What if blaming yourself for the mistakes you’ve made throughout your life wasn’t necessary? Indeed, what if those wrong decisions were not up to you, but up to a parasite comfortably thriving in your brain, manipulating your nervous system and your emotions? This sounds like it could be taken out of a horror movie, doesn’t it? Well, studies have shown that some parasites can control their hosts, manipulating their behaviour often to the benefit of the parasites. Some parasites inhabit the nervous system, manipulating the decision making processes of their host, whereas other parasites dysregulate the hormonal integrity of the host. All these manipulative techniques, whether they are straightforward or sophisticated, exist for one reason: to ensure the parasite’s best chances of survival and reproduction.
We know lots about the disease burdens of parasites, but much less about how parasites might actively alter their host’s behaviour. Toxoplasma gondii is one of the world’s most prevalent parasites. Inhabiting the brain of its host, mounting evidence suggests that Toxoplasma infections lead to changes in the behaviour of an infected individual. The parasite was first discovered in the early 1900s, and shown to infect, and cause disease in, virtually all warm-blooded animals, from mice to humans. The parasite transmits from these hosts into cats and other felines, where the sexual stages of the parasite life cycle occur. Studies in mice and rats infected with Toxoplasma have shown that they lose their aversion to cat urine. This makes sense for the parasite, since such changes in host behaviour make the parasite more likely to continue through its life cycle. One study showed that even our closest relative, the chimpanzee, loses its disgust in the urine of their only natural predator, the leopard. Although infecting approximately one-third of the world’s human population, Toxoplasma infection is usually asymptomatic in healthy subjects. But can the parasite alter our health in other, more subtle ways? People infected with Toxoplasma are more likely to be involved in car accidents, display abnormal aggressive behaviour, or become more introverted. They are also more likely to develop mental illnesses like schizophrenia. How the parasite might cause these changes in human behaviour is still not clear. Inside the brain of its host, parasites replicate slowly. Some studies claim that parasites secrete neurotransmitter molecules to exert their effects on host behaviour. A more likely explanation is that these parasites cause low levels of neuronal tissue damage in the brain, leading to small changes in a person’s behaviour as the parasite exerts its devious influence on an unsuspecting victim.
- Jeremy Debrulle (ANU Undergraduate student) in collaboration with Professor Giel van Dooren