Fatal Attraction: Sex, Death, Parasites, and Cats

By Carl Zimmer | August 17, 2011 11:17 pm

It’s time to revisit that grand old parasite, the brain-infecting Toxoplasma. The more we learn about it, the more marvelously creepy it gets.

Toxoplasma is a single-celled relative of the parasites that cause malaria. It poses a serious risk to people with compromised immune systems (for example, people with AIDS) and fetuses (which is why pregnant women need to avoid getting Toxoplasma infections). If you’ve got a healthy immune system,  it doesn’t cause any immediate harm. (Ed Yong has explained why a purported link to brain cancer is very weak.) All told, perhaps a quarter or a third of all people on Earth carry thousands of Toxoplasma cysts in their heads. Most never become aware of their living cargo.

The Toxoplasma life cycle normally takes the parasite from cats to the prey of cats and back again. In the guts of cats, the parasites have sex and produce egg-like offspring which are shed with cat droppings. They can survive in soil for weeks or months. Rats and other mammals ingest the eggs, which produce cysts mainly in the brain. When the cats eat infected prey, they get infected.

For a little over ten years, scientists have been investigating whether Toxoplasma raises its odds of getting back into cats by manipulating their prey hosts. Oxford researchers kicked thing off by releasing healthy and infected rats into large enclosures. They spritzed corners of the enclosure with various odors, including the urine of rats, rabbits, and cats. Normally rats become anxious the instant they sniff cat urine and explore much less. Wise move.

Not so wise is the response of infected rats: in the enclosure experiments they either became indifferent to the smell of cats, or spent some extra time checking out the feline corner. There was no difference in how the infected rats responded to other smells.

Robert Sapolsky, a neuroscientist at Stanford University, and his colleagues have carried the experimental torch foreward. In 2006, they demonstrated just how precise Toxoplasma’s effects are.  They found that infected rats did not lose their fear across the board. Dog urine still spooked them, and they could be trained to get scared of new stimuli. Only their innate fear of cats changed. Sapolsky’s team then looked at where the parasite actually ended up in the rat brain. They found Toxoplasma cysts clumped around the amygdala, a region of the brain that’s heavily involved in fear and other emotions.

Now Sapolsky and his colleagues have looked even closer at the parasite’s effects. They had rats sniff various odors and then examined their brains to look for a telltale protein called c-Fos. When neurons fire, they produce c-Fos, and so the more active a region of the brain, the more c-Fos accumulates in it. The scientists found two big differences in infected rat brains when they sniffed cat urine, both of which occurred in the region around the amygadala. A circuit in the brain that helps produce defensive behaviors became less active.

Near that circuit is another circuit that triggers sexual arousal.

And the parasite also altered this sexual arousal circuit. It increased the activity of those neurons.

Really, it would have been mind-blowing enough for a parasite to surgically swoop into a host brain and knock out the fear it felt towards a particular animal. We admirers of our parasite overlords would have been satisfied. But the possibility that these hosts are actually attracted to their enemy, that they feel the deepest desire a rat can feel, a desire that could lead them to death, and lead the parasite to live on, to achieve their own deepest sexual desires–well, we can only be grateful.

CATEGORIZED UNDER: The Parasite Files, Top posts

Comments (19)

  1. Bob

    I’d heard a hypothesis that crazy cat lady syndrome might be caused by Toxoplasma. Any word on that?

    [CZ: Just speculation, no science.]

  2. Matt

    Does the same can be found in human? Less fear and desire more sex?

    [CZ: There are some personality shifts associated with Toxoplasma infection, but nothing like this. (See, for example, this paper.)]

  3. andyo

    Couldn’t this just mean that rats wanna make sweet sweet love to each other when they’re around cat urine, instead of to the cat?

  4. pausner

    When I read this thoughts of Cronenberg crossed my mind.

  5. Very very neat research. Parasites are such sneaky little buggers. As to human infection, the research on whether parasites manipulate personality and behavior are tenuous at best. If you’re interested, I covered some of the research in an article a few months ago here: http://bodyhorrors.wordpress.com/2011/04/28/consider-the-carpaccio-looking-at-toxoplasmosis/.

  6. Zi Teng Wang

    Wonderful article! This summarizes the state of the field pretty well. I’m working on making some gene knockouts of Toxo to try and figure out what genes this parasite is using to cause this really cool behavioral shift. It’s a really really really cool example of the manipulation hypothesis at work!

  7. Rhiannon

    This article reminds me of this comic from Saturday Morning Breakfast Cereal that was recently posted: http://www.smbc-comics.com/index.php?db=comics&id=2331#comic

  8. The very real mystery of Toxoplasma gondii’s effect on rats and people is tackled by fictional characters in a new book entitled, Purr, the novel, by F. Wyman Morgan. See an extensive preview at purrthenovel.com. Purchase from Amazon.com. An assistant professor, with a well-deserved reputation as a womanizer, undertakes a program to study Toxoplasma gondi and “solves” the riddle of how the microbe makes rats suicidal and women amorous

  9. Ole Rogeberg

    Are you aware of the book “Peeps” by Scott Westerfeld, which explicitly notes its debt to “Parasite Rex”? It`s a “young adult” book that reinterprets vampires as people infected with a parasite, with an evolutionary rationale for the fear of crucifixes etc. Every second chapter is a short essay on some weird parasite from your book, but he does “overdo” it – suggesting in the toxoplasmosis chapter, for instnace, that the parasite affects people in a way that makes them more likely to be “cat people” (less structured and more messy and laissez faire, if I remember correctly).

    Fun read, though the evolutionary logic that I found most entertaining was unfortunately thrown out the window towards the end…

  10. this is such a fascinating parasite. there’s an essay devoted to this in one of Sapolsky’s books, i believe (Monkeyluv, if i recall correctly). thanks for the post.

  11. seb


    Very interesting report on this incredible research!

    There is one outstanding question that still bothers me. What would be the type of mechanisms that would allow the parasite to specifically attenuate the innate fear for cats? I find it quite to difficult to think of any biological process that would do this and at the same time spare the innate fear of other species, such as dogs for instance, or even spare the acquisition of learned fear.
    Any thoughts or research on this?


  12. can the parasite be passed to kittens from the mother? my cats have always been strictly indoor kitties.


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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.


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