC RANGE

Secernenteans live all over the world.

HABITAT

Most secernenteans live in plant and animal hosts in all types of land habitats. They are rarely found in the sea or in freshwater habitats. Species that are not parasites often live in the soil.

DIET

Secernenteans feed on blood, body fluid, intestinal contents, and mucus in their hosts. They eat bacteria, fungi, and other growths in the soil.

BEHAVIOR AND REPRODUCTION

The life cycle of secernenteans generally goes from fertilization (FUR-teh-lih-zay-shun), or the joining of egg and sperm to start development, in the females through four stages of young and into adulthood. The young worms that hatch from the eggs usually resemble adults except they are smaller and their sex organs are not yet mature. Each of the four stages of young is separated from one another by complete shedding of the outer body layer. Most species of secernenteans have males and females, but in some species male and female organs are in the same worm.

SECERNENTEANS AND PEOPLE

Parasitic secernenteans cause disease and death in people, their food animals, their pets, and their crops. On the other hand, secernenteans that are not parasites are important to the health and survival of humans because they help keep soil healthy.

CLASSIFYING WORMS

Are there 60 families of secernenteans or 89 or a number in-between? On the basis of the number of species found and studied so far—about eight thousand—scientists believe a huge number of species have yet to be discovered. Until they know more about the worms, scientists are not ready to assign them to a particular group.

CARE FOR YOUR PET

One dog can be infected with 25 to 100 heartworms. The heart swells, and lung, liver, and kidney damage can occur. Drugs and surgery are used to remove the worms from an infected dog. People can protect their pets by giving them medication regularly to prevent heartworm.

CONSERVATION STATUS

Secernenteans are not considered threatened or endangered.

CANINE HEARTWORM (Dirofilaria immitis): SPECIES ACCOUNTS

Physical characteristics: Female canine heartworms are 10 to 12 inches (25 to 30 centimeters) long and about 0.2 inch (5 millimeters) wide. Males are about half the size of females.

Geographic range: Canine heartworms live all over the world. Because they are found throughout the world, no distribution map is provided.

Habitat: Canine heartworms live mainly in warm areas. The primary hosts, also called final hosts, are dogs, cats, ferrets, foxes, wolves, sea lions, and humans. In their hosts adult worms live in the right ventricle of the heart and the blood vessels that connect to it. The intermediate hosts are mosquitoes.

Diet: Canine heartworms feed on their hosts' nutrients, primarily through blood in and around the heart and lungs.

Behavior and reproduction: Adult canine heartworms live in the pulmonary arteries of their primary hosts. The pulmonary (PULL-muh-NAIR-ee) arteries take blood from the heart to the lungs. Rather than releasing eggs, the females release large numbers of microscopic young worms—as many as five thousand a day—into the host's bloodstream. These microscopic worms can stay in the host for a year or more but cannot develop further until they enter a mosquito. A mosquito bites the host and sucks in the worms with the hosts' blood. The worms start maturing inside the mosquito, which bites another animal and injects the young worms into it. The worms travel to the heart, become adults, and start releasing microscopic young in the new host. Adults can live in a host and continue to produce microscopic young for several years.

Canine heartworms and people: Canine heartworms are dangerous to people and their animals.

Conservation status: Canine heartworms are not considered threatened or endangered. ∎

RAT LUNGWORM (Angiostrongylus cantonensis): SPECIES ACCOUNTS

Physical characteristics: Adult rat lungworms are 0.8 to 1.3 inches (20 to 34 millimeters) long and 0.01 to 0.02 inch (320 to 560 micrometers) wide. The females are larger than the males.

Geographic range: Rat lungworms live all over the world in warm areas. Because they are found throughout the world, no distribution map is provided.

Habitat: The primary hosts of rat lungworms are rats. The intermediate hosts are animals such as snails, oysters, slugs, and crabs.

Diet: Rat lungworms feed on nutrients in the blood of their hosts, specifically around the lungs and brains of rodents and the lungs of humans.

Behavior and reproduction: Adult rat lungworms live in the blood vessels of the lungs of their hosts. Young worms in their first developmental stage enter the respiratory tract, move up the breathing tubes to the mouth, and then are swallowed. The worms move through the digestive tract and are passed in the host's feces (FEE-seez). They enter intermediate hosts, such as snails, which are eaten by rodents and humans. In rodents the worms travel to the pulmonary arteries and the lungs, where they mature. The adults eventually travel to the brain and travel back to the lungs through veins. In humans, the parasites enter the brain but do not develop further and die.

Rat lungworms and people: Rat lungworms are dangerous to humans because they rupture blood vessels in the brain, causing headache, fever, nerve damage, coma, and death.

Conservation status: Rat lungworms are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Zimmer, Carl. Parasite Rex. New York: Free Press, 2000.

Web sites:

"Filarial Nematodes." Worm Learn.http://home.austarnet.com.au/wormman/nemacont.htm (accessed on February 16, 2005).

"Heartworm: The Parasite." Mar Vista Animal Medical Center. http://www.marvistavet.com/html/heartworm_-_the_parasite.html (accessed on February 16, 2005).

"Illinois, Iowa, Ohio and Wisconsin Warned of Giant African Snails." News-Medical.Net. http://www.news-medical.net/?id=1301 (accessed on February 16, 2005).