288 



TRICHINA WORMS 



small portions from a large number of hogs, and the infection, 

 if any be present among the hogs involved, is necessarily greatly 

 diluted, with the result that no individual consuming the sausage 

 is at all likely to ingest a sufficient number of trichinae to produce 

 an appreciable effect, even though the parasites should happen 

 to survive the curing processes to which the commercially pre- 

 pared sausage is usually subjected." 



Life History. The trichina worm, Trichinella spiralis, occurs 

 in quite a large number of animals, but the readiness with which 



infection occurs in dif- 

 ferent species of ani- 

 mals varies greatly. 

 In America hogs are 

 most commonly in- 

 fected, and infection is 

 common in rats which 

 have access to waste 

 pork; in Europe dogs 

 and cats commonly 

 show a higher percent- 

 age -of infection than 

 hogs in a given local- 

 ity. Man is highly 

 susceptible, in fact so 

 susceptible that he 

 cannot be considered 

 a normal host of the 

 parasite. Rats and 

 mice are sometimes 

 thought to be the pri- 



FIG. 118. Larvae of trichina worms, Trichinella , , ,. ,, 



spiralis, encysted in striped muscle fibers in pork, niary nostfc 

 Camera lucida drawing of cysts in infected sausage, worm, but the f act that 



these rodents succumb 



easily to infection while the parasites are still in the intestinal 

 stage tends to show that rats are not normal hosts. Rabbits 

 and guinea-pigs are easily infected -when fed meat containing 

 the worms, and a number of .- other mammals can occasionally 

 be infected artificially. 



The worms gain entrance to the digestive tract as larvae en- 

 cysted in meat (Fig. 118). In the intestine of the host they are 



