H. Ray Gamble USDA

Publish Date: June 3, 2006

Trichinella spiralis is 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 (Figure 1). 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 17-21 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.

Publish Date: June 3, 2006

Toxoplasma gondii is a protozoan (single-celled) parasite found in muscle and other tissues of many warm-blooded animals including pigs and people. Cats and other felids are the only hosts in which the parasite can complete its entire life cycle (Figure 1), and the only animals that excrete the environmentally resistant and infectious stage called the oocyst (eggs) in the feces. Infection occurs when pigs, and other animals, accidentally ingest oocysts in soil or water or eat tissues of rodents, wildlife, or meat containing cysts. Ingested oocysts or tissue cysts enter the intestine and release sporozoites or bradyzoites, respectively. These stages penetrate intestinal epithelial cells and transform into rapidly dividing tachyzoites. Tachyzoites are dispersed throughout the body by the circulatory and lymphatic systems, eventually entering and encysting as bradyzoites (tissue cysts) in skeletal muscle and other organs of the body (brain, heart, liver). These cysts remain alive in the body for the lifetime of the animal, and are infective when eaten by other hosts, such as humans. Once tissue cysts have formed, most animals are resistant to a second infection. In the cat, a series of asexual stages in the intestine is followed by sexual reproduction of the parasite with the development of gamonts, fertilization, formation of zygotes, and the production of oocysts that are passed in the feces. Cats may shed more than 10 million oocysts per day for 3-10 days after infection. Oocysts must mature (sporulate) in the environment for 1-5 days to become infective for a new host. Transplacental transmission of infection can occur in some hosts, including humans, who become infected during pregnancy.

Publish Date: June 3, 2006

Trichinosis has been a stigma associated with the consumption of pork for years. A recent study indicates that a trichinae-safe pork supply would increase consumer confidence and pork consumption, resulting in additional income to pork producers.