Toxoplasmosis, the disease

Toxoplasmosis is a parasitic disease caused by the obligate intracellular protozoan Toxoplasma gondii. The infection is remarkably widespread: nearly one third of the world’s population carries the parasite. Most people with a healthy immune system never notice it, or experience only mild flu-like symptoms, but toxoplasmosis can cause severe disease in immunocompromised patients and during pregnancy.

Who is at risk of severe disease

People with weakened immune systems, including those living with HIV/AIDS, patients on chemotherapy and organ transplant recipients, are at high risk of acute toxoplasmosis or of reactivation of a latent infection. In pregnancy, a first infection with Toxoplasma gondii can cross the placenta and cause congenital toxoplasmosis. How severe congenital disease becomes depends on when in the pregnancy the mother was infected and, possibly, on the genotype of the infecting strain.

Ocular toxoplasmosis

Toxoplasma gondii is also a major cause of eye disease, particularly in South America, where it is one of the leading causes of blindness. Some Toxoplasma strains cause retinal disease even in people with a normal immune system. Our lab studies how strain-specific variation contributes to disease, with the goal of explaining these clinical outcomes.

Diagnosis

Diagnosis usually relies on serology, that is, on antibodies against Toxoplasma gondii in the blood. Elevated IgM suggests a recent infection, whereas IgG without IgM indicates past exposure or latent infection. Because IgM can persist for months, IgG avidity testing is often used to time the infection: high-avidity IgG points to a chronic stage.

Treatment

Current treatment targets only the acute phase of infection. The standard regimen combines pyrimethamine, sulfadiazine and folinic acid. None of these drugs clears the latent stage, in which the parasite persists as tissue cysts, so there is a clear need for therapies that eliminate latent infection and prevent reactivation in vulnerable people.