Toxoplasma was first discovered in sparrows in 1900 and described in humans in 1908. Now, just over one hundred years later, we know that Toxoplasma is one of the most common parasites among mankind. About one in three people around the world are infected. What’s more, Toxoplasma can infect virtually all warm-blooded animals, from canaries to beluga whales. Toxoplasma is truly a super-parasite. But how did it become so successful?

Toxoplasma is a relative of Plasmodium, the parasite that causes malaria. Plasmodium moves from one human to the next by using mosquitoes as a transportation device. Toxoplasma does not use mosquitoes. But, like malaria parasites, it needs two different animals to complete its lifecycle: a cat and its prey. Only in the intestine of cats can Toxoplasma produce offspring by mixing the genes of different parents, similar to how humans produce children. These offspring are called oocysts and are shed in the cat’s feces. Originally, Toxoplasma could only cause new infections when those oocysts were ingested by new hosts.

However, genetic analyses indicate that some one to ten thousand years ago, the parasite evolved the ability to circumvent cats altogether. At this point the parasite became able to infect new hosts from the cysts it forms in the tissues of existing hosts. In this way, birds of prey, scavengers and humans could become infected by eating Toxoplasma-infected prey. Through this evolutionary innovation, Toxoplasma became able to infect vast numbers of animals. Even today, the risk of Toxoplasma infection is higher in countries where the consumption of undercooked meat is common, such as in France and Ethiopia.

Infection with Toxoplasma makes rodents more active, reckless and attracted to cat urine, which may lead to consumption of the rodent and transmission of the parasite to the cat host. In humans, infection with with Toxoplasma slows reactions time and decreases concentration, which could be responsible for the higher rate of traffic accidents of Toxoplasma-infected people. But how do traffic accidents help the parasite? One hypothesis is that the increased risk of traffic accidents is a consequence of behaviors that originally made people more likely to be eaten by predatory cats. Where loss of concentration and increased reaction times now result in traffic accidents, they may have increased cat predation of our ancestors. 

One recent study suggests this hypothesis may have some bearing. Researchers poured urine samples over the fence of chimpanzee enclosures in Gabon. Interestingly, the chimpanzees behaved much like rats: while uninfected chimpanzees avoided leopard urine, infected animals were actually attracted to it. The chimps did not have any notable responses to lion and tiger urine. Then again, in the wild, chimpanzees never encounter lions and tigers, while they do run into leopards. Thus, Toxoplasma might make chimpanzees attracted to their natural cat predators. Given that chimpanzees are our closest living relatives, it is possible that Toxoplasma could also manipulate humans to display this fatal attraction.

However, the chimpanzee study was small, and its results should be replicated before drawing firm conclusions. In addition, the same scientists who found that Toxoplasma increases accidents in people asked a group of infected and uninfected university students to smell urine samples. While infected men rated domestic cat urine as more pleasant than uninfected men, the opposite was true for women. More surprisingly still, infected students had no preference for the urine of tigers, even though these animals are much more likely, of course, to eat humans than domestic cats. Thus, the altered behaviors of infected humans may be side effects of infection that have nothing to do with increasing parasite success. In this case, the parasite would have evolved the ability to manipulate rodent brains to increase transmission to cats. When it happens to infect humans, it accidentally alters human behavior without helping the parasite.

Want to learn more about brain-manipulating parasites? Watch Jaap de Roode’s TED-Ed lesson How brain parasites change their host’s behavior, and Ed Yong’s TED Talk Zombie roaches and other parasite tales.

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