Toxic House Cats?

Research Blogging / by Dave Munger /

Up to half of all humans are infected by a cat-borne parasite that can cause stillbirth, brain damage, and a host of other subtle neurological effects. Is vaccination the solution?

Credit: Flickr user kevindooley

Toxoplasmosis is a serious disease when transmitted from a pregnant mother to the unborn fetus, potentially resulting in stillbirth, brain damage, or long-term eye damage that can lead to blindness. Even worse, it’s extremely common in a most common animal, the house cat—and it’s easily transmissible from cats to humans. The parasite that causes it, Toxoplasma gondii, is found in one-third to one-half of all humans—over two billion individuals! This potential killer likes to take up residence inside your brain.

So why don’t we hear more about toxoplasmosis? There are a couple reasons. First, it’s a relatively benign disease for most people, comparable to a moderate flu. Second, it’s much less common in its most dangerous form, when transmitted from a pregnant woman to the fetus. This is because it’s only likely to be transmitted if the mother was recently infected herself. Still, it’s estimated that the disease costs as much as eight billion dollars a year in the US; it affects over 1 in 15,000 childbirths in the US and as many as 1 in 500 in parts of the world where the disease is more common.

British science writer Ed Yong gives a good introduction to T. gondii in this post: The parasite is primarily transmitted by cats, through their feces. But cats don’t normally eat cat feces, so path of transmission is often more complicated. In the ideal scenario (for the parasite), rats eat the stuff. Then when a cat preys on an infected rat, the parasite infects the cat, starting the cycle anew. Amazingly, the parasite appears to have evolved to attack the brains of rats, making them easier for cats to catch by suppressing the rats’ fear of cats and motivating them to move around more often so they are easier to spot.

Since few cats prey on humans, the ability of T. gondii to infect people is likely just an unhappy accident—both for us, who can get sick from it, and for the parasite, which is unlikely to extend its life-cycle by being transmitted any further. Humans acquire the parasite by eating raw or undercooked meat from infected animals, or by eating items that have been contaminated with cat feces (let this stand as a reminder to wash your hands after cleaning the litter box!). The paper Yong cites offers controversial speculation that the T. gondii may be responsible for a number of subtle and not-so-subtle mental differences in individuals, ranging from neuroticism to stereotyped gender roles. The research, by Kevin Lafferty and published in Proceedings of the Royal Society B: Biological Sciences, cannot establish causality—it only finds that these conditions are more common in regions of the world where the parasite is more common.

Last week, the amateur scientist and science-themed musician DJ Busby discussed another report that analyzed 11 different studies, nine of which found correlations between toxoplasma infection and several different personality traits. So there seems to be converging evidence—albeit only correlations—that T. gondii isn’t just a benign resident of human brains after the initial toxoplasmosis infection is beaten back by the immune system.

Fortunately, primarily motivated by the more severe effects of toxoplasmosis in fetuses and infants, researchers are making significant progress developing vaccines against T. gondii, Rob Mitchum, a science reporter for the University of Chicago Medical Center, writes on the center’s blog. A team led by University of Chicago researcher Samuel Hutson has created a deactivated form of T. gondii that functions effectively as a vaccine against wild T. gondii in laboratory mice. Like other vaccines, it works by harmlessly activating the animal’s natural immune system, preparing it to defend against the dangerous parasite. But since it’s conceivable that the inactive form of the parasite could also be harmful, most people prefer not to risk being injected with a life vaccine.

In a separate study from the same laboratory and led by Hua Cong, researchers identified key bits of proteins from the parasite that were likely to activate the immune system response, then inoculated mice with only those bits before injecting them with live T. gondii. The vaccine was 80 percent effective in preventing infection. The research was published in Vaccine. There are many steps before this work might result in a practical vaccine, but this are encouraging initial results.

So given the widespread but (possibly) non-harmful nature of T. gondii, would it make sense to vaccinate everyone? The laboratory director, Rima McLeod, thinks so. In addition to the possibility of real danger in transmission from mother to child, people with immune deficiencies such as AIDS are also susceptible to the parasite’s more severe impacts on eyesight and the brain. And since the research isn’t yet conclusive on whether T. gondii is responsible for personality effects and mental disorders, it certainly makes sense to pursue additional research on vaccines that might benefit even more people.

Dave Munger is editor of, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »


Originally published December 15, 2010

Tags behavior health medicine

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