Most mammals and some birds are susceptable to infection by Toxoplasma gondii, but only felines can serve as definitive hosts, in which sexual reproduction and the production of oocysts occurs.



In cats, Toxoplasma gondii may enter intestinal epithelial cells (enteroepithelial phase), undergo asexual reproduction forming multiple merozoites. These then enter new cells and form gametocytes. Fertilization of a microgametocyte (male) with a macrogametocyte (female) forms the zygote which develops into an oocyst. The oocyst bursts the intestinal cell and is passed in the cat's feces. In all other mammals infection with Toxoplasma gondii is extraintestinal (cats also have the extraintestinal phase in addition to the enteroepithelial phase). The sporozoites from oocysts or the bradyzoites from ingested sarcocysts penetrate through the intestinal wall and infect many types of cells and undergo asexual reproduction. In the acute phase, the parasite multiplies rapidly, bursting the infected cells and releasing tachyzoites. The tachyzoites infect new cells and repeat the cycle. As the host immune response increases, the multiplication rate slows and sarcocysts containing many bradyzoites are formed as a cyst wall is deposited. These then may act as infective stages when ingested by other animals.