Feeling a wee bit angry? Brain parasite could have made you a cat's paw

It's the brain parasite that's possibly lodged in your head right now. And it could be making you angry.

Toxoplasma gondii is a single-celled parasite that between 25 and 30 per cent of Australians have. Most probably caught it from their cat.

Mice infected with Toxoplasma gondii  are attracted to the scent of cat urine.
Mice infected with Toxoplasma gondii are attracted to the scent of cat urine. Photo: Natalie Boog

And there is mounting evidence that it is fundamentally changing the way infected people behave – making them angrier, more prone to taking risks, and possibly increasing their risk of psychiatric illness.

A study published in the Journal of Clinical Psychiatry on Wednesday is the latest piece to add to the puzzle.

A toxoplasma gondii cyst, shown here in a mouse brain.
A toxoplasma gondii cyst, shown here in a mouse brain. Photo: Wikipedia

A team of researchers from the University of Chicago recruited 358 adults and studied them for a range of psychiatric disorders including intermittent explosive disorder, schizophrenia, bipolar disorder and suicidal behaviour.

They found across all the subjects studied, people with toxoplasma tested higher on scores of anger and aggression.


Further, they found that the group who had intermittent explosive disorder – recurrent, compulsive outbursts of physical or verbal aggression – were more than twice as likely to test positive for toxoplasma exposure.

Doctor Chris Tonkin from the Walter and Eliza Hall Institute for Medical Research is one of Australia's foremost experts on toxoplasmosis.

Dr Tonkin says the Chicago study is just the latest to draw the link between toxo and increased aggressiveness.

One study even indicated people who were involved in car crashes were more likely to test positive to toxoplasma gondii.

"This is a single-celled organism," he says. "Yet it's able to change our very complicated bodies in some way for its own benefit," he says.

The most interesting study in the field is one of the oldest, and essentially launched the entire field of study.

Toxo can infect many warm-blooded animals, but it can only sexually reproduce within cats.

To achieve this, the parasite has been shown to be capable of rewiring the brains of mice.

Infected mice are attracted to the scent of cat urine, and are more impulsive and exploratory – all to make them better prey for cats, bringing the toxo back inside the feline where it can produce.

The changes to behaviour in people caused by the parasite are subtle. With 25 to 30 per cent of the population infected, the overall change is likely to be much more significant.

How does it change behaviour? That's the subject of a lot of Dr Tonkin's research.

At this stage there are two good candidates: neurotransmitters or immune response.

Several studies point to the ability of the parasite to tamper with fundamental brain chemistry, manipulating the levels of key neurotransmitters within the brain – which may change behaviour.

The other possibility, Dr Tonkin says, is that the parasite could be triggering an immune response in the brain, leading to localised inflammation, which has been proven to have an impact on mental health.