A number of Trichinella nematode (roundworm) species infect humans and cause trichinellosis (trichinosis). 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 (from mammals and birds worldwide), T. nativa (from Arctic bears and walruses), T. nelsoni (from African predators and scavengers), T. britovi (from carnivores, pigs, and horses of temperate Europe and western Asia and northern and western Africa ), T. murelli (from bears and horses in North America), and T. papuae (from wild and domestic pigs and saltwater crocodiles in Papua New Guinea and Thailand). Trichinella zimbabwensis is found in crocodiles and monitor lizards in Africa but there are no known associations of this species with human disease.
Trichinellosis occurs worldwide, but is most common in parts of Europe and the United States. 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. 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. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms. 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 that migrate to striated muscles, where they encyst. 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.
Gottstein et al. (2009) reviewed the epidemiology, diagnosis, treatment, and control of trichinellosis.
(Centers for Disease Control Parasites and Health website; Gottstein et al. 2009)