Boffins here in Blighty say that a brain parasite which is carried by up to 20 per cent of the population is capable of affecting its host's actions for its own benefit – but against the interests of the host.

The parasite in question, Toxoplasma gondii, has now been found to "directly affect" the production of dopamine, a key chemical messenger in the human brain. Dopamine levels are implicated in human illnesses such as attention deficit hyperactivity disorder, Parkinson's disease and schizophrenia.

Toxoplasma gondii's primary host organism is cats, but it can also live in other creatures including humans and rats. Humans are infected with it by eating unwashed vegetables with cat poo on them: it's estimated that between 10 and 20 per cent of Brits carry the protozoan parasite and perhaps 22 per cent of Americans. Normally the hidden brain invader appears to have little effect on a human host, though it can kill in certain cases – for instance in the case of someone whose immune system isn't working.

It's been known for some time that T gondii has a more serious effect on rats and mice, in which they lose their fear of cats or even become attracted to them. This is obviously bad news for the host, as it tends to get killed and eaten, but good news for T gondii as it gets to infect another cat where it can reproduce.

Now, scientists working in Britain and the USA have shown for the first time that T gondii can affect production of dopamine in the brain of a rat host – offering a clue as to how it can manipulate rats' behaviour, and perhaps giving insights into problems of the human brain also. According to a statement issued by Leeds uni announcing the research:

Dopamine is a natural chemical which relays messages in the brain controlling aspects of movement, cognition and behaviour. It helps control the brain's reward and pleasure centres and regulates emotional responses such as fear. The presence of a certain kind of dopamine receptor is also associated with sensation-seeking, whereas dopamine deficiency in humans results in Parkinson's disease.

"Based on these analyses, it was clear that T gondii can orchestrate a significant increase in dopamine production in neural cells," says Dr Glenn McConkey of the Leeds' Faculty of Biological Sciences.

"Humans are accidental hosts to T gondii and the parasite could end up anywhere in the brain, so human symptoms of toxoplasmosis infection may depend on where parasite ends up. This may explain the observed statistical link between incidences of schizophrenia and toxoplasmosis infection."

The full research paper can be read here, published by the journal PLoS ONE. ®