The Evil Genuis of Toxoplasma

Image Credit: Rosa Pomar.

Just when you thought it was safe to clean the cat box, Toxoplasma gondii is back in the news. In the August 17^th issue of the journal PLoS One, scientists reported the latest creepy details about how the Machiavellian parasite tricks host organisms into doing its bidding.

For those unfamiliar with the brilliant and disgusting life cycle of Toxoplasma gondii – the microorganism that causes the disease toxoplasmosis – a brief review (feel free to skip to the next paragraph if you already know this)….

Toxoplasma is a single-celled protozoan with both sexual and asexual phases of reproduction. The sexual phase occurs in one place only – the small intestine of a cat. Here’s how it works: after the micro-sex act, the resulting oocysts make their way into the world in the form of cat feces. There they sporulate for a few days until they’re ready to infect the intermediate host (such as a rat). The rat eats the cat feces, which allows the parasite to complete its last phase of asexual development before lodging itself into the host’s brain or muscle tissue. Then it just hangs out there as a cyst until the host is eaten by a cat and ferried to the feline digestive tract, where a circle of life begins anew.

Getting rats to eat cat shit is no problem; they’ll eat anything. But convincing them to offer themselves up at cat treats is a bit more challenging. Healthy rats have an innate fear of cats, and typically run in the opposite direction the moment they detect catty smells. This is evolutionarily advantageous for rats since cats tend to view them as food. But rats infected with Toxoplasma lose their cat-avoidance instincts. Past experiments exposing normal and Toxoplasma -infected rats to cat urine found that the parasite-carrying rodents not only didn’t show a fear response, they actually seemed somewhat intrigued by the odor of cat pee. Somehow the parasite had tampered with these rats’ brains and was causing them to recklessly place themselves in harm's way, thus ensuring that the parasite-harboring rodents would find their way into a cat’s belly. Curiosity kills the rat.

Image Credit: Luke Hayfield.

In the latest study, researchers showed that infected rats had abnormal responses not just in their behavior, but in subregions of the amygdala – a part of the brain associated with emotions, including both fear and attraction. While healthy rats encountering the smell of cat urine showed more activity in the fear pathway of the amygdala, the Toxoplasma-infected rats also had responses in the attraction pathway. Their brains lit up in pattern more like that of a rat confronted with a potential mate than a potential predator.

While Toxoplasma that make their way into neural tissue do often land in the amygdala, the method by which they confuse its emotional pathways is still unknown. But the behavioral results suggest that the seemingly dormant cysts are turning prudent fear into foolhardy interest. Also rather telling is the observation that infected rats display normal behavior in other situations, even those involving non-feline predators. It would seem that Toxoplasma has no interest in finding its way to the intestine of an eagle or a snake. Just cats please.

So what? Too bad for the rats, you may say, but humans too can harbor Toxoplasma. While most of us don’t go out of our way to eat cat crap, our species can become infected with the parasite through ingesting undercooked meat (remember, the cysts can also end up in muscle tissue) or by just not washing our hands well enough after cleaning the cat box. Once the parasites enter our bodies, they linger just like they do in rodents. Infection rates vary considerably by region, but up to a third of the human population carries the parasite.* Toxoplasmosis is generally mild in otherwise healthy humans, causing little or no symptoms; you might get some flu-ish aches and pains, but that’s about it. However in it can be life-threatening for anyone with a weakened immune system.

But for the healthy, asymptomatic hosts of the parasite, is there anything to be concerned about? Humans have amygdalas too, which can house parasitic cysts just like those of rats. What might Toxoplasma be up to in our brains? Researchers have been increasingly drawn to this question. Studies have linked Toxoplasma in humans to a variety of behavioral changes, cultural differences (since infection rates vary from one country to the next) and even schizophrenia. Such research is hardly at the point of being universally accepted, but it is suggestive of the possibility that toxoplasmosis may not be as asymptomatic as we’ve been led to believe. Perhaps the symptoms are just more psychological than physical. Perhaps Toxoplasma is subtly affecting our behavior in ways that haven’t even occurred to science. I’m not saying that the parasite is causing people to live alone with upwards of ten cats so that it might one day be able to reproduce if, say, the cat owner dies and the pets start eating the body before anyone discovers it.† But we can’t rule it out.

* Before you lock yourself in the house with a box of sani-wipes, please recall that the parasite only reproduces in cats, so you can’t get it from ordinary human contact. However, the disease can be passed from a pregnant mother to her child, or (though this is quite rare) via organ transplants or blood transfusions.

† For a more thorough exploration of “postmortem predation”, have a look at The Straight Dope, where no question is too obscure or too gross to be answered.