Skip directly to search Skip directly to A to Z list Skip directly to site content
CDC Home
For questions about DPDx, contact us

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit


[Trichinella spp.]

Trichinella larvae in pressed bear meat, partially digested with pepsin.

Trichinella larva in pressed bear meat, partially digested with pepsin.

Encysted larvae of Trichinella sp. in muscle tissue, stained with hematoxylin and eosin (H&E). The image on the left was captured at 200x magnification; the one on the right at 400x magnification.

Trichinella larva in tongue muscle of a rat, stained with hematoxylin and eosin (H&E). Image was captured at 400x magnification.

Causal Agents

Trichinellosis (trichinosis) is caused by nematodes (roundworms) of the genus Trichinella. In addition to the classical agent T. spiralis (found worldwide in many carnivorous and omnivorous animals), several other species of Trichinella are now recognized, including T. pseudospiralis (mammals and birds worldwide), T. nativa (Arctic bears), T. nelsoni (African predators and scavengers), T. britovi (carnivores of Europe and western Asia), and T. papuae (wild and domestic pigs, Papua New Guinea and Thailand). Trichinella zimbabwensis is found in crocodiles in Africa but to date there are no known associations of this species with human disease.

Life Cycle

Life cycle of Trichinellosis

Depending on the classification used, there are several species of Trichinella: T. spiralis, T. pseudospiralis, T. nativa, T. murelli, T. nelsoni, T. britovi, T. papuae, and T. zimbabwensis, all but the last of which have been implicated in human disease. Adult worms and encysted larvae develop within a single vertebrate host, and an infected animal serves as a definitive host and potential intermediate host. A second host is required to perpetuate the life cycle of Trichinella. The domestic cycle most often involved pigs and anthropophilic rodents, but other domestic animals such as horses can be involved. In the sylvatic cycle, the range of infected animals is great, but animals most often associated as sources of human infection are bear, moose and wild boar.

Trichinellosis is caused by the ingestion of undercooked meat containing encysted larvae (except for T. pseudospiralis and T. papuae, which do not encyst) of Trichinella species The number 1. After exposure to gastric acid and pepsin, the larvae are released from the cysts The number 2 and invade the small bowel mucosa where they develop into adult worms The number 3. Females are 2.2 mm in length; males 1.2 mm. The life span in the small bowel is about four weeks. After 1 week, the females release larvae The number 4 that migrate to striated muscles where they encyst The number 5. Diagnosis is usually made based on clinical symptoms, and is confirmed by serology or identification of encysted or non-encysted larvae in biopsy or autopsy specimens.

Geographic Distribution

Worldwide. Most common in parts of Europe and the United States.

Clinical Presentation

Light infections may be asymptomatic. Intestinal invasion can be accompanied by gastrointestinal symptoms (diarrhea, abdominal pain, vomiting). Larval migration into muscle tissues (one week after infection) can cause periorbital and facial edema, conjunctivitis, fever, myalgias, splinter hemorrhages, rashes, and peripheral eosinophilia. Occasional life-threatening manifestations include myocarditis, central nervous system involvement, and pneumonitis. Larval encystment in the muscles causes myalgia and weakness, followed by subsidence of symptoms.

Back to Top

For questions about DPDx, contact us
  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
  • Notice: Linking to a non-federal site does not constitute an endorsement by HHS, CDC or any of its employees of the sponsors or the information and products presented on the site. The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO