302 



PARASITES OF THE DOMESTIC ANIMALS 



Life History. — When flesh containing encapsulated living trichinae 

 is taken into the stomach of a suitable animal, the capsule is digested 

 and they are liberated within eighteen to twenty-four hours. The 

 larvae then enter the small intestine and are sexually mature in two to 

 five days. The females with the males are pressed into the crypts of 

 Lieberkuhn where, a week to ten days after the infection, the female 

 deposits living embryos. There is at first an equal number of males 

 and females in the intestine; later the males gradually disappear, so 

 that ten to fourteen days after infection almost all of the worms will be 

 females. These live five to eight weeks, a single female, according to 

 Leuckart, depositing not less than fifteen hundred embryos; according 

 to Braun, the number may reach ten thousand. 



From Lieberkiihn's glands the embryos penetrate the mucosa and, 

 reaching the lymphatics, are probably carried to the blood by way of 

 the thoracic duct. With the blood they are distributed to various parts 

 of the body, passively in greater part, though it is 

 probable that their ultimate lodgment is influenced 

 somewhat by their activity. Embryos deposited by 

 capillary blood in striated muscle with sarcolemma 

 are amid conditions favorable to their further develop- 

 ment. From the capillaries the trichinae force their 

 way through the sarcolemma and into the plasma of 

 the muscle-fiber, where, at first actively motile, they 

 pass to a state of rest and proceed to develop into 

 the larval stage at which, if ingested, they may infect 

 other animals. 



In about three weeks after the occurrence of the 

 infection the larvae in their muscular location have 

 attained a length of eight-tenths to one millimeter, 

 and their growth is completed. At this time they 

 are usually curved in the form of a sickle, later becom- 

 ing coiled spirally (Fig. 163), from which characteristic 

 they derive their specific name, though they may be 

 found in various looped and curved forms. The an- 

 terior portion of the larva is now the thinner; the pos- 

 terior thicker and rounded at its extremity. 

 As a result of this invasion the muscle-fibers undergo certain changes; 

 the transverse striatiop is lost, there is degeneration of the sarcoplasm, 

 and the nuclei increase in number and size, each becoming surrounded 

 by a granular mass. The irritation to the surrounding tissues caused 

 by the presence of the parasites results in the formation of cysts which 

 enclose the trichinae and are fully developed at the end of the third 

 month. The long axis of the capsule is parallel to that of the muscle- 

 fiber. The capsules are usually oval in shape and more or less drawn 



Fig. 163.— Tri- 

 chinella spiralis. 

 Encysted larva in 

 muscle (after Leuc- 

 kart) . 



