Adult tapeworms normally inhabit the bowel. In sheep, how ever, there is a species which lives in the bile ducts. Considering their size, these parasites produce relatively little serious harm. Their presence causes considerable local irritation and diarrhoea. Large numbers may produce obstruction and intussusception (see INTESTINAL OBSTRUCTION) in young animals. Owing to the amount of the nourishment they absorb, their presence is asso ciated with abnormal appetite on the part of the host.
Perhaps the most widespread and dangerous form is the small Taenia echinococcus, only a quarter of an inch long, which lives as an adult parasite in the small intestine of the dog. Its im portance lies in the dangerous character of its larval development. This may occur in a very large number of mammals including man, sheep and cattle. The eggs are passed in the dog's faeces. When swallowed by the intermediate host they grow into cysts varying in size from a tangerine to a football. Within these cysts thousands of infective forms develop. When eaten by a dog these infective young attain sexual maturity. Echinococcal cysts usu ally develop in the liver, lung or spleen and cause considerable pressure atrophy of the tissues. Escape of fluid from ruptured cysts causes severe shock. Two other tapeworms of interest are Taenia solium and T. sagilurta. The larva of the former develops as a cyst in the muscles of the pig while that of the latter similarly develops in cattle. A search for these cysts is one of the objects of meat inspection. Sheep and rabbits frequently harbour cysts which develop into adult Taenias in the dog. One of these occurs in the brain producing symptoms of "gid" in sheep, see Plate, fig. 9. Cattle, sheep, horses and rabbits frequently harbour adult intestinal cestodes belonging to the family Anoplocephalidae. Although of economic importance, no information is available con cerning their life history and intermediate hosts. In towns, dogs and cats are quite frequently heavily infested with tapeworms of relatively small size which are made up of lentil-shaped segments and belong to the genus Dipylidium. The lice and fleas which infest these animals are their intermediate hosts. A peculiar type of life history is that seen in the minute Hymenolepis nana, one of the commonest tapeworms in the United States. Man appears to act as intermediate as well as definitive host for this parasite.
Larval development takes place in the wall of the small intestine in which the adult subsequently lives.
The cestodes so far described belong to the division Cyclo phyllidea and are characteristically armed with four round suckers for purposes of attachment. The other main subdivision is named the Pseudo-phyllidea. In this group the typical round suckers are replaced by two elongated slits. The most important member of this small group is Dibothriocephalus latus a parasite of man in Finland, Sweden, Latvia and Switzerland. It also occurs in other countries where lake fish form a staple article of diet. Infection is acquired from infected fish of the family Salmonidae, perch, pike, etc. These intermediate hosts acquire their infection from minute fresh water Crustacea belonging to the family Cyclopidae, which in turn have swallowed and been infected by the swimming larva which hatches from the eggs passed by infected man. For many years an anaemia resembling pernicious anaemia has been attributed to infection with D. latus, but recent studies have failed to bring evidence in support of these earlier clinical deductions.
BIBLIOGRAPHY.—General text books: A. Railliet, Traite de zoologie medicale et agricole (1885. 2nd ed., 1895) ; P. Manson, Tropical Dis eases (1898. 8th ed., by P. Manson-Bahr, 1925) ; M. Braun and 0. Seifert, Die tierischen Parasiten des Menschen (Wiirzburg, 1903. 5th ed., 1915, Eng. trans. by P. Falche 1906) ; E. Brumpt, Précis de Para sitologie (Iwo) ; A. Castellani and A. J. Chalmers, Manual of Tropical Medicine (191o. 3rd ed., 1919) ; M. Neven-Lemaire, Parasitologie des Animaux Domestiques (1912) ; H. B. Fantham and A. Porter, Some Minute Animal Parasites (1914); H. B. Fantham, J. W. W. Stephens and F. V. Theobald, The Animal Parasites of Man (1916) ; A. C. Chandler, Animal Parasites and Human Disease (1918. 3rd ed., rev. 1926) ; C. M. Wenyon, Protozoology (1926) ; W. Yorke and P. A. Maplestone, The Nematode Parasites of Vertebrates (1926) ; R. N. Chopra and A. Chandler, Anthelmingics and their Uses (1928) ; L. Rogers, Recent Advances in Tropical Medicine (1928). See also: Archiv fur Protistenkunde (Jena, 1902, etc.) ; Parasitology, a supple ment to the Journal of Hygiene (Cambridge, 1908, etc.) ; Journal of Parasitology (Urbana, 1914, etc.) ; American Journal of Hygiene, Publ. by the School of Hygiene and Public Health of the Johns Hopkins Univ. (Baltimore, 1921, etc.) ; Annales de parasitologie humaine et comparee (Paris, 1922, etc.) ; Journal of Helminthology, Publ. by the School of Tropical Medicine, Lond. Univ. (1923, etc.). For summaries and reviews of current literature see Tropical Diseases Bulletin, Publ. by the Bureau of Hygiene and Tropical Diseases (London, 1912, etc.).
(R. T. L.)