PARASITISM 813 



calized in a lower part of the body such as a leg or the scrotum which 

 may become tremendously enlarged. 



Trichinella. Another kind of nematode, Trichinella spiralis, bur- 

 rows in the host body during a portion of its life cycle (Fig. 39.14). The 

 adult is a small intestinal parasite, females 3 to 4 mm. long, males 1.5 

 mm. long. They are ovoviviparous, and the female usually burrows 

 slightly into the intestinal wall so that the young are released mto the 

 tissues. These larvae (0.1 mm. long) are distributed throughout the body 

 by the circulatory system and eventually burrow into striated muscles. 

 Within the muscle they grow rapidly to a length of 1 mm. and then 

 roll into a spiral form embedded in cysts between the muscle cells. This 

 is a waiting stage, for the worms will develop no further unless the meat 

 is eaten by another host. They will survive in this condition for periods 

 ranging from several months to several years. If the meat is eaten by an 

 appropriate host (man, swine, rodents, cats, sometimes other mammals) 

 the worms are digested free of the cyst and mature in about four days 

 in the new host's intestine. The disease trichinosis is caused by a sudden 

 heavy infestation and is manifested in two stages. While the adult 

 females are burrowing into the intestinal wall various intestinal and 

 systemic disorders, including diarrhea, pain and fever, may result. The 

 second stage is caused by the activities of the larvae as they penetrate 

 the muscles, and is accompanied by intense muscular pain, disturbances 

 of muscular activity, and sometimes death. Unlike most parasites Tri- 

 chinella is most abundant in temperate climates. Although its natural 

 reservoir is probably in rodents, wild pigs and carnivorous mammals, 

 it is common only where it has found especially suitable conditions on 

 swine farms where pigs are fed raw garbage, including pig scraps and 

 dead rodents. It is more abundant in this country than elsewhere. 



Botflies. Maggots of many botflies burrow throughout the body. 

 The skin botfly described previously stays beneath the skin but others, 

 such as cattle bots, burrow deep into the body and wander at will. 

 Eventually they migrate to the skin of the back and produce blisters 

 or warbles. When full grown they drop off and pupate in the ground. 

 Head bots of sheep and goats penetrate the lining of the nose and 

 burrow in the face, sometimes destroying an eye. 



365. Intracellular Parasites 



Only the protozoa and nematodes have given rise to intracellular 

 parasites. Probablv the first parasites were ones living within the cells of 

 other protozoa, possibly forms like some of the dinoflagellates that are 

 endoparasites of ciliates. Among the intracellular parasites of metazoans 

 are a genus of flagellates related to trypanosomes, Leishmama, and the 



entire class of sporozoans. 



Trypanosomes themselves are to some extent mtracellular, espe- 

 cially in the arthropod host where they may grow and reproduc^ ni the 

 cells lining the intestine. One species {T. cruzi) is intracellular m 

 the vertebrate host, but several species are completely extracellular in 

 both hosts. In the related genus, Leishmama, the parasites are entirely 



