Friday, July 6, 2012

T. gondii in the news again

     It's been bat shit crazy in the Not-So-Dark Laboratory (otherwise known as my day job), so my apologies for getting behind on the real crazy science.  One of my first posts (zombie science) talked about zombie science and the potential role of Toxoplasma gondii. T. gondii is a neurotropic protozoan parasite that has been linked to a variety of mental disorders. When rats are infected, they lose many of the behavioral adaptations that protect them from cats and aggressively try to get themselves eaten.  Sadly, the effect of T. gondii on humans seems more subtle. But there is a new paper out on the relationship between T. gondii and suicide, which provides stronger evidence that this parasite is a potentially serious global problem.
     Several recent studies have shown that T. gondii affects human behavior. A meta analysis published in 2007 (abstract) found that there is a correlation between schizophrenia and serum antibody levels against T. gondii.  The predictive power of the association was weak, since more that one third of the entire population is thought to be seropositive for T. gondii, and aside from a few ex-girlfriends and that driver on the 15 the other day, not all of them are crazy. One flaw with the meta-analysis is that it was not clear when the patients were infected relative to the onset of disease.  If they all got T. gondii after they became schizophrenic, then the association is meaningless.  The new study just published by Pedersen et al. (abstract) tries to account for that by measuring T. gondii antibody levels when women gave birth (using samples from heel-stick cards in the birth records) and then looking at the risk of depression and suicide later in their life. Seropositivity was 26.8% at delivery, consistent with the notion that 1/3 of the population has already been infected (remember that infants don't start making their own antibodies for about three months after birth, so these are the mom's antibodies).

From Pedersen et al, Arch. Gen. Psych. 2012.
Pedersen et al. found that seropositive women had a 1.53-fold greater risk of self-directed violence (ie, suicide) than seronegative.  Women with the highest antibody titers had nearly a 2-fold higher risk.  The risk is small, but significant and is also consistent with other recent studies on the relationship between T. gondii antibody titers and mental illness (for example, see this).  I'm generally not a big fan of these types of analyses, because the data could simply be a case of "true, true, and unrelated".  There is no mechanistic hypothesis for why antibodies against T. gondii would alter behavior, or whether the infection caused permanent damage to the brain.  However, evidence continues to accumulate that T. gondii infection can cause permanent changes in human behavior, and with billions of people having been infected at some point, it isn't too hard to see the beginnings of a zombie apocalypse.


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