It might sound like the plot of a Hollywood film, but there is a parasite that invades the brain of its host and alters their behavior in an attempt to get them killed. The parasite, known as Toxoplasma gondii, is actually much more common than you might think, with over half of humans thought to be infected, though thankfully for most people it is harmless. But a new study, published in Current Biology, shows that when the parasite infects chimpanzees, it makes them attracted to leopard pee, in theory making them more likely to fall prey to the predator.
The host-parasite relationship is intricate and convoluted. While a parasite doesn’t usually want their host to die, for the protozoan Toxoplasma gondii it can actually be advantageous. Indeed, while the parasite needs to spend time in a feline host – be it a domestic moggy or their wild counterparts – in order to sexually reproduce, T. gondii also spends part of its life cycle in an intermediate host, which can be any mammal, from mice to humans.
Leopards are the only natural predator of chimps, and the parasite plays on this. Eduard Kyslynskyy/Shutterstock
Now, when mice are infected with the parasite that then wants to get inside a cat in order to reproduce, it has to somehow bridge the gap between the natural enemies. Like something straight out of a horror movie, the parasite takes over the region of the brain responsible for smell in mice. Usually, the rodents avoid cat urine at all costs, in the fear that they might bump into the animal that did the deed, but when T. gondii takes over, it makes the mice attracted to the pee. The purpose is to increase the chance the mice will get eaten, and thus return the T. gondii back to a cat where it can complete its life cycle.
The parasite, a protozoa, is thought to enter the brain, and hijack the region responsible for smell. Omar Cera/Flickr CC BY-NC-ND 2.0
Incredibly, it turns out that the same process might be going on with Toxoplasma-infected chimpanzees. In the wild, the ape’s only natural predators are leopards. In a series of experiments on 33 captive chimps in the central African country of Gabon, researchers have found that chimps with T. gondii are actually attracted to the urine of these predators, but importantly not to the pee from other big cats or humans. When they ran the tests with uninfected chimps, they found that the apes would actively avoid the urine from the leopards, in a similar way to how mice steer clear of domestic cat pee.
The researchers hope that their study might be able to shed light on why the parasite also seemingly, though controversially, causes behavioral changes in humans, when we could be considered a dead-end host as we no longer have any natural predators. They suggest that these shifts in the way that some infected people act, such as reduced reaction times and altered risk-taking behavior, might be an evolutionary throwback to when we as a species were also hunted by big cats on the African savannah.