Helminth infections acquired through the gastrointestinal tract

Trichinosis. Trichinella spiralis is the cause of trichinosis in man. The nematode circulates between rats and pigs with man becoming infected from eating raw or inadequately cooked pork products. Encysted larvae in the meat excyst (hatch) in the intestine and develop into minute adults in the mucosa. These mature and the females deposit larvae that then migrate through the tissues to reach skeletal muscles in which they encyst. Human infections may be asymptomatic but can include fever, orbital oedema, myalgia and eosinophilia. In the extreme, infection can be fatal through myocarditis or encephalitis.
Enterobiasis. Enterobias vermicularis is a small thread-like "pinworm" mainly infecting young children. The female emerges to the perianal region usually at night and lays some 10,000-15,000 eggs and then dies. In the process they cause severe pruritis (itching). The embryonated eggs are infectious on ingestion and hatch in the duodenum. The larvae pass to the caecum where they mature into adults. Because of the pruritis, children often re-infect themselves from eggs under their fingernails. Bedding is also a source of infection and can be a means of spreading the organism in families and institutions such as orphanages and boarding schools.
Taenia solium (pork tapeworm). The adult lives in the small intestine of man that is the definitive host. Segments of the worm pass through the anus and release large numbers of eggs that can survive for long periods outside of the body. When ingested by pigs, the eggs hatch and each releases an onchosphere that migrates through the intestinal wall and blood vessels to reach striated muscle where encystment occurs. When inadequately cooked pig meat is eaten by man, excystment occurs in the small intestine and an adult cestode (worm) develops. If the eggs are released into the upper intestine of man (e.g. through regurgitation) they can invade the host setting up a potentially dangerous larval infection known as cysticercosis in muscle and other sites.
Taenia saginata (beef tapeworm). This also infects man through cattle. The life cycle is similar to T. solium and in both species the adult tapeworm can grow up to 10 meters in length.
Hydatidosis. This is caused by the tapeworm Echinococcus granulosus. The adult worm inhabits the small intestine of dogs from which the eggs of the species are passed. These eggs can be ingested by herbivorous animals and hatch in the duodenum. The embryos enter the circulation where they are carried to various sites to develop into cysts. Dogs become infected when they eat contaminated offal. Humans are infected if they accidentally ingest eggs from infected dogs and the liver is the most common site of infection in which hydatid cysts form.

References: Baker, J.R. and Muller, R (Eds). Advances in Parasitology. Academic Press, London. Knight, R. (1982). Parasites Diseases in Man. Churchill Livingstone, London. Peters, W. and Gilles, H.M. (1995) Colour Atlas of Tropical Medicine and Parasitology. Mosby-Wolfe.