Toxoplasmosis test could lead to learning how parasite affects behaviour

An international research team has developed an online test that they say is capable of detecting toxoplasmosis infection, commonly spread to humans by cats.

While the test is not designed to replace a medical diagnosis, the researchers hope it will open the door for more large-scale studies into the effects of toxoplasmosis on the population. 

An estimated 30 to 50 per cent of the world's population is infected with Toxoplasma gondii, a single-celled parasite capable of infecting humans and any warm-blooded animal or bird.

In Australia, the main host of the parasite is cats, which typically become infected by eating small mammals such as rodents or birds.

The disease associated with the parasite, toxoplasmosis, can cause a range of health problems, from minor flu-like symptoms to, in rare cases, inflammation of the brain, seizures, and blindness. 

Toxoplasmosis can alter behaviour in animals, and some research suggests it may also affect how humans think. 

A new international study published in Acta Psychologica may provide a new way to study these effects. 

Hijacking neurochemistry 

According to neuroeconomist Michele Garagnani, from The University of Melbourne and study co-author, toxoplasmosis affects our neurochemistry by influencing neurotransmitters such as dopamine, serotonin, and adrenaline, which regulate behaviour. 

Unlike many parasitic infections, being infected with T. gondi has been linked to a range of mental health conditions and behavioural changes, including bipolar disorder, delayed reaction times, and schizophrenia. 

The infection has been found to make mice impatient and willing to take risks. 

In some cases, mice actively seek out cats, which makes them more likely to be eaten. 

While it is still unknown how the parasite causes these changes, it raises important questions about the broader impacts of toxoplasmosis on human health and behaviour. 

"When we estimate that toxoplasmosis affects 2.4 billion people worldwide, that's a very large portion of the global economy who are having their decisions at least partially influenced by a parasite," Dr Garagnani said. 

A new way of detection

Much about how toxoplasmosis affects human behaviour remains unknown. 

One major obstacle is that medical testing for the parasite is expensive and invasive, requiring blood or saliva samples.

"Our research was trying to develop a method that is very cheap and easy to test on a large number of people," Dr Garagnani said.

In the study, researchers ran a laboratory experiment with 79 participants with RhD-negative blood lacking the RhD protein on the surface of red blood cells, asking them to complete a reaction-time task.

People with the RhD-negative blood type, about 20 per cent of Australians, have slower response times when infected by the parasite. 

While it is unclear why RhD-negative individuals have slower reaction times when infected, some theorise that the RhD antigen has protective capabilities against the parasite, and in its absence, patients experience elevated symptoms. 

Despite the small sample size, the team found that their method matched medical testing with 97 per cent accuracy.

The delay in reaction time is small, between 0.16 and 0.25 seconds, but Dr Garagnani said even marginal effects could have large-scale consequences.

"On a societal scale, having toxoplasmosis infection with that particular blood type leads to an increase in traffic accidents, for example," he said. 

The team then replicated the experiment using an online survey with more than 1,000 participants in the UK who had RhD-negative blood.

Based on the results, the research team deemed 18 per cent of the respondents to be infected with toxoplasmosis.

The team then asked participants a series of questions about their risk preferences, employment status, income, and completed other cognitive tests. 

"Those who were deemed infected were more likely to smoke, drink alcohol, and experience mental health problems such as anxiety, stress and depression compared to the non-infected participants," Dr Garagnani said.

More to be done

While the new online test does not replace a medical diagnosis, Dr Garagnani hoped it would allow researchers to measure the impact of toxoplasmosis on humans.

"Our new method is an opportunity for the world as a whole to understand how many of us are infected, how it affects our behaviour, and what steps we can take to manage its impact," he said. 

Veterinary parasitologist Tharaka Liyanage said the study posed an innovative approach to exploring the potential behavioural consequences of an infection. 

"I find it fascinating that a microscopic cat parasite could influence not only human health, but also cognition, behaviour, and even economic decision-making," he said. 

"This tool may be particularly useful in low-resource settings or as a preliminary step in large-scale epidemiological studies."

But, Dr Liyanage cautioned that a lot of work needed to be done before the tool made its way into a clinical setting. 

"I believe further validation, particularly in consultation with medical and clinical experts, would be necessary before this method can be considered for broader application." 

The test also requires people to have RhD-negative blood, which only 15 to 20 per cent of people have.

Rima McLeod, an internationally recognised toxoplasmosis expert from the University of Chicago, echoed this sentiment and called for further research. 

"Proper, accurate serologic controls documenting toxoplasma infection or its absence for each participant would be needed to implicate toxoplasma in the associations they propose," Professor McLeod said. 

Dr Garagnani hoped to conduct further tests on the diagnostic tool. 

"We are hoping that with future study we verify further that the method works, and whether it could be used as a public policy type of intervention where we could firstly test for toxoplasmosis using our method and then try to understand how to effectively target the population where the prevalence is more diffuse."