Answer: Trichinella spp. Shown here are multiple L1 larvae in skeletal muscle. Note that each larvae is contained within a 'nurse' cell derived from the invaded host myocyte (muscle cell).
There are at least 3 species of Trichinella that infect humans, of which T. spiralis is the most common. T. spiralis has several subspecies, which some may be genetically distinct enough to warrant being a separate species. Larvae of T. pseudospiralis do not encapsulate within a nurse cell, but are otherwise morphologically similar to larvae of T. spiralis. Regardless of the species, the symptoms and prognosis seem to be the same, and are related to the parasite burden in the host (more larvae, worse disease).
Trichinosis (aka trichinellosis and trichiniasis) is classically acquired from ingestion of undercooked pork containing Trichinella spp. larvae; however, bear, horse, and walrus meat are also major sources of infection. After ingestion, the larvae mature in the host intestine and mate. The adult female then releases thousands of first stage (L1) larvae which penetrate the intestine and migrate to skeletal muscle via the blood stream. When the larvae reach skeletal muscle, they penetrate the individual muscle cells and remain coiled up inside the cell until ingested by another carnivore. So each host serves first as a definitive host, and then an intermediate host!
The moral of the story - don't eat undercooked meat.
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