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 pregnancy with Toxoplasma gondii from seroconversion or seroprevalence data meets with various difficulties. Because of the high risk of transmission of the infection to the fetus such infections are however a major concern in pregnancy-related health planning.
METHODS: The expected annual percentage of pregnant women infected with Toxoplasma was calculated using models with varying assumptions with regard to the infection rate, assumed to be independent of age but dependent on calendar time. Three situations were studied: a stable situation, a sudden fall in the infection rate and a gradually declining (slower or faster) infection rate over the lifetime of the pregnant women.
RESULTS: With a constant 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 between age and seropositivity related to only one period of time tend to overestimate the number of affected pregnancies by as much as 60%.
CONCLUSIONS: In countries in transition from high to low infection rates, it is likely that the influence of decreasing immunity will, at least temporarily, more than outweigh the influence of the falling infection rates, resulting in a higher number of infected pregnant women. The models used can also describe situations with age-dependent variation in the infection rate, and may well apply to other infectious diseases relevant to pregnancy.