Trichinosis

This fact sheet was updated on 12/20/20.

Other names: Trichinellosis

Cause

Trichinosis, also called Trichinellosis, is a zoonotic disease of mammals, birds, and reptiles caused by a nematode (roundworm) parasite in the group, or genus, Trichinella.

Significance

Humans contract trichinosis by eating undercooked meat from many species of wild and domestic animals, such as pigs, horses, crocodiles, bears, walruses, seals, and other wild game. People who contract trichinosis can exhibit mild (vomiting, fatigue, fever), to severe (difficulty breathing, loss of coordination) symptoms that may last for a month or more, and Trichinosis can be fatal. There has been a significant decrease in the number of human cases of trichinosis in the United States due to improvements made in the pork industry and increased public awareness of the dangers of eating raw or undercooked meat, but cases still occur frequently in Europe and Russia.

Species Affected

Trichinella can infect over 150 different hosts. Most mammals are susceptible to trichinosis, but it is primarily a disease of carnivores. In North America, the parasite has been detected in black bears, coyotes, cougars, gray wolves, skunks, bobcats, raccoons, wolverines, fishers, lynxes, walruses, red foxes, grizzly bears, polar bears. Trichinella has also been found in rodents, beavers, opossums, whales, crocodiles, and carnivorous birds. Trichinella is a significant parasite of domestic and feral swine. Humans can also become infected.

Distribution

Trichinosis is found worldwide, except Antarctica. Different species of Trichinella are associated with different geographic regions and different host species. While the prevalence of Trichinella in domestic swine in the US is <1%, the prevalence in black bears in the northeast varies between 1-5%.

Several Trichinella species are known to occur in North America and humans are susceptible to every species of the parasite.

Transmission

Trichinosis is transmitted when carnivores and omnivores ingest meat or meat products of infected animals that have Trichinella larvae encysted within their muscle cells. Once ingested, the larvae imbed themselves within the intestinal lining of the new host and develop into adults. The adult worms mate and the females release live larvae. The larvae migrate to muscle tissue and enter individual muscle cells where they grow, destroy the cells, and encase themselves within cysts. Some larvae end up in the intestines and are released in the feces. These larvae can then infect new hosts. The life cycle repeats when meat containing these cysts is consumed by a human or another animal.  Infective larvae encysted in muscle can survive freezing and for several weeks in decomposing carcasses. Person-to-person transmission does not occur.

Clinical Signs

Trichinella is not known to cause clinical signs of illness in wildlife. However, the parasite may be associated with abnormal behavior and decreased reproductive success.

Diagnosis

Trichinella larvae may be found by microscopic examination of muscle tissue. The tongue, diaphragm, the muscles used for chewing, and the muscles between the ribs usually contain the most larvae. Antibody and antigen tests are also available for diagnosis. 

Treatment

Treatment of trichinosis is generally not attempted in animals.

Management

The wide host range and distribution of this parasite makes  control of trichinosis in wildlife unrealistic. This disease can be more easily controlled in domestic animals by preventing them from feeding on uncooked meat and wildlife carcasses and proper cooking of all meat. Exposure to Trichinella was once much more common in domestic pigs, but control efforts have nearly eliminated the parasite in these animals in North America. Currently wild game meat consumption is emerging as the primary route of Trichinella zoonosis in humans. In order to prevent contracting trichinosis, meat should always be cooked thoroughly (a meat thermometer should read at least 165° F).

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