PARASITES — SCHWARTZ 421 



enable the larvae that hatch from these eggs to establish contact with 

 the first intermediate host. Such pollution apparently has taken place 

 in the Great Lakes region during the past few decades. Finlanders 

 and other northern Europeans in that area who have retained their 

 native fondness for raw or smoked or salted fish in which live plero- 

 cercoids may be present have contributed greatly to the pollution of 

 lakes and streams to which infestations with this tapeworm have been 

 traced in this country. 



Although it was predicted in 1897 that the fish tapeworm would be- 

 come endemic in various sections of the upper peninsula of Mich- 

 igan, where immigi'ants from Finland, Sweden, and other Baltic 

 countries were working as miners, it was not until 1906 that the first 

 case of this tapeworm in a native American was reported from Minne- 

 sota. Before 1906 persons harboring this tapeworm in the United 

 States were foreigners who either brought, or certainly could have 

 brought, this parasite with them from their place of origin in Europe, 

 especially the Baltic countries. In North America endemic areas are 

 Minnesota and Michigan in the United States and Manitoba in 

 Canada. 



The heef and pork tapeworms. — The beef tapeworm, known to zoolo- 

 gists as Taenia saginata, and the related pork tapeworm. Taenia so- 

 lium^ are so-named because the larval or cysticercus stage (known also 

 as a bladder worm) of these two parasites occurs in cattle and swine, 

 respectively. The adult parasite occurs in the intestine of man. 



The adult beef tapeworm (pi. 1), which may attain a length of 40 

 feet or more, consists of a chain of proglottids or segments anchored 

 to the intestinal wall by a so-called head which bears four cup-shaped 

 suckers. It has been estimated that the entire tapeworm colony, or 

 strobila, consists of between 1,000 and 2,000 proglottids. Those far- 

 thest from the head are gravid, those nearest to it are inunature, and 

 those between these extremes are sexually mature. The gravid pro- 

 glottids filled with the tapeworm eggs become detached from the rest 

 of the strobila either singly or in small chains and are voided to the 

 outside with the bowel evacuations. If the proglottids reach pastures, 

 barnyards, feed lots, or other places to which cattle have access, the 

 segments disintegrate and liberate the eggs. Cattle are likely to swal- 

 low hundreds of eggs and become infected with the cystic stage of 

 the tapeworm. The cysticerci, which sometimes occur in large num- 

 bers, localize principally in the muscles of the bovine intermediate 

 host (pi. 2, fig. 1) and develop into bladder worms, ranging from about 

 one-fifth to three-fifths of an inch in length by about half that in width. 

 The thin-walled bladder worm contains a fluid into which the head of 

 the future tapeworm is invaginated. 



