Retrospectacle: A Neuroscience Blog

A parasite called Toxoplasma gondii has a unique mechanism to help it spread: “tricking” rats into delighting in the smell of its predator, cats. This is an important adaptation since this parasite can only sexually reproduce in the gut of cats, so the parasite needs to get the infected rats into the cats’ gut. Yeah, the cat eats the rat, and the parasite lives happily ever after.

Normally rats have a strong aversion to the scent of cat urine, but infected rats completely lose their aversion to cat pee but retain all their other fears and phobias.

“Toxoplasma affects fear of cat odors with almost surgical precision,” Vyas concluded. “A large number of other behaviors remain intact.”

What happens is that Tomoplasma infection induces cysts to form in the brain, specifically in the amygdala. This region of the brain is involved in fear response, and the cysts disrupt that normal response. In fact, it goes even further–the rats are actually *attracted* to the cat urine. The team is now examining how the parasite can target just one behavior so specifically.

Below: Representative scatter plots depicting the location of a control rat (C) and an infected rat (D) during trial, with respect to bobcat and rabbit urine.
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Vyas et al. 2007. Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. PNAS. Early Edition April 5th. (

For more on this, check out Carl Zimmer’s post on The Loom.


  1. #1 Left_Wing_Fox
    April 5, 2007


    I wonder, there were some papers discussing the role of Toxoplasma gondii‘s role in human evolution as well. Perhaps the effect it has in human beings is not a direct efffect on it’s own evolution,but rather a side effect on the selective process towards making it a more effective “brain sugeon” in rats?

  2. #2 Kurt
    April 5, 2007

    Hmmm, maybe that’s why some cat owners after a while seem to no longer notice the litter box odor…

  3. #3 Shelley
    April 5, 2007

    Fox, do you know which papers discuss Toxo and humans? Sounds interesting.

    Kurt: Hopefully those cat owners aren’t *attracted* to the smell! 🙂

  4. #4 Kurt
    April 5, 2007

    I think Carl Zimmer had a few posts here last year on different aspects of toxoplasma. There was one about a link with schizophrenia and another about possible cultural side-effects. Here’s a post from The Loom that has some links to articles.

  5. #5 brtkrbzhnv
    April 5, 2007

    Shelley Batts:

    this parasite can only reproduce in the gut of cats

    Carl Zimmer:

    It’s only in cats that Toxoplasma can reproduce sexually. […] But Toxoplasma can also reproduce like a clone, splitting in two, either in cats or outside them.

    SB: Ahhh. Thanks for the clarification!

  6. #6 D'oh!
    April 5, 2007

    My favorite Science News article from a few years ago was about this. No only is the subject fascinating, but I loved the ending of the article, in which the speculations of the Oxford researchers on the effect of an infection in humans were reported:

    In people, a latent infection can flair up and cause mental decline. Now, the researchers speculate that low-grade infections may result in more subtle effects, such as odd behavior and IQ dips. They estimate the parasite infects 22 of the UK residents and 88 percent of the French.

    To this day, I don’t know whether the estimates were real or not, but I prefer to believe that it was a joke that got by the editors of Science News.

  7. #7 DoggieMeow
    April 6, 2007

    Pretty cool story. Adaptation.

  8. #8 Lab Lemming
    April 7, 2007

    Has anyone studied the mechanism behind the behavioural effects of viruses like rabies?

  9. #9 Eric
    April 20, 2007

    Fascinating! Cysts! I didn’t know that T. Gondii had could turn into a cyst form. I must have missed that study abstract when I was recently reading about toxoplasma (maybe it was Toxo in my brain making me miss it! lol)

    And yes apparently a large majority of the population have antibody titers towards toxoplasma, and a lot of people could be infected to a certain degree.

    It is believed toxoplasma can alter dopaminergic functionality, amongst other things. There are a few studies attempting to correlate personality changes to toxoplasma.

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