Latent infection with the apicomplexan Toxoplasma gondii (Nicolle et Manceaux, 1908) has been associated with schizophrenia, bipolar disorder and self-harm behaviour. However, the potential relationship between T. gondii immunoglobulin G antibody (IgG) seropositivity and generalised-anxiety disorder (GAD) and panic disorder (PD) has not been investigated. The associations between serum reactivity to T. gondii and major depressive disorder (MDD), GAD and PD were evaluated in a total sample of 1 846 adult participants between the ages of 20 and 39 years from the United States Center for Disease Control's National Health and Nutrition Examination Survey (NHANES). Approximately 16% of the overall sample was seropositive for T. gondii and 7% of the sample met criteria for MDD, 2% for GAD and 2% for PD. There were no significant associations between T. gondii IgG seroprevalence and MDD (OR = 0.484, 95% CI = 0.186-1.258), GAD (OR = 0.737, 95% CI = 0.218-2.490) or PD (OR = 0.683, 95% CI = 0.206-2.270) controlling for sex, ethnicity, poverty-to-income ratio and educational attainment. However, limited evidence suggested a possible association between absolute antibody titres for T. gondii and GAD and PD but not MDD. Toxoplasma gondii seroprevalence was not associated with MDD, GAD or PD within the context of the limitations of this study, although there may be an association of T. gondii serointensity with and GAD and PD, which requires further study.
Toxoplasma gondii reportedly manipulates rodent behaviour to increase transmission to its definitive feline host. We compared the effects of mouse infection by two Type II strains of T. gondii, Prugniaud (PRU) and ME49, on attraction to cat odour, locomotor activity, anxiety, sensorimotor gating, and spatial working and recognition memory 2 months post-infection (mpi). Attraction to cat odour was reassessed 7 mpi. At 2 mpi, mice infected with either strain exhibited significantly more attraction to cat odour than uninfected animals did, but only PRU-infected mice exhibited this behaviour 7 mpi. PRU-infected mice had significantly greater body weights and hyperactivity, while ME49-infected mice exhibited impaired spatial working memory. No differences in parasite antibody titres were seen between PRU- and ME49-infected mice. The present data suggest the effect of T. gondii infection on mouse behaviour is parasite strain-dependent.
A convincing body of evidence now exists, from both human and animal studies, and encompassing epidemiological to experimental, to indicate that the common protozoan Toxoplasma gondii can cause specific behavioural changes in its host. Such behavioural alterations are likely to be the product of strong selective pressures for the parasite to enhance transmission from its intermediate host reservoir, primarily rodent, to its feline definitive host, wherein sexual reproduction can occur and the parasite's life cycle completed. Here we consider what the available data to date may reveal about the potential mechanisms involved, the future research that needs to be performed, and the subsequent implications for animal and human health.