When Patrick House gets to know someone, he often finds himself silently assessing them for Toxoplasma gondii, a single-celled parasite doctors and scientists are linking to increased aggression and recklessness, among other things. “I find myself typing people as, ‘Oh, they probably have it,’ or, ‘They probably don’t,’ ” the Stanford University neuroscientist says. “I imagine whether or not people have it in their brains, based on personality profiles.”
About one-third of the world’s human population carries a Toxoplasma infection, the origin of which is overwhelmingly associated with an unlikely sinister source: the common house cat.
Though it can only reproduce sexually in a cat’s gut, Toxoplasma gondii can make its way into humans through contact with cat feces, eating undercooked infected beef or chicken, or drinking untreated water. (It can also be passed on in the womb—which is why pregnant women are told to stay away from cat litter.) Its effects vary: many people with Toxo don’t even know they have it; others can take seriously ill and are at risk for everything from suicide to schizophrenia.
Parasitic mind control sounds like the stuff of science fiction, but in fact that’s along the lines of what happens when Toxo infects the body. The parasite needs to reproduce, so it overrides the normal behaviours of its host to achieve its own goals. Rats infected with Toxo lose their fear of cats, and in fact become attracted to their scent because it would bring the parasite closer to its natural habitat.
When Stanford University professor Robert Sapolsky first read about Toxoplasma gondii, he was “blown away” by the parasite’s ability to manipulate a host’s behaviour. Other parasites affect human behaviour, he says, “but in terms of the behavioural profile—one cluster of traits related to increased risk of schizophrenia, another cluster related to neuropsychological and behavioral dis-inhibition—there’s nothing else like [Toxo].” Findings linking Toxoplasma infections to increased risk of car accidents are of particular interest to him. If people infected with Toxoplasma become less inhibited and more reckless, they’re more likely to do things like speed. Sapolsky recalls talking to Toxoplasma clinicians over lunch one day. One doctor piped in about his own medical experience with the parasite. “It had to do with when he was a medical resident, doing a rotation in transplant surgery,” Sapolsky says. “He remembers an old surgeon telling the residents, ‘Whenever you get an organ from someone killed in a motorcycle accident, check it for being Toxo-positive. Don’t ask me why, but there tends to be high rates of Toxo in those donors.’ ”
Two recent independent studies showed that when compared to people with the same severity of depression, people infected with Toxoplasma were more likely to attempt or commit suicide. These ﬁndings point to impulsiveness, Sapolsky says. Doctors have also identified a correlation between Toxoplasma infections and schizophrenia, says Dr. Robert Yolken, director of developmental neurovirology at the Johns Hopkins Children’s Center. “What a number of researchers have found is that you take a population of people with schizophrenia, wherever they are, the rate of Toxoplasma is somewhat higher than in the controlled population,” Yolken says. Most people infected with Toxo aren’t schizophrenic. “But there are a small number of people where evidently it contributes to the disease,” he says.
Perhaps the strangest theory surrounding the parasite comes from House, who argues that soccer teams from countries with high Toxoplasma infection rates fare better at the World Cup. Infection increases testosterone in male brains, he says, making them more aggressive and less inhibited. “Brazil, France, Germany and African countries all have some of the highest infection rates in the world, and generally that’s associated with cultures of eating raw meat, which is the most common way of getting Toxo,” he says. “I was watching the World Cup and I recognized that the countries that had more Toxo seemed to be winning.” House admits the relationship is not foolproof. Ghana, with an infection rate of 92 per cent, ranked well below first-place team Spain, which has an infection rate of 44 per cent.
House first became interested in Toxo after reading an article in Discover called “Do parasites rule the world?” He says if they do, Toxoplasma gondii is just the beginning. “Conceivably, we’re going to learn there are many more of these types of parasites influencing behaviour that we just weren’t aware of,” he says. “A tiny neurotransmitter, a tiny drug, or a tiny parasite that potentially changes neurotransmitter levels can have profound impacts on your personality, both individually and at large culturally.”