There's a cat-borne parasite that has, in the past, been linked to increased rates of suicide, depression, and mental illness in infected humans. However, a new study in PLOS ONE concludes that the connection between these ailments and Toxoplasma gondii may be overblown. Contrary to what previous studies have suggested, this parasite is probably not controlling your mind.
T. gondii is a particularly deceptive parasite known to alter the behavior of mice and, more recently, maybe even chimpanzees too. Although mice normally avoid the smell of cat urine – a sure sign that they are entering the territory of one of their predators – infected mice actually seek it out, severely reducing their life expectancy as a result. Cats are the only known animals in which the parasite can sexually reproduce, so this suicidal behavior in mice increases the chance that T. gondii successfully infects a feline host.
Cat-owners are likely to pick up this parasite from their pets. While it’s not able to reproduce in a human host, it is known to sometimes cause toxoplasmosis, a condition that’s dangerous to pregnant women or those with a weakened immune system. But can this parasite – which may be hiding within 50 percent of the human race – also adversely affect our psychology?
Previous research had linked the parasite to mental disorders, including schizophrenia, but this could also be somewhat coincidental. The team from Duke University decided to find more direct evidence of the parasite’s supposed mind-altering effects within infected humans.
Blood samples from over 800, 38-year-old New Zealanders were taken, and 28 percent tested positive for the parasite. All were registered participants of the Dunedin Longitudinal Study, a long-term project that has been following their health since birth. Using this database, the team could rigorously compare the mental and physical conditions of the infected with the uninfected.
A T. gondii tissue cyst within a mouse brain. Jitinder P. Dubey/USDA
First off, infection with T. gondii was not shown to be linked to neurological disorders, including major depression or schizophrenia. Additionally, significant personality differences between infected and uninfected participants were indistinguishable. The parasite was also not shown to lead to an increase in self-injury behaviors, criminal convictions or traffic-related accidents, contrary to previous studies.
However, suicidal behavior – the key trait exhibited in infected mice – was found to be more common in infected humans, but only very slightly. Similarly, after being given 14 separate measures designed to test memory, intelligence and general neurological functioning, the infected cohort were found to do marginally worse on just one of them.
Overall, however, the link between the parasite and negative alterations to human cognition was shown to be extremely limited. Nearly 1,000 people is a decent size for this type of medical study, but if you’re unconvinced, a previous study using 7,440 participants also reported mostly no link between the parasite and various mood disorders.
So it appears that at present there is no strong, direct evidence linking the parasitic infection in humans to suicidal tendencies, depressiveness, or schizophrenia. Good news for cat hoarders, then.