MODELS FOR PREDICTION OF THE FREQUENCY OF TOXOPLASMOSIS IN PREGNANCY IN SITUATIONS OF CHANGING INFECTION-RATES

Background. Estimation of the number of women infected during pregnanc y with Toxoplasma gondii from seroconversion or seroprevalence data me ets with various difficulties. Because of the high risk of transmissio n of the infection to the fetus such infections are however a major co ncern in pregnancy-related health planning. Methods. The expected annu al percentage of pregnant women infected with Toxoplasma was calculate d using models with varying assumptions with regard fo the infection r ate, assumed to be independent of age but dependent on calendar time. Three situations were studied: a stable situation, a sudden fall in th e infection rate and a gradually declining (slower or faster) infectio n rate over the lifetime of the pregnant women. Results. With a consta nt infection rate, a maximum number of affected pregnancies occurs at a yearly infection rate of 4%. In countries with a strongly decreasing annual infection rate, estimates based on data on the relation betwee n age and seropositivity related to only one period of time tend to ov erestimate the number of affected pregnancies by as much as 60%. Concl usions. In countries in transition from high to low infection rates, i t is likely that the influence of decreasing immunity will, at least t emporarily, more than outweigh the influence of the falling infection rates, resulting in a higher number of infected pregnant women. The mo dels used can also describe situations with age-dependent variation in the infection rate, and may well apply to other infectious diseases r elevant to pregnancy.