Turns out it’s probably not your cat that’s making you crazy

Proven innocent, for now.
Proven innocent, for now.
Image: AP Photo/Amy Sussman
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Cats often get a bad rap. They’re thought to be aloof and uncaring, and some have even speculated that their presence might alter their owner’s behavior.

Fortunately, researchers have found evidence that this last aspect of their reputation probably has no actual scientific basis.

It all started with a parasite—Toxoplasma gondii, to be precise, which is responsible for an infection called toxoplasmosis. T. gondii can live a number of different mammals (like us) and birds. But cats—both wild and domestic—are the only animals in which T. gondii can produce eggs, which are circulated when they are shed in the cats feces. Studies have shown that mice who become infected with the parasite lose their fear of cats, which suggests that the parasite hijacks the mouse’s brain to make it more likely to be eaten by a cat, all in the name of reproduction.

Naturally, scientists have wondered if T. gondii affects people in a similar way. The most common transmission of the T. gondii parasite in people is through eating contaminated meat, although it’s also possible to get it through accidentally ingesting cat scat if you don’t wash your hands after changing a litter box. Most people infected with the parasite never even know they have it: In healthy individuals, the immune system is able to keep the parasite at bay, or the infection only presents with temporary flu-like symptoms. Some estimates suggest that a third of all people are infected with T. gondii.

But, the parasite has a darker side. In rare cases, toxoplasmosis can cause detrimental birth defects, including epilepsy and blindness in infants whose mothers contract the illness while pregnant. Additionally, in recent years, some have investigated whether toxoplasmosis is responsible for a host of mental health complications, including schizophrenia, manic depression, and even risk-taking behavior (resulting in a number of headlines asserting that “your cat might be making you crazy.”)

New research published in PLoS ONE suggests that this is not actually the case. Researchers from Duke University found that “on the whole, there was little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations, or neurocognitive impairment.”

The study followed over 800 New Zealand men and women over the course of 38 years. About 30% of these individuals tested positive for antibodies used to fight off T. gondii at the end of the trial, suggesting they were infected with the parasite. Researchers collected data about the mental health of these patients at 12 different intervals over the course of their lives, and found that being infected with T. gondii was not significantly related to any cognitive or behavioral problems.

The researchers appear grateful to finally have evidence that might put to rest the theory that there is a correlated association between T. gondii and mental health problems. They cite that the already large number of scientific papers attempting to link the two is at least in part a result of  a “frustrating scientific search for biological causes with large effects in common mental disorders and processes.”

Pregnant women or anyone with a severely compromised immune system should still be wary of a toxoplasmosis infection, and avoid changing litter boxes and eating any possibly contaminated meat. But for the most part, the newest evidence suggests that the likelihood of your cat making you mad through T. gondii is not something you should worry about. As for making them be nicer to you, you’re on your own.