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.