We humans like to think that we are in charge of our lives, that our actions are the consequences of what we choose to do. But are we actually always responsible for our deeds?
Recent findings indicate that it might be very common for parasites to influence the behaviour of their hosts. The influence can be of a subtle nature, like parasitic wasp behaviour influenced by a certain type of virus, or of prominent influence, which is the case for so-called zombie ants infected with Cordyceps fungi.
Toxoplasma gondii is a parasite with a life cycle which oscillates between cats (as definitive hosts) and other warm-blooded animals (usually rodents) acting as intermediate hosts. In humans, T. gondii causes the disease toxoplasmosis – it is estimated that over two billion people are hosts, yet only immunocompromised patients (e.g. HIV/AIDS sufferers) develop symptoms.
Rodents have an inborn fear of feline odour, yet when infected with the parasite they appear to be attracted to the scent. This is due to T. gondii cysts located in a certain part of brain, associated with fear. This process both helps cats catch their prey, and gives the parasite a route of entry back into their preferred feline hosts, where they can replicate. Similarly to rodents, humans too appear to find cat’s odour more pleasant. Additionally, individuals harbouring T. gondii tend to react more slowly than unaffected ones. Toxoplasmosis in humans has been linked to many car or work related accidents where quick response has failed. Apart from obvious physical symptoms, certain changes in personality like risk-taking behaviours in men or carelessness in women have been observed.
Need an idea for your next zombie blockbuster? Probably the best idea would be to take an example from Mother Nature. In the Play Station 3 game “The Last of Us”, 60% of the world population is wiped out by a mysterious fungus that transforms humans into mindless creatures with vivid fruiting bodies on their faces. These deadly fungi belong to the genus Cordyceps.
In nature, spores of the Cordyceps fungus penetrate the body of an ant and hijack its nervous system. The ant is then forced by the fungi to climb plant stems and reach leaves a certain height above the forest floor. The zombie ants then clamp their jaws down on a leaf (known as a bite of death) and remain in this position until they die. The leftover body is now used for nourishment of the fruiting body, which emerges in a horrid fashion through the ant’s head. The fruiting body is filled with new spores, which after a short time burst out, covering a large area and making it probable that other ants will be infected.
Another example of ‘mind controlling abilities’ is uncovered when analysing the relationship between parasitic wasps, viruses and fruit fly larvae. The wasp lays its eggs in fruit fly larvae, but as only one wasp can emerge from a singular larva, it is very unlikely that another parasitic wasps will lay their eggs in already parasitized larvae. However, this is not the case for female wasps harbouring a certain virus. The behaviour of laying eggs in already parasitized larvae is referred to as superparasitism and is induced by the LbFV virus. Although it is unfavourable for wasps, as the probability of the offspring emergence is reduced, it is favourable for the virus as it has the opportunity to spread to new wasp hosts.
Should we be worried about a possible The Last of Us scenario? Is it possible that we might lose control of our bodies and become mindless zombies? Thankfully, not at the moment. The human brain is far more complex than any of the Cordyceps hosts and trying to control it is doomed to fail. Although this scenario is improbable, other examples like Toxoplasma gondii prove that our mind can be easily tricked and that we might not control our minds to extent that we would like to.