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Toxoplasma gondii is a protozoan parasite that infects most species of warm-blooded animals (for example, cats, pigs, sheep, and humans) and causes the disease toxoplasmosis. The only known host animal that allows the parasite to complete its life cycle is the cat (domestic cats and other relatives in the family Felidae). After primary infection, cats shed millions of oocysts in their feces for about one to three weeks; the oocysts take one to five days to sporulate, which then can infect mice and birds (termed intermediate hosts) when these animals ingest water, plants, or soil containing the sporulated oocysts. The oocysts can remain viable in the environment for about a year. These sporulated oocysts become tachyzoites when ingested and migrate into muscle and neurological tissues where they further develop into bradyzoites. When a cat ingests an infected mouse or bird, the ingested bradyzoites develop into either tachyzoites or oocysts. The life cycle of Toxoplasma is completed when oocysts are shed in the cat's feces. Humans and other animals are not part of the complete life cycle (unless eaten by a cat); the majority of infections occur when humans, domesticated or other animals ingest food, soil, or other animals that contain either sporulated oocysts or animal tissue containing Toxoplasma bradyzoites. Humans usually become infected by ingesting undercooked infected meat, food, or water. Infection may also be transmitted by contaminated blood transfusions, transplantation of infected organs, or from an infected mother to fetus. Finally, the disease may be acquired by directly ingesting cat feces, which may occur when cleaning out litter boxes.
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