Trichinella spiralisis a parasitic nematode (roundworm) which is found in many warm-blooded carnivores and omnivores, including pigs. Trichinella has a direct life cycle, which means it completes all stages of development in one host. Transmission from one host to another host can only occur by ingestion of muscle tissue which is infected with the encysted larval stage of the parasite. When ingested, muscle larvae excyst and enter tissues of the small intestine, where they undergo development to the adult stage. Male and female adult parasites mate and produce newborn larvae which leave the intestine and migrate, through the circulatory system, to striated muscle tissue. There, they penetrate a muscle cell, modify it to become a unique cyst, and mature to become infective for another host. The total time required for this development is from 17 to 21 days. Adult worms continue to produce larvae in pigs for several weeks before they are expelled. Once adult worms are expelled and larvae reach and encyst in musculature, no further contamination can occur. An animal that is infected with Trichinella is at least partially resistant to a subsequent infection due to a strong and persistent immunity.