Epidemiologic implications of limited-duration seropositivity after toxoplasma infection

In the Netherlands, the prevalence of immunoglobulin G antibodies against Toxoplasma shows a plateau at older ages. The titer distributions in the indirect fluorescent antibody (IFA) test do not change after the age of 40 years. In recent serologic studies, it has been observed that enzyme-linked immunosorbent assay of toxoplasma antibodies yields much higher seroprevalence values at older ages. These findings indicate that IFA seropositivity is not lifelong. It has been suggested in the literature that the IFA data in the older age groups reflect a dynamic balance between reversions to seronegativity and conversions to seropositivity. In this paper, the epidemiologic implications are examined by means of a mathematical model. The seroprevalence data linked to the model indicate that the mean duration of IFA seropositivity after infection is 40 years and that the annual infection risk in the reproductive age group (15-40 years) is 4-6%. A further implication of a limited duration of IFA seropositivity is that a certain fraction of IFA-seronegative persons have experienced an infection in the past. The model-based analysis indicates that the predictive value of a negative IFA test is very low at the older ages. The model is consistent with the available data, and it provides a framework for further theoretical and data-related research.