22 July 2021

Toxoplasmosis linked to cognitive decline

Clinical Public Health

Infection with the parasite Toxoplasma gondii is associated with cognitive impairment, according to a recent metanalysis published in JAMA Psychiatry.

Researchers said the small but statistically significant effect could have a considerable impact on public health given around 30% of the global population was infected.

In an analysis of 13 studies including more than 13,000 otherwise healthy people, results of neuropsychological tests were compared among those with antibodies to T. gondii and those without.

Those without T. gondii seropositivity performed better in four areas: processing speed, working memory, short-term verbal memory and executive functioning.

Previous studies had shown an association between infection and neuropsychiatric disorders including schizophrenia. But it had been unclear whether – and to what extent – seropositivity was associated with changes in cognitive performance.

The authors of this current study said public health programs to prevent T. gondii infection were now warranted, “based on the present findings and those of previous meta-analyses examining the association of T. gondii seropositivity with motor vehicle crashes, suicide attempts, and the prevalence of psychiatric disorders”.

“Hygienic measures are often already undertaken to prevent other infectious diseases,” they wrote. “However, these measures are not sufficient to prevent quiescent T. gondii infection.”

The parasite reproduces in the intestines of cats, with infected felines excreting oocysts in their faeces. Humans can become T. gondii hosts if they ingest contaminated food or water.

But parasitologist associate professor Chris Tonkin told The Medical Republic he did not think the results were cause for immediate alarm as neither it nor previous studies could show cause or effect.

“Are people with these disorders more likely to be infected because they have behavioural abnormalities? That’s largely where the problem lies,” said Professor Tonkin, who leads a laboratory at the Walter and Eliza Hall Institute in Melbourne studying how toxoplasmosis persists in the brain.

He said this study and previous work suggested toxoplasmosis may be a disease modifier when it came to the mind and brain.

“This is one of many studies, and it’s saying similar things to what other studies have suggested: that there are small, but measurable impacts of this parasite. But I think it’s important to see it in the context of our environment really playing a role in cognitive health. And pathogens, and our microbiome, all play a role in this.”

Ophthalmologist Professor Justine Smith of Flinders University in Adelaide told TMR that although the new findings lacked a cause-effect link, enough was known about its other health effects to call for increased public awareness.

“Toxoplasma is an extremely successful parasite, often called the most successful in the world,” said Professor Smith, an internationally recognised expert in uveitis with an interest in ocular toxoplasmosis. 

“It infects many of us, and actually causes disease most often in healthy adults – the most common condition is retinal inflammation and scarring.”

In elderly patients and unborn children, infection and resulting disease was often more severe.

Preventive measures were particularly important given there was not an approved vaccine against infection and once infected, a person carries the parasite for life, she added.

“So I don’t need any new information to strongly recommend public health measures including: cooking meat to an internal temperature of 66°C; washing utensils that have touched raw meat; washing or peeling fresh fruit and vegetables; using gloves and washing hands after gardening or handling the cat litter – and not handling it at all if pregnant – and feeding cats cooked meat and preventing access to prey.”

JAMA Psychiatry 2021, 14 July

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