Health and Medicine

How parasites and bacteria could be changing the way you think and feel

The ConversationGiven recent events around the world, you could be forgiven for thinking that people have been acting in a very odd and unpredictable manner. There has been much research across psychology and economics to explain why we behave the way we do and to explore what our motivations may be. But what if there are other unseen influences at play? As science uncovers more about the influence of parasites and bacteria on human behaviour, we may well begin to see how they also shape our societies.

Mind control is a very real and prevalent threat to humans. We already know it is used by many organisms throughout the animal kingdom and how essential it is for the transmission and reproduction of many diverse parasitic species. The Cordyceps fungus, for example, infects ants before making them travel to the top of the tree canopy where they die. The fungus then reproduces and its offspring float down to the forest floor to infect more ants.

Nematomorph worms, meanwhile, make their cricket hosts commit suicide by jumping into water and drowning in order to get back to where they normally live. And parasitic trematodes infect snails so that their eyestalks bulge and change colour to red, blue and yellow. The next host, a bird, sees a juicy maggot and pecks off the eyestalks so the trematode can complete its lifecycle in the bird’s gut.

Little critter. Shutterstock

These horror stories are not restricted to invertebrates – and humans are not immune. When we learned how to farm and select strains of crops that grew best in certain environments, we sometimes made a surplus that could be stored for the future. This brought wild mice and rats and with them cats and a hidden danger: the protozoan parasite, Toxoplasma gondii.

This parasite can’t complete its lifecycle in humans, but we can be infected by it through coming into contact with cat faeces (or eating uncooked meat). The percentage of people estimated to be infected worldwide is between 30 and 40%. France has an infection level of a staggering 81%, Japan 7%, and the US 20%.

Just a friendly cat. Shutterstock

T. gondii does strange things to rats and mice to make sure they come into contact with cats. They lose their inhibition of cats and cat urine. They become more exploratory and spend more time in daylight. But even stranger things happen when humans inadvertently come into contact with T. gondii. Men are more likely to be in car crashes due to riskier behaviour. They also are more aggressive and more jealous.

Women, meanwhile, are more likely to commit suicide. It has even been suggested that T. gondii could potentially be involved in dementia, bipolar disorder, obsessive-compulsive disorder and autism. There is even evidence from more than 40 studies that people suffering from schizophrenia have elevated levels of IgG antibodies against T. gondii.

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