. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. 288 TRICHINA WORMS small portions from a large number of hogs, and the infection, if any be present among the hogs involved, is necessarily greatly diluted, with the result that no individual consuming the sausage is at all likely to ingest a sufficient number of trichinae to produce an appreciable effect, even though the parasites should happen to survive the curing processes to which the commercially pre- pared sausage is usually ; Life History. — The trichina worm, Trichinella spiralis
. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. 288 TRICHINA WORMS small portions from a large number of hogs, and the infection, if any be present among the hogs involved, is necessarily greatly diluted, with the result that no individual consuming the sausage is at all likely to ingest a sufficient number of trichinae to produce an appreciable effect, even though the parasites should happen to survive the curing processes to which the commercially pre- pared sausage is usually ; Life History. — The trichina worm, Trichinella spiralis, occius in quite a large number of animals, but the readiness with which infection occurs in dif- ferent species, of ani- mals varies greatly. In America hogs are most commonly in- fected, and infection is common in rats which have access to waste pork; in Europe dogs and cats commonly show a higher percent- F 'i ^^RM^^H^H^^^' '^^^^^ ^^^ °^ infection than k f[ iMIw l«^akSByls2^/'^a^l hogs in a given local- ity. Man is highly t '?''J-HtllW/^k'i^B'llr*^ "*^Jfil susceptible, in fact so ;'P^M»f(y^^H^\r^i^l/^B susceptible that he r '.; ^f/^^i^Mlvi^JWF&X^Jiklliffr/^S cannot be considered a normal host of the parasite. Rats and mice are sometimes thought to be the pri- mary hosts of the worm, but the fact that these rodents succumb easily to infection while the parasites are still in the intestinal stage tends to show that rats are not normal hosts. Rabbits and guinea-pigs are easily infected when fed meat containing the worms, and a number of other mammals can occasionally be infected artificially. The worms gain entrance to the digestive tract as larvae en- cysted in meat (Fig. 118). In the intestuie of the host they are. Fig. 118. Larvae of trichina worms, Trichinella spiralis, encysted in striped muscle fibers in pork. Camera lucida drawing of cysts in infected sausage. Please note that these images are extracted from scanned page images that may hav
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