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
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

Guru Careers: Software Developer / C# Developer

£40-50K: Guru Careers: We are seeking an experienced Software / C# Developer w...

Guru Careers: Software Developer

£35 - 40k + Benefits: Guru Careers: We are seeking a Software Developer (JavaS...

SThree: Trainee Recruitment Consultant / Resourcer

£18000 - £23000 per annum + Commission: SThree: As a Trainee Recruitment Consu...

Ashdown Group: UI Developer - (UI, HTML, CSS, JavaScript, AngularJS)

£25000 - £40000 per annum: Ashdown Group: UI Developer - (UI, JavaScript, HTML...

Day In a Page

Sun, sex and an anthropological study: One British academic's summer of hell in Magaluf

Sun, sex and an anthropological study

One academic’s summer of hell in Magaluf
From Shakespeare to Rising Damp... to Vicious

Frances de la Tour's 50-year triumph

'Rising Damp' brought De la Tour such recognition that she could be forgiven if she'd never been able to move on. But at 70, she continues to flourish - and to beguile
'That Whitsun, I was late getting away...'

Ian McMillan on the Whitsun Weddings

This weekend is Whitsun, and while the festival may no longer resonate, Larkin's best-loved poem, lives on - along with the train journey at the heart of it
Kathryn Williams explores the works and influences of Sylvia Plath in a new light

Songs from the bell jar

Kathryn Williams explores the works and influences of Sylvia Plath
How one man's day in high heels showed him that Cannes must change its 'no flats' policy

One man's day in high heels

...showed him that Cannes must change its 'flats' policy
Is a quiet crusade to reform executive pay bearing fruit?

Is a quiet crusade to reform executive pay bearing fruit?

Dominic Rossi of Fidelity says his pressure on business to control rewards is working. But why aren’t other fund managers helping?
The King David Hotel gives precious work to Palestinians - unless peace talks are on

King David Hotel: Palestinians not included

The King David is special to Jerusalem. Nick Kochan checked in and discovered it has some special arrangements, too
More people moving from Australia to New Zealand than in the other direction for first time in 24 years

End of the Aussie brain drain

More people moving from Australia to New Zealand than in the other direction for first time in 24 years
Meditation is touted as a cure for mental instability but can it actually be bad for you?

Can meditation be bad for you?

Researching a mass murder, Dr Miguel Farias discovered that, far from bringing inner peace, meditation can leave devotees in pieces
Eurovision 2015: Australians will be cheering on their first-ever entrant this Saturday

Australia's first-ever Eurovision entrant

Australia, a nation of kitsch-worshippers, has always loved the Eurovision Song Contest. Maggie Alderson says it'll fit in fine
Letterman's final Late Show: Laughter, but no tears, as David takes his bow after 33 years

Laughter, but no tears, as Letterman takes his bow after 33 years

Veteran talkshow host steps down to plaudits from four presidents
Ivor Novello Awards 2015: Hozier wins with anti-Catholic song 'Take Me To Church' as John Whittingdale leads praise for Black Sabbath

Hozier's 'blasphemous' song takes Novello award

Singer joins Ed Sheeran and Clean Bandit in celebration of the best in British and Irish music
Tequila gold rush: The spirit has gone from a cheap shot to a multi-billion pound product

Join the tequila gold rush

The spirit has gone from a cheap shot to a multi-billion pound product
12 best statement wallpapers

12 best statement wallpapers

Make an impact and transform a room with a conversation-starting pattern
Paul Scholes column: Does David De Gea really want to leave Manchester United to fight it out for the No 1 spot at Real Madrid?

Paul Scholes column

Does David De Gea really want to leave Manchester United to fight it out for the No 1 spot at Real Madrid?