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Toxoplasmosis: could that latent infection affect people’s behavior?

This is disturbing news. What if eating undercooked meat, gardening, or having a cat could give you an infection which stayed with you for life, nested in cysts in your brain? Worse, those cysts somehow affected your personality, possibly putting you at increased risk of suicide, schizophrenia, or car accident? And here’s the stop-you-in-your-tracks point: up to one third of humanity may harbor this protozoan. That is one very successful parasite.

Infection rates are frighteningly high: as much as 10-20% of the population in the US, 30-40% of Czechs, and 55% in France.

Sounds like a plot for a bad sci-fi but we know that Toxoplasma gondii makes small mammals take life threatening risks – an infected rat finds cat-urine “attractive” and runs about more, being fearless, but getting eaten: good for the parasite, not so good for the rat. Our brains may be  larger but they are made with much the same building blocks and tools.  Rabies is known to stir larger mammals into a rage, and there are “a truckload” of examples from the insect and fish world of parasitic mind control. Hmmm.

Toxoplasmosis is thought to be a nasty parasite that affects pregnant women and immune-compromised people, but evidence of an insidious nature is accruing. Jaroslav Flegr, an evolutionary biologist from Prague, has a theory that this protozoan might be rewiring our brains.

‘There is strong psychological resistance to the possibility that human behavior can be influenced by some stupid parasite,” he says. “Nobody likes to feel like a puppet. Another more obvious reason for resistance, of course, is that Flegr’s notions sound an awful lot like fringe science, right up there with UFO sightings and claims of dolphins telepathically communicating with humans.’

Flegr has been ignored or years, but other researchers are now getting results that support his theory. Studies show that mental illness is 2-3 times as common in people who have the parasite compared to people in the same region who don’t. It could be that schizophrenia runs in families because of a genetic predisposition to a weaker immune response to parasites.

According to psychiatrist E. Fuller Torrey, schizophrenia did not become common until around the same time as people started having cats for pets. Schizophrenia did not rise in prevalence until the latter half of the 18th century, when for the first time people in Paris and London started keeping cats as pets. The so-called cat craze began among “poets and left-wing avant-garde Greenwich Village types,” says Torrey, but the trend spread rapidly—and coinciding with that development, the incidence of schizophrenia soared.”

Now that I ponder the unpleasant implications, it seems inevitable that evolution could toss up a parasite-host relationship which shifts human behavior. If Toxo isn’t doing it, sooner or later something else will. (Another reason we ought throw more money at medical research, rather than at bat-killing windmills.)

Those who test positive for the latent infection have significantly delayed reaction times and here’s an especially weird effect, infected men are more likely to be introverted and suspicious and to disregard rules, but infected women are the opposite: more trusting, outgoing and obedient.

‘Compared with uninfected people of the same sex, infected men were more likely to wear rumpled old clothes; infected women tended to be more meticulously attired, many showing up for the study in expensive, designer-brand clothing. Infected men tended to have fewer friends, while infected women tended to have more. And when it came to downing the mystery fluid, reports Flegr, “the infected males were much more hesitant than uninfected men. They wanted to know why they had to do it. Would it harm them?” In contrast, the infected women were the most trusting of all subjects. “They just did what they were told,” he says.’

‘Why men and women reacted so differently to the parasite still mystified him. After consulting the psychological literature, he started to suspect that heightened anxiety might be the common denominator underlying their responses. When under emotional strain, he read, women seek solace through social bonding and nurturing. In the lingo of psychologists, they’re inclined to “tend and befriend.” Anxious men, on the other hand, typically respond by withdrawing and becoming hostile or antisocial. Perhaps he was looking at flip sides of the same coin.’

Because reaction times were slowed he looked at car accident statistics and found that people who tested positive for Toxoplasmosis were 2.5 times more likely to have a car accident. Turkish studies showed similar results.

To put this in perspective Flegr says he cannot say who is infected and who is not by doing a personality test. He needs at least 100 people to find a statistically meaningful disparity. (Only 100 thinks Jo?)

“The vast majority of people will have no idea they’re infected.” It may be that the parasites most detrimental effects occur in a smaller genetically susceptible part of the population. One quarter of a group suffering from schizophrenia showed shrinkage of their cerebral cortex in MRI scans, and these were the same people who tested positive for Toxo. So most schizophrenia is caused by something else, but some cases appear to be caused by the parasite.

Joanne Webster, a parasitologist at Imperial College London:

‘I don’t want to cause any panic,” she tells me. “In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior. But in a small number of cases, [Toxo infection] may be linked to schizophrenia and other disturbances associated with altered dopamine levels—for example, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and mood disorders. The rat may live two or three years, while humans can be infected for many decades, which is why we may be seeing these severe side effects in people. We should be cautious of dismissing such a prevalent parasite.’

Flegr has the infection himself, and this was part of what got him interested in studying the area. He noticed that he seemed unusually happy to take risks that others around him did not, getting honked at while crossing roads, being calm when gunfire broke out in the strife torn region.

‘He also made no effort to hide his scorn for the Communists who ruled Czechoslovakia for most of his early adulthood. “It was very risky to openly speak your mind at that time,” he says. “I was lucky I wasn’t imprisoned.’

So an odd side effect, a potential benefit in a weird way, is that a large slab of the population might be less afraid to speak up against tyranny. Common Toxo infections might make at least males in the population more likely to rebel.

The bottom line

For meat eaters: if you like it rare, better make sure that meat was frozen before you cook it; otherwise, learn to like it well done (freezing and cooking both kill the parasite). For gardeners: wear gloves. For cat owners, change that kitty litter daily, wash your hands. (Cats don’t post that great a threat. Indoor cats don’t have it, and outdoor cats are only infectious for the first three weeks after they catch it, not to mention that there are plenty of studies showing benefits from pet ownership.)

Do read the full (much longer and well written article) in a The Atlantic. Gripping, provocative stuff — an example of a real problem we ought be dealing with instead of all the faked ones. There is no treatment for Toxo, no way to remove those cysts, not yet.

H/t to Tim Blair who found this report in, and called it SCHIZOKITTIES.

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