PARAGONIMUS SPECIES : PARASITOLOGY
Several Paragonimus species may infect humans. P. westermani, which is widely distrib-uted in East Asia, is the species most frequently involved. The short, plump (10 by 5 mm), reddish-brown adults are characteristically found encapsulated in the pulmonary parenchyma of their definitive host. Here they deposit operculate, golden-brown eggs, which are distinguished from similar structures by their size (50 by 90μm) and promi-nent periopercular shoulder. When the capsule erodes into a bronchiole, the eggs are coughed up and spat out or swallowed and passed in the stool. If they reach fresh water, they embryonate several weeks before the ciliated miracidia emerge through the open opercula. After invasion of an appropriate snail host, 3 to 5 months pass before cercariae are released. These larval forms invade the gills, musculature, and viscera of certain crayfish or freshwater crabs; over 6 to 8 weeks, the larval forms transform into metacercariae. When the raw or undercooked flesh of the second intermediate host is ingested by hu-mans, the metacercariae encyst in the duodenum and burrow through the gut wall into the peritoneal cavity. The majority continue their migration through the diaphragm and reach maturity in the lungs 5 to 6 weeks later. Some organisms, however, are retained in the intestinal wall and mesentery or wander to other foci such as the liver, pancreas, kidney, skeletal muscle, or subcutaneous tissue. Young worms migrating through the neck and jugular foramen may encyst in the brain, the most common ectopic site. In addition to humans, other carnivores, including the rat, cat, dog, and pig, may serve as definitive hosts. Immature ectopic adults in the striated muscles of the pig may infect humans after ingestion of undercooked pork.