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
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 significance of this approach is that, if the white blood cells in the small intestine have been exposed to Trichinella antigens (through vaccination), when an individual does get infected, the immune system will respond to expel the worms from the small intestine fast enough to prevent the female worms from releasing their larvae. A DNA vaccine tested on mice "induced a muscle larva burden reduction in BALB/c mice by 29% in response to T. spiralis infection". Epidemiology
About 11 million humans are infected with Trichinella; T. spiralis is the species responsible for most of these infections. Infection was once very common, but this disease is now rare in the developed world, but two known outbreaks occurred in 2015. In the first outbreak, around 40 people were infected in Liguria, Italy, during a New Years Eve celebration. The second outbreak in France was associated with pork sausages from Corsica, which were eaten raw, affecting 14 people in total. The incidence of trichinosis in the U.S. has decreased dramatically in the past century from an average of 400 cases per year mid-20th century down to an annual average of 20 cases per year (2008–10). The number of cases has decreased because of legislation prohibiting the feeding of raw meat garbage to hogs, increased commercial and home freezing of pork, and the public awareness of the danger of eating raw or undercooked pork products.China reports around 10,000 cases every year, so is the country with the highest number of cases.