304 



PARASITES OF THE DOMESTIC ANIMALS 



fibers of the heart. Certain muscles are peculiarly liable to invasion 

 by the parasites, and these in the order of frequency may be listed as 

 follows, — pillars of the diaphragm, muscles of the larynx and tongue, 

 abdominal and intercostal muscles, psoas muscles, and muscles of the 

 back. They are usually found in greatest number toward the extrem- 

 ities of the muscles in the neighborhood of tendons, a fact probably to 

 be accounted for in the arrest offered by these locations to their migra- 

 tions. 



The number of cysts which an infested individual may harbor is 

 capable of reaching an enormously high figure. Neumann states that 

 Leuckart has counted between twelve hundred and fifteen hundred in a 

 gram (15.43 grains) of muscle, while Fielder, 

 according to the same author, estimated the 

 number found in the body of a young woman 

 as ninety-four million. 



Occurrence. — Adult trichinae are only found 

 _ -^^^K » ■ ft. ^" ^^^ intestines, especially the upper part of 

 I^Bj^^'^JX^Jl the small intestine, of mammals and birds which 

 ^^r ^t^ have recently eaten flesh containing the en- 



^ I cysted larva?. In fishes and other cold-blooded 



164. — Trichineiia vertebrates the trichina cvsts are not acted 





n human upon by the digestive canal and they pass 

 through without change. Of the animals com- 



FlG, 



spiralis. Cyst 

 muscle (from 

 tograph by Hoedt) 



monly used for human food only the hog harbors 

 muscle trichinae by natural infection, and trichinosis of man is usually 

 acquired by eating the trichinosed flesh of this animal. Rats are peculiarly 

 susceptible to trichina, and probably one of the most frequent sources 

 of the infection of hogs is by eating trichinous rats. Transmission to 

 herbivorous animals, as cattle, sheep, and horses, is difficult. After 

 experimental feeding of flesh containing the cysts to these animals 

 there is usually a development of intestinal trichinae but no muscle 

 trichinae. Intestinal trichinae have been experimentally developed in 

 birds, but birds do not harbor the encysted larvae. 



Only encysted living larvae are capable of producing trichinosis in 

 their suitable hosts. Ingested larvae which are unprotected by a cyst 

 are destroyed in the stomach by the direct action of the gastric juice. 



Symptoms in Hogs. — Symptoms of trichinosis by natural infection 

 are rarely observed in hogs, though where a considerable quantity of 

 the cysts have been ingested it is probable that such symptoms follow, 

 their true cause being unrecognized. Feeding experiments have shown 

 that after massive infestation intestinal trichinosis is manifested by 

 the third to the eighth day. There is then depression, loss of appetite, 

 g;rinding of the teeth, and a disposition to remain crouched in the 

 bedding or to stand about with back arched and abdomen retracted. 



