Trichinella spiralis larvae in muscle tissue Animal tissue infected with the parasite that causes the disease<unk> : Most parasites are shown in cross section, but some randomly appear in long section. The classical agent is T. spiralis (found worldwide in many carnivorous and omnivorous animals, both domestic and sylvatic (wild), but seven primarily sylvatic species of Trichinella also are now recognized: Species and characteristics * T. spiralis is most adapted to swine, most pathogenic in humans, and is cosmopolitan in distribution. * T. britovi is the second-most common species to infect humans; it is distributed throughout Europe, Asia, and northern and western Africa, usually in wild carnivores, crocodiles, birds, wild boar, and domesticated pigs. * T. murrelli also infects humans, especially from black bear meat; it is distributed among wild carnivores in North America. * T. nativa, which has a high resistance to freezing, is found in the Arctic and subarctic regions; reservoir hosts include polar bears, Arctic foxes, walruses, and other wild game. * T. nelsoni, found in East African predators and scavengers, has been documented to cause a few human cases. * T. papuae infects both mammals and reptiles, including crocodiles, humans, and wild and domestic pigs; this species, found in Papua New Guinea and Thailand, is also nonencapsulated. * T. pseudospiralis infects birds and mammals, and has demonstrated infection in humans; it is a nonencapsulated species. * T. zimbabwensis can infect mammals, and possibly humans; this nonencapsulated species was detected in crocodiles in Africa. Taxonomy * Kingdom: Animalia * Phylum: Nematoda * Class: Adenophorea * Order: Trichurida * Family: Trichinellidae * Genus: Trichinella Lifecycle Lifecycle of Trichinella spiralis The typical lifecycle for T. spiralis involves humans, pigs, and rodents. A pig becomes infected when it eats infectious cysts in raw meat, often porcine carrion or a rat (sylvatic cycle). A human becomes infected by consuming raw or undercooked infected pork (domestic cycle). In the stomach, the cysts from infected undercooked meat are acted on by pepsin and hydrochloric acid, which help release the larvae from the cysts into the stomach. The larvae then migrate to the small intestine, and burrow into the intestinal mucosa, where they molt four times before becoming adults. Thirty to 34 hours after the cysts were originally ingested, the adults mate, and within five days produce larvae. Adult worms can only reproduce for a limited time, because the immune system eventually expels them from the small intestine. The larvae then use their piercing mouthpart, called the "stylet", to pass through the intestinal mucosa and enter the lymphatic vessels, and then enter the bloodstream. The larvae travel by capillaries to various organs, such as the retina, myocardium, or lymph nodes; however, only larvae that migrate to skeletal muscle cells survive and encyst. The larval host cell becomes a nurse cell, in which the larva will be encapsulated, potentially for the life of the host, waiting for the host to be eaten. The development of a capillary network around the nurse cell completes encystation of the larva.<unk>  is not soil-transmitted, as the parasite does not lay eggs, nor can it survive long outside a host.
Trichinosis
Dermatomyositis
Trichuriasis
Measles
Trichiasis
Gastroenteritis
Trichomoniasis
Naegleriasis