ROUNDWORMS 



257 



Trichina. Trichinel'la spira'lis (Fig. 134, 1) is one of the 

 most dangerous of parasites. Like the tapeworm it requires 

 two hosts to complete its development. The adult trichina 

 may be found in the intestine of a pig, rat, or of man. The 

 female trichina, about three millimeters (one eighth of an 

 inch) long, bores into the wall of the 

 intestine and brings forth her young 

 alive. The young worms are carried 

 by the blood and lymph streams to 

 all parts of the body. When they 

 reach muscle tissue they come to rest, 

 and inclose themselves in a thick, 

 tough membrane or cyst (Fig. 134, 2), 

 remaining encysted until the muscle 

 is eaten by another animal. Then the 

 cyst dissolves, permitting the young 

 and still undeveloped trichinae to 

 grow and reach maturity in the in- 

 testine of the second host. If the 

 first host is the pig, the second host 

 may be the human being. 



The danger to the human being 

 comes while the young are making 

 their way into the muscular tissue. 

 The boring of thousands and some- 

 times millions of these larval parasites 

 in the muscles disintegrates the tissue and causes a fever, 

 which is very frequently fatal. A simple preventive remedy, 

 which everyone should apply, is to see that all the pork that 

 is eaten is thoroughly cooked. Human beings are infected 

 less frequently by the trichina since the Bureau of Animal 

 Industry of the United States Department of Agriculture 

 established the system of inspecting packing houses (see 

 page 266). 



Fig. 134. Trichina. 

 (Much enlarged) 



1, parasite; 2, membrane 



of cyst ; 3, muscle fiber of 



pig. (After Leuckart) 



