Once outside the cat s body, sporogony occurs for up to 21 days, resulting in development of infectious oocysts. In warm, moist soil, these oocysts can remain viable for more than 1 year. Trophozoites Tachyzoites are the actively proliferating trophozoites, which are observed during the acute stage of infection. Tachyzoites can be found in any organ but occur most commonly in the brain, skeletal muscle, and heart muscle. Intracellular infection can occur in all mammalian cells except anuclear erythrocytes.
Intracellular multiplication continues until host cells lyse or a tissue cyst is formed.
In an immunocompetent host, tachyzoites are eliminated and tissue cysts form. Tissue Cysts Tissue cysts are found most commonly in the brain, skeletal, and cardiac muscle but can occur in any organ. These cysts contain slowly growing trophozoites known as bradyzoites. In an immunocompromised host, tachyzoite replication results in development of focal necrosis, such as necrotizing encephalitis, pneumonitis, or myocarditis. Tissue cysts usually are observed during the chronic or latent stage of infection causing little, if any, inflammatory response.