304 PARASITES OF THE DOMESTIC ANIMALS 



fibers of the heart. Certam muscles are pecuharly 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 probabh' to 

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

 tions. 



The nmuber 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, 



jJBHIIjjHj according to the same author, estimated the 



^^^^B^^^H number found in the body of a young woman 



^^HflH^^^S as ninety-four million. 



l^^^^^^^^B^ Occurrence. — Adult trichinae are only found 

 J^^H^^^^^ in the intestines, especially the upper part of 

 ^^i^H^^l^J^^JI the small intestine, of mammals and birds which 

 ^^^ Wm^ have recently- eaten flesh containing the en- 

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



Fig. 164. — Trichinella vertebrates the trichina cysts are not acted 

 spiralis. Cyst in human ^jpon by the digestive canal and thev pass 



muscle (from micropho- . , i • , i , i r^t- l^ • i" 



tograph by Hoedt). through Without change. Ui the animals com- 



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 experimentalh^ 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 b}^ a cyst 

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



Symptoms in Hogs. — Sj'mptoms 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 s\anptoms 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, 

 grinding of the teeth, and a disposition to remain crouched in the 

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



