Parasite uses the power of sexual attraction to trick rats into becoming cat food

Aug 17, 2011 By Louis Bergeron
Individual Toxoplasma parasites (green) are shown invading neurons (red) grown in a petri dish in the lab. The blue areas are fluorescently tagged cell nuclei.

(PhysOrg.com) -- Could it be love? Rats infected with the parasite Toxoplasma seem to lose their fear of cats – or at least cat urine. Now Stanford researchers have discovered that the brains of those infected, fearless male rats show activity in the region that normally triggers a mating response when they meet a female rat. But that does not necessarily mean the rats find cats sexually appealing. It's a trick that Toxoplasma plays to have the rats eaten by cats, a clever manipulation of rat behavior that is part of the parasite's reproduction scheme.

When a male rat senses the presence of a fetching female rat, a certain region of his lights up with neural activity, in anticipation of romance. Now Stanford University researchers have discovered that in male infected with the parasite Toxoplasma, the same region responds just as strongly to the odor of cat urine.

Is it time to dim the lights and cue the Rachmaninoff for some cross-species canoodling?

"Well, we see activity in the pathway that normally controls how male rats respond to female rats, so it's possible the behavior we are seeing in response to cat urine is behavior, but we don't know that," said Patrick House, a PhD candidate in neuroscience in the School of Medicine. "I would not say that they are definitively attracted, but they are certainly less afraid. Regardless, seeing activity in the attraction pathway is bizarre."

For a rat, fear of cats is rational. But a cat's small intestine is the only environment in which Toxoplasma can reproduce sexually, so it is critical for the parasite to get itself into a cat's digestive system in order to complete its lifecycle.

Thus it benefits the parasite to trick its host rat into putting itself in position to get eaten by the cat. No fear, no flight – and kitty's dinner is served.

House, the lead author of a paper about the research published in the Aug. 17 issue of Public Library of Science ONE, works in the lab of Robert Sapolsky, a professor of biology and, at the medical school, of neurology and neurological sciences.

Scientists have known about Toxoplasma's manipulation of rats for years and they knew that rats infected with Toxoplasma seemed to lose their fear of cats.

It is an example of what is called the "manipulation hypothesis," which holds that some alter the behavior of their host organism in a way that benefits the parasite. There are several known examples of the phenomenon in insects.

But the details of how the little single-celled protozoan Toxoplasma, about a hundredth of a millimeter long, exerts control over the far more sophisticated rat have been a mystery.

Sapolsky's group previously determined that although the parasite infects the entire brain, it shows a preference for a region of the brain called the , which is associated with various emotional states. Once in the brain, the parasite forms cysts around itself, in which it essentially lies dormant.

House was interested in how the amygdala is affected by the parasite, so he ran a series of experiments with both healthy and Toxoplasma-infected rats. He exposed each male rat to either cat urine or a female rat in heat for 20 minutes before analyzing its brains for evidence of excitation in the amygdala.

For the experiments, he used cat urine he purchased in bulk from a wholesaler. No actual cats participated in the experiments.

House analyzed certain subregions of the amygdala that focus on innate fear and innate attraction.

In healthy male rats, cat urine activated the "fear" pathway.

But in the infected rats, although there was still activity in the fear pathway, the urine prompted quite a bit of activity in the "attraction" pathway as well. "Exactly what you would see in a normal rat exposed to a female," House said.

"Toxoplasma is altering these circuits in the amygdala, muddling fear and attraction," he said.

The findings confirmed observations House made during the experiments, when he noticed that the infected rats did not run when they smelled cat urine, but actually seemed drawn to it and spent more time investigating it than they would just by chance.

Although House doesn't have the data yet to speculate on just how the cysts in the rats' brains are causing the behavioral changes, he is impressed with what Toxoplasma can accomplish.

"There are not many organisms that can get into the brain, stay there and specifically perturb your behavior," he said.

"In some ways, Toxoplasma knows more about the neurobiology of fear than we do, because it can specifically alter it," Sapolsky said.

Because Toxoplasma reproduces in the small intestine of cats, the parasites are excreted in feces, which is presumably how rats get infected. Rats are known to be extremely curious, tasting almost everything they come in contact with. Toxoplasma is also frequently found in fertilizer and can infect virtually any mammal.

Approximately one third of the world's human population is infected with Toxoplasma. For most people, it appears to present no danger, although it can be fatal in people with compromised immune systems. It also can cross the placental barrier in a pregnant woman and lead to many complications, which is why pregnant women are advised not to clean cat litter boxes.

House said humans acquire the parasite by eating undercooked meat or "eating little bits of cat poop, which I suspect happens more often than people want to admit." Or know.

Although Toxoplasma has not been shown to have any ill effects in most people, one can't help but wonder whether it truly has no effect in humans.

"There are a couple dozen studies in the last few years showing that if you have schizophrenia, you are more likely to have Toxoplasma. The studies haven't shown cause and effect, but it's possible," House said. "Humans have amygdalae too. We are afraid of and attracted to things – it's similar circuitry."

Explore further: Meteorite that doomed dinosaurs remade forests

More information: House PK, Vyas A, Sapolsky R (2011) Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats. PLoS ONE 6(8): e23277. doi:10.1371/journal.pone.0023277

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User comments : 9

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2020
3 / 5 (3) Aug 17, 2011
Mother Nature is one HELL of a Bioengineer!

word-to-ya-muthas
Isaacsname
5 / 5 (3) Aug 17, 2011
1/3 of people tested. So maybe cats' popularity is really due to brain parasites ? I wonder how many other popular pets might also carry similar parasites.
Pete1983
5 / 5 (1) Aug 18, 2011
Actually the crazy part is that we're finding more and more of these brain affecting parasites all the time, and it seems quite logical to assume that humans can be affected quite regularly by these as well...

Hmmm....

To all non-US residents - (I think this might explain the US)

To US residents - Sorry!
Bob_Kob
not rated yet Aug 18, 2011
Any way to get rid of it?
M_N
1 / 5 (1) Aug 18, 2011
Sounds like a good reason to get rid of cats from society.

Other studies have shown a strong correlation between infection with Toxoplasma and being involved in car accidents - this is a nasty parasite that can really screw with your brain.
Y8Q412VBZP21010
5 / 5 (2) Aug 18, 2011
To all non-US residents - (I think this might explain the US)


There was a study, I recall, that compared human Toxoplasma infections in the UK to those in France, finding French rates much higher, somewhat matching the differential in schizophrenia rates. "Correlation is not causality" but the study was suggestive for further investigation.
Isaacsname
5 / 5 (2) Aug 18, 2011
When I was a child I always thought there was something weird going on with cats. I thought that they were manipulating their owners somehow to provide them with a comfortable lackadaisical existence. Toxoplasmosis seems to trump them both, lol.

I wonder how many irrational decisions in history have been made under the sway of brain parasites..?
that_guy
5 / 5 (1) Aug 18, 2011
but it also changes the way certain subregions of the amygdala respond to cat odor - specifically, by increasing neural activity in the presence of cat odor in regions normally activated by exposure to a female rat.


The funniest image popped into my head. A lovesick rat chasing around a cat like pepe le pew.

I bet there are some crazy situations that happen on any occasions when a cat is not interested in eating or killing the rat.
Vendicar_Decarian
1 / 5 (2) Aug 21, 2011
Common street hookers and Republicans do pretty much the same thing.