A parasite spread by cats may be manipulating half of the world’s population’s brains, causing them to take more risks and behave more aggressively.
A new review article brought together multiple studies to highlight how important it is to consider the mental, not just physical, impacts of parasitic infection.
The review, published in Frontiers in Psychiatry, focused on the most common parasitic infection found in humans, Toxoplasma gondii.
The parasite can cause the brain to create more of the ‘feel-good’ hormone dopamine, which rewards people for doing what will help them survive and thrive, such as sleeping, eating, social interaction and sex.
Having more dopamine might sound like a good thing, but it actually means the brain rewards behaviour that it shouldn’t, leading to poor impulse control, increased aggression and even riskier sexual behaviour.
“The significance of these findings for society is considerable,” the study states. “The possible behavioural changes caused by a widespread parasitic infection such as T. gondii underscore that such infections can pose not only medical but also psychological and social challenges.
“In particular, the potential for parasitic infections to exacerbate aggressive and impulsive behaviour raises important questions for understanding and preventing violence and behavioural disorders.”
It’s thought that somewhere between 30 and 60 per cent of humans worldwide are infected with T. gondii (though rates in the US are much lower – around 10 per cent).
The infection largely goes undetected, as most adults with healthy immune systems exhibit no obvious physical symptoms, though it causes host animals to release dopamine as part of its complex life cycle.
The parasite’s primary hosts are cats, as they reproduce inside the feline digestive system, creating an egg-like structure known as oocysts.
When the cat poops, it releases these oocysts into the wild, allowing them to spread into the soil, water and plants. These are then eaten by other animals, including cats that prey on such as birds and rodents.
As the parasite aims to end up back inside a cat’s digestive tract, T. gondii has evolved to interfere with these prey species’ brain chemistry, including reducing their fear response. This means they are less likely to run away from a cat and more likely to end up inside its stomach.
Humans are mostly infected with T. gondii through the handling of cat litter and the consumption of untreated food. Fortunately, there are several easy ways to protect yourself from infection:
- Wash hands, knives and chopping boards thoroughly after handling raw food, particularly meat
- Make sure meat is properly cooked through, especially lamb, pork and venison
- Wash fruit and vegetables thoroughly under running water
- Avoid unpasteurised milk and related products, particularly goat milk
- Wear gloves while gardening
- Take care when emptying cat litter trays, always washing hands afterwards
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