Larvae of Trichinella
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),
four other species of Trichinella are now recognized: T. pseudospiralis (mammals
and birds worldwide), T. nativa (Arctic bears), T. nelsoni (African
predators and scavengers), and T. britovi (carnivores of Europe and western Asia).
Trichinosis is acquired by ingesting meat containing cysts (encysted larvae) of Trichinella. 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 (female 2.2 mm in length, males 1.2 mm; life span in the small bowel: 4 weeks). After 1 week, the females release larvae that migrate to the striated muscles where they encyst . (Trichinella pseudospiralis, however, does not encyst). Encystment is completed in 4 to 5 weeks and the encysted larvae may remain viable for several years. Ingestion of the encysted larvae perpetuates the cycle. Rats and rodents are primarily responsible for maintaining the endemicity of this infection. Carnivorous/omnivorous animals, such as pigs or bears, feed on infected rodents or meat from other animals. Different animal hosts are implicated in the life cycle of the different species of Trichinella. Humans are accidentally infected when eating improperly processed meat of these carnivorous animals (or eating food contaminated with such meat).
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 blood 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.
Worldwide. Most common
in parts of Europe and the United States.
The suspicion of
trichinosis, based on clinical symptoms and eosinophilia, can be confirmed by specific
diagnostic tests, including antibody detection, muscle biopsy, and microscopy.
Thiabendazole and mebendazole are
active only against the intestinal stages, but not the encysted larvae. Corticoids may
decrease the severity of symptoms during the muscle invasion phase.
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