I love my cats. But sometimes when Bonnie and Clyde are living up to their names, puking up hair balls twice a day, peeing on the guest bed (yes, contrary to popular belief even vets can’t stop their own cats from doing this sometimes), caterwauling at 3 AM, or begging for food all afternoon, they do make me c-r-a-z-y crazy – but they’re not making me suicidal.

In yet another example of how the media will present study results in the manner that will sell the most newspapers or magazines, rather than the way that helps people interpret the results in a logical manner, comes an article entitled "Is Your Cat Hosting a Human Suicide Parasite?" The article talks about a study recently published in the Archives of General Psychiatry (Pedersen et al. 2012) which looked at a cohort of 45 788 women in Denmark who gave birth between 1992-1995, and found a statistically significant association between self-directed violence (including suicide attempts) in these women and their antibody titre to Toxoplasma gondii at the time of birth.  The risk in seropositive women was 1.53 times greater than the risk in seronegative women.

Toxoplasma gondii is a parasite that is shed in the feces of cats, which are the parasites definitive host.  Most house cats only ever shed significant amounts of the parasite the first time they’re exposed to the parasite (typically when they’re young).  Depending on where people live and various cultural practices, transmission of the parasite from scooping out litterboxes may actually be relatively uncommon compared to other possible sources including exposure from soil (e.g. working in the garden and then not washing one’s hands), eating unwashed vegetables, or eating some types of undercooked meat.

The most glaring limitation of the Pedersen study is that they didn’t control for any other factors that may have resulted in the women who committed acts of self-directed violence being more likely to be seropositive for Toxoplasma than others.  For example, women with mental illness may be less likely to practice good hand hygiene (one of the most important factors for reducing the risk of parasite transmission), and therefore more likely to be exposed to Toxoplasma, or there may be other factors about their health or their lifestyle that make them more prone to infection. The point is the authors only found an association in a specific subset of the population (Danish women who had given birth to at least on child).  This does not mean that the relationship is causative – they can’t say that Toxoplasma infection makes people more prone to self-directed violence, only that women – in this particular group – who were seropositive for the parasite were also at increased risk for this kind of behaviour.  It’s a somewhat subtle but very important difference.  The authors of the study clearly acknowledge the limitations of their work, but the news article does not do quite as good a job of pointing this out, until right at the very end where it does finally get mentioned.

Does Toxoplasma infection cause behavioural changes in rats that may make them more likely to wander into a cat’s territory and be eaten?  According to an experimental study it can, and it does make a certain amount of ecological sense that the parasite could have an effect on its intermediate host (the rat) that makes it more likely to be able to continue its life cycle (via being eaten by a cat) by reducing fear in the rat. Could infection of the brain in humans cause subtle behavioural changes?  I can’t deny the possibility, but humans are not rats and I would be very wary of extrapolating results from one species to the other. But is this parasite likely to "drive our brains off the highway" as the news article says?  I’m not ready to buy that, certainly not based on this study.  As the authors clearly state in the first line of the paper "Suicide is a tragic multifactorial outcome of mental illness, with complex biopsychosocial underpinning…"  There are so many things that contribute to such an unfortunate outcome that a lot more work is needed before anyone can justifiably blame a "suicide parasite" in cats.

Whether you believe Toxoplasma infection can result in behavioural changes in people or not, there are some very simple steps everyone can take to help decrease the risk of becoming infected with this parasite regardless.  These are particularly important for individuals who are immunosuppressed and women who are pregnant, because it is very well established that toxoplasmosis in these high-risk individuals certainly can have severe repercusions to either the individual or the unborn fetus.  However, it is by no means necessary for such individuals to get rid of their cats if they take these simple precautions:

  • Clean your cat’s litter box every day. The oocysts shed in cat feces usually take about 24 hours to become infective once they’ve been passed, so daily cleaning helps remove them before they reach this stage.
  • Always wash your hands with soap and water after cleaning your cat’s litter box, after working in the garden or in any soil, and after handling raw meat.
  • Keep your cat indoors. Outdoor cats are more likely to be exposed to Toxoplasma and shed oocysts in their stool. 
  • Keep sandboxes covered so outdoor cats don’t contaminate them with stool.
  • Cook all meat, especially pork, lamb, mutton and wild game, to an internal temperature of 67ºC/153ºF or higher.

More information about Toxoplasma can be found on the info sheet on the Worms & Germs Resources page.