trichina, common name for species of roundworm of the phylum Nematoda. The species Trichinella spiralis is an important parasite, occurring in rats, pigs, and man, and is responsible for the disease trichinosis. The small adult worms mature in the intestine of an intermediate host such as a pig. Each adult female produces batches of up to 1,500 live larvae, which bore through the intestinal wall, enter the blood and lymphatic system, and are carried to striated muscle tissue. Once in the muscle, they encyst, or become enclosed in a capsule. Larvae encysted in the muscles remain viable for some time. When the muscle tissue is eaten by a human, the cysts are digested in the stomach; the released larvae migrate to the intestine to begin a new life cycle. Female trichina worms live about six weeks and in that time may release 15,000 larvae. The migration and encystment of larvae can cause fever, pain, and even death. Encysted larvae in pork are destroyed by thorough cooking or long periods of low-temperature storage. Trichina are classified in the phylum Nematoda.

Species (Trichinella spiralis) of parasitic nematode, found worldwide, that causes the disease trichinosis. Trichinae (or trichinas), 0.06–0.2 in. (1.5–4 mm) long, mate in the host's small intestine. Fertilized females penetrate the intestinal wall and release larvae, which the blood carries throughout the body. The larvae grow, mature, and become encysted within muscle tissue. The digestive juices of an animal that eats the muscle tissue break down the cyst, liberating the larvae for further development; the worms mature, and the cycle begins again.

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