Diagnosis of infection with Toxoplasma gondii in pregnant women, neonates and immunocompromised patients

Abstract

Infection with Toxoplasma gondii poses unique diagnostic problems like long-term persistence of specific IgM-antibodies, which makes it difficult to use the presence of Toxoplasma-specific IgM-antibodies alone as a sign of acute infection. The importance of determining the time of infection in pregnant women is also a unique diagnostic challenge in Toxoplasma diagnostics.

The first paper in this thesis compares the performance of different enzyme immuno assays, immunofluorescence assays, immunosorbent agglutination assays and IgG-avidity assays. The study showed that a combination of an IgM assay followed by an IgG-avidity test was the best combination to estimate the time interval in which infection had taken place. Diagnosis of infection in newborns without Toxoplasma-specific IgM- or IgA antibodies is difficult and a two-dimensional immunoblot assay to distinguish between maternal and child IgG-antibodies with different specificities was developed having higher sensitivity than previous assays.

A new IgG-avidity assay based on recombinant antigens was developed, which effectively abolished the problem of long-term low IgG-avidity seen in samples analysed by assays using whole cell, lysed antigen. Enzyme immuno assays with whole cell, lysed antigen pose problems with poor discrimination between IgG negative and low-positive samples and recombinant antigens should provide assays with less background, however, the sensitivity may be reduced.

Two studies show how combinations of recombinant antigens perform in assays of Toxoplasma-specific IgG- and IgM-antibodies. The assays do not yet have the same sensitivity as whole cell, lysed antigen based assays, but the concept is promising and should be further explored. T. gondii infection is a problem in immunodeficient hosts as the parasite remains alive in the chronically infected.

Current strategies for diagnosing these infections rely on regular testing for Toxoplasma-specific DNA by PCR in blood and other fluids including bronchioalveolar lavage (BAL). We show that a new, real time PCR based assay can be used to detect Toxoplasma in BAL fluids from HIV patients.