Trichinella spiralis : Parasitology

TRICHINELLA

Trichinella spiralis : PARASITOLOGY

Adult Trichinella live in the duodenal and jejunal mucosa of flesh-eating animals through-out the world, particularly swine, rodents, bears, canines, felines, and marine mammals.

Originally thought to be members of a single species, arctic, temperate, and tropical strains of Trichinella demonstrate significant epidemiologic and biologic differences and have recently been reclassified into seven distinct species. Only two species, T. spiralis and the arctic species T. nativa, display a high level of pathogenicity for humans. This discussion focuses on the former, while highlighting the unique epidemiologic and clinical character-istics of the latter.

The tiny (1.5-mm) male copulates with his outsized (3.5-mm) mate and, apparently spent by the effort, dies. Within 1 week, the inseminated female begins to discharge off-spring. Unlike those of most nematodes, these progeny undergo intrauterine embryonation and are released as second-stage larvae. The birthing continues for the next 4 to 16 weeks, resulting in the generation of some 1500 larvae, each measuring 6 by 100μm.

From their submucosal position, the larvae find their way into the vascular system and pass from the right side of the heart through the pulmonary capillary bed to the systemic circulation, where they are distributed throughout the body. Larvae penetrating tissue other than skeletal muscle disintegrate and die. Those finding their way to striated muscle continue to grow, molt, and gradually encapsulate over a period of several weeks. Calcifi-cation of the cyst wall begins 6 to 18 months later, but the contained larvae may remain viable for 5 to 10 years. The muscles invaded most frequently include the extraocular muscles of the eye, the tongue, the deltoid, pectoral, and intercostal muscles, the di-aphragm, and the gastrocnemius. If a second animal feeds on the infected flesh of the original host, the encysted larvae are freed by gastric digestion, penetrate the columnar epithelium of the intestine, and mature just above the lamina propria.