Toxoplasma gondii IgG serointensity and cognitive function in bipolar disorder

Alongside affective episodes, cognitive dysfunction is a core symptom of bipolar disorder. The intracellular parasite T. gondii has been positively associated with both, the diagnosis of bipolar disorder and poorer cognitive performance, across diagnostic boundaries. This study aims to investigate the association between T. gondii seropositivity, serointensity, and cognitive function in an euthymic sample of bipolar disorder.

A total of 76 participants with bipolar disorder in remission were tested for T. gondii-specific IgG and IgM antibodies and for cognitive performance using neuropsychological test battery. Cognitive parameters were categorized into three cognitive domains (attention and processing speed, verbal memory, and executive function). Statistical analysis of associations between continuous indicators of cognitive function as dependent variables in relationship to T. gondii, included multivariate analyses of co-variance for seropositivity, and partial correlations with IgG serointensity in IgG seropositives. All analyses were controlled for age and premorbid IQ.

In seropositives (n = 27), verbal memory showed significant inverse partial correlations with IgG antibody levels (short delay free recall (r=-0.539, p = 0.005), long delay free recall (r=-0.423, p = 0.035), and immediate recall sum trial 1-5 (r=-0.399, p = 0.048)). Cognitive function did not differ between IgG seropositive and seronegative individuals in any of the cognitive domains (F (3,70) = 0.327, p = 0.806, n = 76). IgM positives (n = 7) were too few to be analyzed.

This investigation is the first to show an association between T. gondii IgG serointensity and memory function in a well-diagnosed bipolar disorder sample. It adds to the existing literature on associations between latent T. gondii infection and cognition in bipolar disorder, while further research is needed to confirm and expand our findings, eliminate potential sources of bias, and establish cause-effect relationships.