Cat parasite toxoplasma uses 'Trojan horse' to infect human brain and may cause suicidal thoughts and risk-taking

Toxoplasma able to pass from the gut into the central nervous system, says report

A food-borne parasite that infects domestic cats can get inside the human brain by commandeering special cells of the immune system which it uses as a Trojan horse to enter the central nervous system, a study has found.

Scientists believe they have finally discovered the mechanism that allows Toxoplasma gondii – a single-celled parasite – to pass from the human gut to the brain where it may cause behavioural changes.

Researchers have shown that the parasite can infect the dendritic white blood cells of the immune system causing them to secrete a chemical neurotransmitter that allows the infected cells, and the parasite, to cross the natural barrier protecting the brain.

Toxoplasma gondii can live in many different species but it can only complete its life cycle in cats, which secrete the parasite in their faeces. Studies have shown that toxoplasma affects the behaviour of rats and mice, making them more likely to be eaten by cats, thereby completing parasite’s complex life-cycle.

Latest figures released in September by the Food Standards Agency show about 1,000 people a day in Britain – 350,000 a year – are being infected with toxoplasma, probably from either direct contact with cats or by eating poorly-cooked meat or vegetables.

Up to 40 per cent of the British population are believed to be infected with toxoplasma and although the vast majority show no apparent symptoms, there is a risk to unborn children if their mothers become infected for the first time during pregnancy.

However, recent studies have also suggested that toxoplasma may be a trigger for psychological disturbances in humans, including schizophrenia, although the research has fallen well short of showing a cause-and-effect.

Antonio Barragan of Sweden’s Centre for Infectious Diseases at the Karolinksa Institute in Stockholm said that when infected with toxoplasma human dendritic cells, which are not part of the central nervous system, begin to secrete a neurotransmitter called GABA which is normally produced by brain cells.

“For toxoplasma to make cells in the immune defence to secrete GABA was as surprising as it was unexpected…This was unknown before. It means that the parasite had the capacity potentially to manipulate the central nervous system,” Dr Barragan said.

The study, published in the on-line journal Plos Pathogens, used human dendritic cells growing in a test tube, but it also showed that infected dendritic cells pass more easily than uninfected cells into the brains of laboratory mice.

“We’ve shown that it happens in human dendritic cells taken from healthy donors and also proved that the same thing happens in the mouse model. It shows that the parasite is using dendritic cells as a sort of Trojan horse to transport itself from the human gut to the brain,” Dr Barragan said.

“We’ve not looked at behaviour changes in people infected with toxoplasma, as that’s been dealt with by previous studies. Instead, we’ve shown for the first time how the parasite behaves in the body of its host, by which I mean how it enters the brain and manipulates the host by taking over the brain’s neurotransmitters,” he said.

GABA, or gamma aminobutyric acid, is involved, among other things, in inhibiting the sense of fear and anxiety. Rats and mice infected with toxoplasma show little fear of cats and Dr Barragan suggested that infected dendritic cells may continue to stimulate the production of GABA once the cells have entered the brain.

However, other scientists have shown that toxoplasma is capable of producing another nerve substance called L-dopa which is a chemical precursor to the dopamine neurotransmitter, which may be another route to altering mammalian behaviour.

“Many neuropsychiatric disorders implicate a dysregulation of several neurotransmitters. If one is affected, this may affect the others, or the balance between neurotransmitters. How GABA specifically acts in the equation is a question for the future,” Dr Barragan said.

Scientists emphasised that the jury is still out on whether toxoplasma is capable of influencing the behaviour or mental state of infected people given the preliminary nature of the studies showing a tentative link between the parasite and human behaviour.

“We believe that this knowledge may be important for the further understanding of complex interactions in some major public health issues that modern science still hasn’t been able to explain fully,” Dr Barragan said.

“At the same time, it’s important to emphasise that humans have lived with this parasite for many millennia, so today’s carriers of toxoplasma need not be particularly worried,” he said.

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Cleaner

£15000 - £16500 per annum: Recruitment Genius: If you've got first class custo...

Recruitment Genius: Mobile Applications Developer / Architect - iOS and Android

Negotiable: Recruitment Genius: This is a great opportunity to join a medium s...

Recruitment Genius: Telesales Account Executive - £40K OTE

£11830 - £40000 per annum: Recruitment Genius: Working in a friendly, sales ta...

Recruitment Genius: Web Designer

£15000 - £27000 per annum: Recruitment Genius: One of the UK's leading web des...

Day In a Page

Greece debt crisis: What happened to democracy when it’s a case of 'Vote Yes or else'?

'The economic collapse has happened. What is at risk now is democracy...'

If it doesn’t work in Europe, how is it supposed to work in India or the Middle East, asks Robert Fisk
The science of swearing: What lies behind the use of four-letter words?

The science of swearing

What lies behind the use of four-letter words?
The Real Stories of Migrant Britain: Clive fled from Zimbabwe - now it won't have him back

The Real Stories of Migrant Britain

Clive fled from Zimbabwe - now it won’t have him back
Africa on the menu: Three foodie friends want to popularise dishes from the continent

Africa on the menu

Three foodie friends want to popularise dishes from the hot new continent
Donna Karan is stepping down after 30 years - so who will fill the DKNY creator's boots?

Who will fill Donna Karan's boots?

The designer is stepping down as Chief Designer of DKNY after 30 years. Alexander Fury looks back at the career of 'America's Chanel'
10 best statement lightbulbs

10 best statement lightbulbs

Dare to bare with some out-of-the-ordinary illumination
Wimbledon 2015: Heather Watson - 'I had Serena's poster on my wall – now I'm playing her'

Heather Watson: 'I had Serena's poster on my wall – now I'm playing her'

Briton pumped up for dream meeting with world No 1
Wimbledon 2015: Nick Bollettieri - It's time for big John Isner to produce the goods to go with his thumping serve

Nick Bollettieri's Wimbledon Files

It's time for big John Isner to produce the goods to go with his thumping serve
Dustin Brown: Who is the tennis player who knocked Rafael Nadal out of Wimbeldon 2015?

Dustin Brown

Who is the German player that knocked Nadal out of Wimbeldon 2015?
Ashes 2015: Damien Martyn - 'England are fired up again, just like in 2005...'

Damien Martyn: 'England are fired up again, just like in 2005...'

Australian veteran of that Ashes series, believes the hosts' may become unstoppable if they win the first Test
Tour de France 2015: Twins Simon and Adam Yates have a mountain to climb during Tour of duty

Twins have a mountain to climb during Tour of duty

Yates brothers will target the steepest sections in bid to win a stage in France
John Palmer: 'Goldfinger' of British crime was murdered, say police

Murder of the Brink’s-MAT mastermind

'Goldfinger' of British crime's life ended in a blaze of bullets, say police
Forget little green men - aliens will look like humans, says Cambridge University evolution expert

Forget little green men

Leading evolutionary biologist says aliens will look like humans
The Real Stories of Migrant Britain: An Algerian scientist adjusts to life working in a kebab shop

The Real Stories of Migrant Britain

An Algerian scientist struggles to adjust to her new life working in a Scottish kebab shop
Bodyworlds museum: Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy

Dying dream of Doctor Death

Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy