How Parasites Commandeer and Change Our Neurocircuits | Kathleen McAuliffe | Big Think

We know that parasites can make us sick, of course, and siphon off our nutrients, but it's very surprising to hear that some of them, in fact, there may be a large number, at least a hundred are known about at this point in time manipulate the behavior of their host in order to enhance their own transmission.

And the best way to understand this phenomenon, I think, is with an example. One is a cat parasite called Toxoplasma Gondii, or just Toxoplasma for short. I first learned about this parasitic manipulation while just reading about scientific research and I came across a study that showed that rodents that are infected with this parasite can pick up the parasite from the ground.

Cats defecate this parasite, so rodents, as they are scavenging around, can pick up the parasite, and it then invades their brain. It actually tinkers with the animal's neural circuits in such a fashion that it makes it attracted to the scent of cat urine.

And when I say attracted, I mean sexually attracted. The rodents become sexually aroused by the scent of cat urine, so they approach, and needless to say, they're not long for this world. They soon end up in the belly of a cat, and that's the only place where this parasite can sexually replicate. So that's its little trick.

And it does many other things as well. For example, the same parasite goes to the testicles and jacks up production of the sex hormone testosterone. In females, by means nobody's figured out yet, it can increase the level of the sex hormone progesterone.

And in both cases, these changes make the rodent more emboldened and cause the rodent to sort of lower its guard and to act in foolish ways around cats. So that's yet another example of other tricks it has for getting back into the belly of a cat.

This parasite can also infect us. One of the ways we can get it is by changing a cat's litter box. The current thinking in medicine is that the parasite mainly poses a threat to a developing fetus and can harm the developing baby's nervous system or even cause blindness.

And it's also well known to be a threat to people who are immuno-compromised; for example, people who have received transplanted organs or are being treated with chemotherapy. It's still assumed that for most healthy people it poses no threat, and once the parasite gets inside the brain, it just hunkers down inside neurons, never again to cause any problems.

But there are now several labs, both in Europe and the United States, that are challenging that dogma. They have uncovered a lot of evidence that for a small percentage of people, the dormant infection may indeed have adverse consequences.

Nobody yet has a handle on what percentage, but about 20 percent of all Americans are infected with the parasite. So people's guess is that we're only talking about a small percentage of people who have these adverse responses.

But, among other things, it's linked to mental illness. For example, people with schizophrenia are two to three times more likely to have antigens against the parasite. It's also been linked to manic depression, and it's been linked to suicide.

There was actually a study done in 22 nations in Europe, and the researchers found that suicide increased in direct proportion to the prevalence of the parasite in each country. It's been linked to dangerous driving as well.

Several studies in a few different countries have shown that people who test positive for the parasite are more likely to be in car accidents. So one theory—nobody knows for sure what the reason for this association is—but one theory is that just as rodents lower their guard and behave in a cocky way, maybe people behind the wheel of a car are less vigilant.

Or there's also research that shows that infected people have slightly slower reaction times, so that's perhaps another factor that may influence their driving. I should emphasize these are all correlational studies, but as scientists have learned more about what this parasite does to the rodent brain, it does make them think it's plausible that the dormant infection is indeed causing...