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Common parasite increased risk for self-directed violence and suicide in women

Women infected by toxoplasma gondii, a widespread neurotropic protozoan parasite, were at an increased risk for self-directed violence and suicide, according to study results.

Toxoplasma gondii, or T. gondii, affects approximately one-third of people worldwide, according to researchers. The parasite is spread through contact with cat feces or ingesting unwashed vegetables or undercooked meat, working from the intestine to muscles and the brain, where it hides within neurons and glial cells. T gondii has been associated with behavioral, affective and cognitive abnormalities in humans, and has been consistently associated with schizophrenia.

Marianne G. Pedersen, MSc, of the National Centre for Register-based Research, and colleagues examined whether T. gondii-infected mothers were at increased risk for self-directed violence, violent suicide attempts and suicide, depending on the level of T. gondii immunoglobulin G (IgG) antibodies measured in newborn children. The researchers chose to study the blood tests of children since IgG antibodies pass through the placenta and infected children will not begin to produce T. gondii-specific IgG until approximately 3 months of age. The antibodies are maternal in origin and are a reliable representation of T. gondii levels in mothers.

The researchers analyzed data collected on 45,788 women followed up from the date of delivery in 1992 and 1995 and whose children were screened for T. gondii infection, or toxoplasmosis. The women were part of a Denmark register-based cohort study, the same cohort in which Pedersen and colleagues found that high levels of IgG antibodies were associated with a significantly elevated risk for schizophrenia spectrum disorder.

A heel stick blood sample was taken from each child 5 to 10 days after birth to measure IgG antibody levels. The researchers used the Danish Cause of Death and National Hospital Registers to assess suicide, self-directed violence and violent suicide attempts. The study population was linked with the Danish Psychiatric Central Research Register, which provided information on history of mental illness.

Women infected with T. gondii had a relative risk of self-directed violence of 1.53 (95% CI, 1.27-1.85), compared with women who were not infected, and the risk appeared increase alongside IgG antibody levels, according to the researchers. The relative risk for violent suicide attempts was 1.81 (95% CI, 1.13-2.84), and the relative risk for suicide 2.05 (95% CI, 0.78-5.20), in women infected with T. gondii, compared with uninfected women. Researchers found a similar association for repetition of self-directed violence, with a relative risk of 1.54 (95% CI, 0.98-2.39).

Among women with a history of mental illness, infected women had a relative risk of 1.25 (95% CI, 0.94-1.66), and among women without mental illness infected women had a relative risk of 1.56 (95% CI, 1.21-2.00). The effect of T. gondii infection was not significantly different in the two groups, and therefore the lower relative risk among women with a history of treated psychiatric disorders should be interpreted with caution, the researchers wrote.

Pedersen and colleagues said that one possible explanation for the connection between toxoplasmosis and risks for suicide and self-directed violence could involve the neuroimmune path. In the latent form of infection, T. gondii is contained immunologically, and immunological alterations have been reported in individuals with a history of suicidal behavior.

“Even though, for the first time to our knowledge, the T. gondii antibody level was measured prior to first registered occurrence of self-directed violence, we cannot say with certainty whether the observed association between T. gondii infection and self-directed violence is causal,” the researchers wrote. “T. gondii infection is likely not a random event and it is conceivable that the results could be alternatively explained by people with psychiatric disturbances having a higher risk of becoming T. gondii infected prior to contact with the health system.”

Disclosure: The researchers report no relevant financial disclosures.