Determining if host serum IgG titer is associated with Toxoplasma gondii virulence

Toxoplasma gondii is a protozoan parasite that causes the disease toxoplasmosis in animals and humans. One third of the world’s human population is chronically infected with this parasite (1). Toxoplasma gondii has two infectious forms: oocysts shed in the feces of felids (domestic and wild cats) and tissue cysts in chronically infected animals (Figure 1). Humans may become infected via ingestion of food or water contaminated with oocysts, the consumption of tissue cysts in undercooked meats, or vertical transmission from the mother to the fetus. Previous studies have found that T. gondii strains from South America are genetically diverse from and tend to be more virulent and causing more severe diseases in humans than strains from elsewhere (2). Also, a previous study found that higher anti-Toxoplasma IgG levels were correlated with severity of toxoplasmosis in humans (3). We hypothesize that more virulent strains can induce a stronger immune response to chronic infection than less virulent strains, resulting in higher IgG antibody levels in the hosts. To test this hypothesis, we used the modified agglutination test (MAT) to measure anti-Toxoplasma IgG levels in serum samples from animals (cats & dogs) and humans from South America and elsewhere. The IgG titers were then compared to determine if IgG level can be used as a marker to predict parasite virulence at the population level.