Toxoplasma: A Common, Mind-Controlling Parasite

Parasites are unnerving at best, and, at worst, they’re fatal. The Toxoplasma gondii parasite has a mixture of these traits. Probably the unicellular protozoan’s most unnerving characteristic is being able to change its host’s behavior. And as if that weren’t enough – what’s also unsettling about Toxoplasma is that it’s pretty common. In fact, it’s thought that about one third of everyone has had it. Why? It comes from some common sources – cats and undercooked meat. Luckily, most people’s immune system can keep it in check, but this doesn’t stop the parasite from potentially permanently changing the host (i.e., one third of us!) in largely unknown neurological ways.

Studies in mice help give an idea of how a Toxoplasma infection might affect someone. Normally, mice stay away from areas that have been marked by cat urine, but a mouse infected with Toxoplasma is less likely to avoid them. The infected mice basically lose their innate fear of cats. And what’s more, a recent study revealed that this effect may be somewhat permanent – at least four months after the parasite is gone, the previously-infected mice still retain the odd behavior. While this works out well for the cat, who gets an easy snack, this clearly has some concerning implications for people.

In most people, the immune system can fight off the Toxoplasma parasite, and initial infection simply causes mild flu-like symptoms, or no symptoms at all. However, it can be fatal, or cause serious problems, in someone who is immune-compromised or pregnant (and it can cause fetal complications, so it’s often checked for using a prenatal blood draw to look for existing antibodies). But even if the immune system fights it off, the parasite just goes into a latent phase and can form cysts in muscle and nervous tissue. We’re not really sure what they do there, but, like in the mouse, they’re likely to affect people neurologically – for example, Toxoplasma infection has been suspected to be connected to schizophrenia and suicidal behavior, among other neurological conditions.

If you want to avoid infection, it’s best to cook meat thoroughly and don’t touch cat feces. Handling and eating undercooked pork in particular is often a source of infection – cysts in the animal’s tissues can be spread this way. Cats also shed the parasite cysts in their feces, so pregnant women (who have never been infected) are often advised to avoid cleaning litter boxes and other areas where feces might be, such as in garden soil and sandboxes. (About 30 to 40% of domestic cats in the U.S. are thought to have Toxoplasma, but it varies a lot depending on the area and the cat’s lifestyle – outdoor hunters are more likely to have caught it from wild prey.)

At least next time you find yourself at a loss for explaining someone’s odd behavior, perhaps it will turn to be due to Toxoplasma, or a different, yet-to-be-characterized brain-controlling parasite.

For further reading:

• Wendy Marie Ingram, Leeanne M. Goodrich, Ellen A. Robey, and Michael B. Eisen’s article “Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even After Extensive Parasite Clearance” at PLOS One
• ScienceDaily’s article “Toxoplasma Infection Permanently Shifts Balance in Cat and Mouse Game“
• Wikipedia’s article “Toxoplasmosis”