10 February 2012

Is your behavior controlled by a parasite ?

Excerpts from an essay in The Atlantic:
Starting in the early 1990s, [Jaroslav Flegr] began to suspect that a single-celled parasite in the protozoan family was subtly manipulating his personality, causing him to behave in strange, often self-destructive ways. And if it was messing with his mind, he reasoned, it was probably doing the same to others. The parasite, which is excreted by cats in their feces, is called Toxoplasma gondii (T. gondii or Toxo for short) and is the microbe that causes toxoplasmosis—the reason pregnant women are told to avoid cats’ litter boxes...

If Flegr is right, the “latent” parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia...

But after years of being ignored or discounted, Flegr is starting to gain respectability. Psychedelic as his claims may sound, many researchers, including such big names in neuroscience as Stanford’s Robert Sapolsky, think he could well be onto something. Flegr’s “studies are well conducted, and I can see no reason to doubt them,” Sapolsky tells me. Indeed, recent findings from Sapolsky’s lab and British groups suggest that the parasite is capable of extraordinary shenanigans. T. gondii, reports Sapolsky, can turn a rat’s strong innate aversion to cats into an attraction, luring it into the jaws of its No. 1 predator...

What’s more, many experts think T. gondii may be far from the only microscopic puppeteer capable of pulling our strings. “My guess is that there are scads more examples of this going on in mammals, with parasites we’ve never even heard of,” says Sapolsky.
Familiar to most of us, of course, is the rabies virus. On the verge of killing a dog, bat, or other warm-blooded host, it stirs the animal into a rage while simultaneously migrating from the nervous system to the creature’s saliva, ensuring that when the host bites, the virus will live on in a new carrier...

But even if we were never part of the parasite’s life cycle, Flegr reasoned, mammals from mouse to man share the vast majority of their genes, so we might, in a case of mistaken identity, still be vulnerable to manipulations by the parasite...

Compared with uninfected men, males [humans] who had the [toxo] parasite were more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.

Twelve of 44 schizophrenia patients who underwent MRI scans, the team found, had reduced gray matter in the brain—and the decrease occurred almost exclusively in those who tested positive for T. gondii. After reading the abstract, I must look stunned, because Flegr smiles and says, “Jiri had the same response. I don’t think he believed it could be true.” When I later speak with Horacek, he admits to having been skeptical about Flegr’s theory at the outset. When they merged the MRI results with the infection data, however, he went from being a doubter to being a believer. “I was amazed at how pronounced the effect was,” he says. “To me that suggests the parasite may trigger schizophrenia in genetically susceptible people.”  ...

Human-genome studies, both scientists believe, are also in keeping with that finding—and might explain why schizophrenia runs in families. The most replicated result from that line of investigation, they say, suggests that the genes most commonly associated with schizophrenia relate to the immune system and how it reacts to infectious agents. So in many cases where the disease appears to be hereditary, they theorize, what may in fact be passed down is an aberrant or deficient immune response to invaders like T. gondii...

For example, she and Chris Reiber, a biomedical anthropologist at Binghamton University, in New York, strongly suspected that the flu virus might boost our desire to socialize. Why? Because it spreads through close physical contact, often before symptoms emerge—meaning that it must find a new host quickly. To explore this hunch, Moore and Reiber tracked 36 subjects who received a flu vaccine... the flu shot had the effect of nearly doubling the number of people with whom the participants came in close contact during the brief window when the live virus was maximally contagious...

Reiber has her eye trained on other human pathogens that she thinks may well be playing similar games, if only science could prove it. For example, she says, many people at the end stages of AIDS and syphilis express an intense craving for sex. So, too, do individuals at the beginning of a herpes outbreak. These may just be anecdotal accounts, she concedes...
Out of fairness to cat owners (which includes our household), it's worth emphasizing that toxo can also be acquired through the touching of raw or undercooked meat, especially if you have broken skin on your hands, and via materno-fetal transmission during pregnancy.  Other animals that can carry toxo are rabbits, birds, sheep, goats, cattle, pigs and poultry; dogs have lower infection rates.

If you've read all this, you might as well read the whole story at The Atlantic.


  1. That is so creepy yet so fascinating at the same time. It's like the plot of some Michael Crichton novel.

    I remember hearing about the cat/mouse toxo reaction years ago (similar to a parasite that grazing mammals get - it somehow forces its intermediary host to climb blades of grass to ensure it gets eaten by another grazing animal) and being fascinated by it, but it was brand new research at the time so the story I read wasn't very extensive.

    Thanks for the link, Stan - nice to be able to follow up on this!

    1. "somehow forces its intermediary host to climb blades of grass"

      from the link:

      A drop in temperature normally causes ants to head underground, but the infected insect instead climbs to the top of a blade of grass and clamps down on it, becoming easy prey for a grazing sheep.

      That's probably the research you remember.

    2. Thx! Hadn't had the chance to read the Atlantic story yet.

  2. Radiolab had this in an episode. It completely makes sense, how many crazy cat ladies do you know? I know more than a handful that live within 3 miles of me...

  3. Toxoplasmosis has been an interesting talking point among epidemiologists and biologists for a long time. Google it and you'll come across loads of information. It's not like it's just discovered, BTW; it's why pregnant women have been advised for decades to not get near cat feces.

    The Scientist just recently had an article with some popular examples of apparently adaptive behavior modification by parasites:



    1. "... researchers have observed countless examples of parasites hijacking the autonomy of their hosts ..."

      *cough* democracy *cough*

    2. You should really see a doctor about that cold, King George III.

  4. "It completely makes sense, how many crazy cat ladies do you know?"

    Interesting point.


    1. "Parasite makes men dumb, women sexy"

      Does that remind anyone else of the Republican Party?


  5. Correlation or causation?

    I submit to you that cat lovers tend to be more introverted people, and tend to be the ones exposed to cat poo. Did the toxo really make them introverted?

    Similarly with the flu shot stuff - perhaps people who know they are immune to the flu feel more courageous about socializing during flu season?


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