These tissue cysts may reactivate if the immune system becomes compromised. Also, women who contract the disease during pregnancy may transmit the parasite to the fetus. Congenital infections may cause fetal brain and ocular lesions and, in a small percentage, death. Susceptibility to infection is universal, but immunity is readily acquired. Patients undergoing cytotoxic or immunosuppressive therapy or patients with AIDS are at high risk of developing illness from reactivated infection.
Diagnosis is based on clinical signs and supportive serologic results.
Rising antibody levels are indicative of active infection. High antibody levels may persist for years with no relation to active disease. Only felines harbor the parasite in the intestinal tract where the sexual stage of its life cycle takes place, which results in the excretion of the oocysts in feces for 10 to 20 days or, rarely, longer. Tissue cysts remain viable for long periods, perhaps for the life of the animal. The oocysts sporulate 1 to 5 days after shedding in the feces and are then infectious.