Chap. 26 ROUNDWORMS THE TUBULAR PLAN 527 



Massachusetts, and on the west coast. Essentially, it occurs wherever pigs are 

 fed on garbage that contains bits of infected pork. However, marketing of meat 

 products into different regions of the country does not leave any locality free 

 from suspicion. Uninspected pork from farms and small butchering places has 

 proven more dangerous than government inspected pork. Trichinae have not 

 been eliminated anywhere. More effective than inspection is the fact that pork 

 is usually refrigerated for long intervals which kills trichinae. 



Life History. Trichina worms are usually swallowed as immature larvae 

 enclosed in cysts embedded in pork muscle (Fig. 26.6). The cysts are digested 

 off and the microscopic larvae bore into the intestinal wall where they grow to 

 maturity, mate and reproduce within five to seven days after they are swal- 

 lowed. The adults may or may not cause intestinal disturbances depending 

 upon the number of larvae that were swallowed. An ounce of heavily infected 

 pork sausage may contain 100,000 encysted larvae. 



The embryo trichinae develop in the uterus of the mother. The microscopic 

 larvae are born alive, burrow into the capillaries and become numerous in the 

 blood between two and three weeks after their parents were swallowed. They 

 are distributed all over the body but finally settle into muscles that have a large 

 blood supply, those of the diaphragm, the thorax, the legs, but not the heart 

 (Figs. 26.5, 26.6). 



After they enter the muscles, the larvae grow rapidly but are still practically 

 microscopic. They are then in the infective stage. Their only chance for life is 

 that the muscle which they occupy may be eaten by an animal in which they 



Fig. 26.6. Drawing of microscopic cyst of trichinae about three weeks old. 

 The walls of cysts contained in infected pork are digested off in the human 

 stomach and the larvae develop into adults within five to seven days. Mating 

 occurs and the females produce living young, larvae that invade the body within 

 about three weeks, finally settling into muscles and other tissues in the encysted 

 state shown here. See also figure 26.1. The harm to the body is done by the 

 migrations of larvae, rather than by the cysts. (Courtesy, Craig and Faust: 

 Clinical Parasitology, ed. 5. Philadelphia, Lea and Febiger, 1951.) 



