420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



as trichostrongyles, normally acquired by their hosts through the 

 ingestion of forage on which their larval stages may be present. It 

 is not surprising therefore that these parasites are largely accidental 

 sojourners in man, whereas among those he apparently acquired from 

 carnivores and onmivores, some have become so well adapted to him 

 that they are among the more common species he can harbor. 



In this paper only a few of the parasites that animals share with 

 man will be considered. The parasites selected for discussion are 

 among the most important ones in temperate regions, including the 

 United States. 



HUMAN PARASITES ACQUIRED THROUGH INGESTION OF ANIMAL FOOD 



The parasites that man commonly acquires through the consumption 

 of animal food include four species, of which three are cestodes, or 

 tapeworms, and one is a roundworm, or a nematode. 



CESTODES 



The hroad tapeworm. — One tapeworm, commonly referred to as the 

 broad tapeworm, or the fish tapeworm, and known to zoologists as 

 DiphyUohothrium, latum^ occurs as an adult in the small intestine of 

 man, the dog, and certain wild carnivores. In the intestine of man 

 this parasite may reach a size of up to 60 feet in length and about 

 fourth-fifths of an inch in width. 



The life cycle of the fish tapeworm involves two intermediate hosts. 

 The first one is a copepod, or so-called water flea, of the genera Di- 

 aptomus and Cyclops., and the second is a fresh-water fish. In the 

 United States the wall-eyed pike, the sand pike, the burbot, and the 

 pickerel, among others, are common second intermediate hosts. The 

 first intermediate host becomes infected by swallowing the free- 

 swimming, microscopic larvae that hatch from the tapeworm eggs 

 in water. The fish or second intermediate host becomes infected by 

 swallowing parasitized copepods in which the tapeworm larvae have 

 meanwhile undergone further development, or by eating smaller fish 

 that have recently ingested parasitized copepods, or by ingesting 

 smaller fish harboring the still more advanced tapeworm larvae, known 

 as plerocercoids, in their viscera and muscles. Man and other defini- 

 tive hosts become infected with the adult or strobilate tapeworm by 

 eating raw, infected fish. Once the plerocercoid reaches the intestine 

 of man or of another suitable definitive host, it grows rapidly. When 

 mature, the tapeworm discharges enormous numbers of eggs which 

 hatch in water, following a period of incubation therein, into the 

 free-swimming, ciliated larvae. 



For the propagation of the fish tapeworm lakes and streams have 

 to be polluted by sewage containing the tapeworm eggs in order to 



