It is a common infection worldwide, but it is seldom seen in the United States because of regulations regarding the feeding of domestic animals and meat processing inspections.

The muscle of infected animals contain cysts that, when ingested, hatch within the human intestinal tract to produce an adult roundworm. The adult then produces numerous larvae that migrate through the gut wall and bloodstream to muscle tissue, where they form cysts.

The cyst form is part of the parasite's life cycle in which the tissue is eaten and the parasite is passed on to the next animal. Humans are an end host for the parasite because Trichinella spiralis encysted in human muscle will never be eaten.

Early symptoms of infestation with Trichinella spiralis are usually gastrointestinal -- cramping and diarrhea can occur. The larvae migrate to muscle, where they cause an inflammatory reaction that produces muscle pain (myalgia).

Pain is most notable on movement and in muscles that are constantly in motion (the breathing muscles such as the diaphragm and rib muscles), and it can be very painful.

During migration of the larvae through tissue, the infected person can develop swelling in the face and around the eyes. Larvae can invade the muscle of the heart and cause erratic heart rhythm (arrhythmias) or damage to the heart muscle.

Domestic meat animals (hogs) raised specifically for consumer consumption under USDA guidelines and inspection can be considered safe. Wild animals, especially carnivores or omnivores, should be considered a possible reservoir for roundworm infestation.

There are approxiamately 40 cases a year in the U.S.

Risk factors include eating meat from wild game.

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