A Game of Cat and Mouse… and Human: Toxoplasmosis and how it messes with your mind

Are you a cat lover? If so, you are not alone. Over the eons, cats have worked their way so deeply into human society that they are rivaled only by dogs in terms of their pervasiveness. Even if you’re not a fan, it is virtually certain that you have come into contact with Felis catus at some point in your life. Estimates vary but it is generally accepted that there are between 400 million and 500 million domestic cats on Earth today, ranging from the ball of fur currently curled up on your keyboard to its feral family members that lay claim to Coliseum in Rome. But what if the human affinity for cats isn’t the innocent story of interspecies friendship that it seems? Could there be a more nefarious explanation for our attraction?

It would explain a lot. Aside from keeping your house clear of rodents, cats generally don’t offer much. They don’t even seem especially fond of humans when compared to their goofier fun-loving roommates – dogs. So, why do we love these untrainable, aloof houseguests as much as we do? The answer could be in their poop.

As it turns out, cats are the sole natural host of a microscopic parasite called Toxplasmosis gondii. While the parasite is able to move from cats to infect basically any kind of mammal or bird it comes into contact with, it reproduces only in cat guts. In essence, your cat is a four-legged toxoplasmosis factory and its litter box is a warehouse.

The effects of toxoplasmosis infection in rodents are well-documented and insanely interesting. These parasites may be among the most well-evolved creatures on the planet for the ingenious way they complete their lifecycle. To completely understand what I’m on about, it is helpful to view the world from the point of view of a toxoplasmosis parasite.

Born in a cat’s intestine, things are very good for our infancy. There is plenty of food to eat and we grow up in peace, until one day our host makes a fateful trip into the garden and leaves us behind in a pile of unpleasantness. Left outside, we are washed by the rain into a nearby pond and before we know it the water we now inhabit is slurped up by a thirsty mouse. We can live happily enough inside the mouse for a while, but eventually, there comes a time in our parasitic lives when we want to settle down and have a family. A mouse, ironically, for is toxoplasmites (not a real word) is like the big city; it’s fine for a while, but it’s no place to raise kids.

Now we have a problem. The only place we want to reproduce is in the gut of a cat, presumably because of our fond childhood memories, but how are we supposed to get this damn mouse to deliver us there? Mice are stubbornly known for their desire to avoid cats, as anyone who has ever watched Tom and Jerry knows. Our only recourse is a trip to the mouse’s brain.

This is where the details get fuzzy. There are a number of theories about the various mechanisms toxoplasmosis uses to alter a rodent’s brain, but no one can argue the results. Mice infected with the parasite lose their fear of cat odour. They scurry their little behinds right into the waiting jaws of their natural enemy. Successful in its mission, toxoplasmosis’ homecoming is complete and they can spawn the next generation.

But if toxoplasmosis can attract mice to cats, what effect does it have on humans? Fortunately, the health effects are pretty unnoticeable in most people. You may get a fever or headache or swollen lymph nodes, but unless you have a compromised immune system or are a fetus growing in an infected mother, toxoplasmosis is unlikely to be life threatening. Though, there is some evidence that is messes with our brains.

Researchers at Charles University in Prague conducted a study in 2011 where they asked both infected and uninfected men and women to rate the pleasantness of various samples of animal urine. While it is fair to assume that none of the ratings were off the charts, infectedmen rated the pleasantness of cat urine significantly higher than uninfected men did, while the ratings for other animals were unaffected. Curiously, the findings were the opposite for women, with infected women finding cat urine less pleasant.

In the end, the effects are small and we aren’t lining up to have our cats eat our flesh, but there is evidence of an effect. Given that recent research shows that mice continue to demonstrate the behavioural changes even after they have been cured of infection, it is also likely that the changes in human brains are long-term or permanent. Ultimately, no one seems to mind and the internet will continue to be made up primarily of cat-related content for the foreseeable future. It is at least worth acknowledging that we owe most of our amusement to brain-infecting parasites, though.