|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
|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.