Pathogenesis
Toxoplasma gondii is an obligate intracellular protozoan
existing in three forms: the oocyst, the tachyzoite and the bradyzoite.
Replication of the parasite takes place in the intestine of the cat
family Felidae, as a result the production of oocysts. Millions of
oocysts are shed into the feces of cats. After sporulation, oocysts
containing sporozoites are infective when ingested by mammals, including
humans. At this point the tachyzoite stage starts. The rapidly dividing
form of the parasite is represented by the stage of tachyzoites. Active
penetration is the way that the parasite contaminates all the nucleated
cells. After that, a continuous proliferation leads to host cells
destruction and tachyzoites invade and damage adjacent cells (2).
Furthermore, they are disseminated via bloodstream and infect many
tissues. This situation causes a strong inflammatory response, tissue
damage and, therefore clinical manifestations. The host’s immune
response can put pressure and limit the growth of tachyzoites, and so
they are transformed into bradyzoites, which they form tissue cysts (in
the brain and skeletal and heart muscles). Bradyzoites stay inside cysts
for the entire life of the host and although they are morphologically
identical to tachyzoites, multiply slowly. In immunocompromised patients
(patients with AIDS, organ transplantation, cancer, or taking
immunosuppressants), bradyzoites can be released from cysts and
transform back into tachyzoites (1). In addition, tissue cysts are
considered to be infective stages for intermediate hosts (2).