You like cats because they’re beautiful, elegant creatures, right? Or is it because you’ve been infected by a parasite that influences your brain?
Bizarrely, new research raises that question, finding that the parasite Toxoplasma gondii, which lives in cats, actually makes cats attractive — rather than scary — to their natural prey (in this case, rats). Earlier research also suggests that this parasite, which can infect humans, may affect personality and possibly even the risk of schizophrenia.
The one-cell critter T. gondii is probably better known as a major cause of food-borne illness (toxoplasmosis). However, the parasite doesn’t usually cause obvious disease in healthy people: some 10% of Americans carry the bug, but few have symptoms because their immune systems keep it at bay. Still, certain groups are vulnerable; if pregnant women are infected, it can cause birth defects, and the parasite can be dangerous to people with AIDS and other diseases that compromise the immune system.
While it’s possible to pick up the bug by handling dirty cat litter, T. gondii is more commonly transmitted to humans by contact with raw or undercooked meat, particularly pork.
T. gondii has a complicated life cycle. It can live in almost any mammal, but it reproduces sexually only in cats. Consequently, being able to manipulate cats’ prey — i.e., rats — so that they are more likely to be eaten is in the parasite’s genetic interest.
The new research explored rats’ response to cat urine. For obvious reasons, rats are typically terrified of cat pee, and exposure to it activates rats’ brain regions that process fear. But researchers led by Stanford’s Patrick House found that when rats with T. gondii infection were exposed to the urine, they showed activity in brain areas associated with sexual arousal instead.
This occurred only in response to urine from cats, not from other predators; the exposure also did not affect other types of fears.
By making a normally threatening scent sexy, T. gondii creates a fatal attraction in its host. So how does it perform this wicked feat? By going directly to the brain. Prior research suggests that T. gondii raises levels of dopamine in mouse brains by 15%; using drugs to block dopamine in turn eliminates infected animals’ attraction to cat urine.
(Scarily, there are a number of parasites in nature that produce behavior that is detrimental, even deadly, to the host, while helping the parasite survive. If you want to know the sickening but fascinating details, Discover’s Carl Zimmer, author of Parasite Rex, is the best journalist on this beat.)
T. gondii may have behavioral effects in humans too, though the research is preliminary. A recent study in Mexico found that people with schizophrenia, a disorder that is associated with elevated dopamine activity in the brain, were four times more likely than healthy controls to be infected with the parasite; other recent research has found no effect, however.
More controversially, research has linked T. gondii infection to normal variations in human personality. One large study found that infected women were warmer and friendlier than non-infected women, while infected men were more suspicious and antisocial. Infected people also have a higher risk of being involved in car accidents, according to two studies.
So does T. gondii also cause cat ladies to collect furry felines? No research appears to have been done directly on this subject, though one study did find a connection between the bug and obsessive-compulsive disorder, of which animal hoarding is one form.
As for me, I just like my kitty because she’s pretty.