MELISSA BLOCK, HOST:
Scientists say a parasite carried by cats appears to influence the behavior of humans, in this case, women infected with the parasite were slightly more likely to attempt suicide.
NPR's Jon Hamilton reports this is just the latest study suggesting that parasites can cause subtle changes in our brains.
JON HAMIILTON, BYLINE: This parasite is called Toxoplasma and its primary home is in the intestine of a cat. People can get infected when they eat under-cooked meats or sometimes when they change the litter in a cat box.
Teodore Postolache, from the University of Maryland, says earlier studies linked Toxoplasma infections with mental illness and risky behavior. So, he and a team of researchers analyzed decades of health records on women in Denmark - they'd been tested for the parasite when they gave birth. Postolache says infected women were different in at least one way.
DR. TEODOR POSTOLACHE: What we found is a significant increase in the risk of attempting suicide.
HAMIILTON: The risk was still very small. The chance that an infected woman would attempt suicide in a given year was about one in 5,000. But that was 1.5 times higher than in uninfected women. Postolache says that suggests the parasite affected their brains. And he says there are hints that the problem is in a specific area of the prefrontal cortex.
POSTOLACHE: That area is like a braking mechanism, like it's a brake in a car. That what we are thinking is that it's possible that the parasite, in fact, interferes with the capacity of the prefrontal cortex to apply brakes.
HAMIILTON: So people might become more impulsive, more likely to act on a suicidal impulse. That's speculation, though. Scientists are only beginning to understand how parasites affect behavior in people. But there are lots of dramatic examples in animals.
Janice Moore, a biologist at Colorado State University, says one example is a tiny worm that infects pill bugs.
JANICE MOORE: And when those worms mature in the body cavity of those pill bugs, then the pill bugs behavior changes and it roams around out in exposed areas. It no longer cares about crawling under things, which is ultimately really, really bad for it.
HAMIILTON: But good for the worms; reckless pill bugs are more likely to end up in the stomach of a bird which is where the worms complete their lifecycle. Moore says she doubts a parasite could cause such a dramatic change in a person's behavior.
MOORE: I absolutely do not believe that an animal like a person, with a long life and a big brain, is going to be a robot that just responds mechanically to every influence out there.
HAMIILTON: But Moore says the parasite might suddenly affect something like a person's willingness to take risks.
MOORE: Those are very impulsive kind of behaviors. And the direction that a person leans in his or her life, at a certain time, might very well be influenced by lots of things including parasites.
HAMIILTON: Scientists say parasites also could help explain why people still carry genes that increase the risk of mental illness. Robert Yolken, of Johns Hopkins School of Medicine, says that's baffled evolutionary biologists for a long time.
ROBERT YOLKEN: It's a little hard to understand how genes involved in conditions, such as schizophrenia or bipolar disorder or suicide, may have persisted in humans because they're obviously detrimental. An individual who commits suicide doesn't pass on their genes.
HAMIILTON: But Yolken says genes that harm a person could help a parasite. That's because many parasites can't complete their lifecycle until their host dies and is eaten by another animal, like a lion or some other member of the cat family. Yolken says people shouldn't be afraid of their own cats even though they can carry the parasite linked with suicide.
YOLKEN: I have two cats. I enjoy them. And I would not advise people to get rid of their cats. I would advise them to take care of their cats.
HAMIILTON: By keeping them from eating rodents which is how most cats get infected by Toxoplasma. The new research appears in the Archives of General Psychiatry.
Jon Hamilton, NPR News. Transcript provided by NPR, Copyright National Public Radio.