Image caption Mice have a natural aversion to cats, although it wouldn't appear so from this scene

Mice may permanently lose their innate fear of cats after they have been infected with a microscopic parasite, researchers say.

The single-celled Toxoplasma gondii parasite is already known to inhibit their hard-wired fear of cats.

But even after they were cleared of the infection, mice no longer reacted with fear to a bobcat's urine.

The team, writing in the journal Plos One, says the infection could cause a permanent change to their brains.

Wendy Ingram and colleagues at the University of California, Berkeley, measured how mice reacted to a bobcat's urine.

Cats typically mark their territory with urine, which helps mice detect and avoid an area in which predators might lurk.

Those that remained uninfected with Toxoplasma gondii showed an aversion to the urine, whereas those that were infected walked freely around the test area.

"It is remarkable that even after the infection has been largely or completely cleared, a profound behavioural change persists," said Ms Ingram.

"Simply having a transient infection resulting in what is potentially a permanent change in host biology may have huge implications for infectious disease medicine."

In rodents, the infection is usually contracted from eating cat faeces. The parasite then works itself into every organ in the body, especially the brain, where it forms cysts.

Pregnancy complications

The infection can also be spread to humans, with recent estimates suggesting that in the UK 350,000 people a year contract toxoplasmosis.

It can create serious complications in pregnancy and adversely affect people with already weakened immune systems. The parasite has also been linked to mood changes in the mental illness schizophrenia.

Ms Ingram told BBC News that the results highlight how current thinking on infectious disease may need to change.

"Typically if you have a bacterial infection, you go to a doctor and take antibiotics and the infection is cleared and you expect all the symptoms to also go away.

"Now we have an example where there is no obvious damage done by the parasite, yet major changes in the neurobiology of the mouse remains after the parasite is gone," she said.

The way forward, she added, would be to look at all the antibodies present in a patient's blood.

"This would show all the parasites and bacteria a person has ever been exposed to and may end up playing a bigger role in explaining illnesses."

Joanne Webster at Imperial College London, who was not involved with the research, said the work was a valuable addition to the scientific literature on this subject.

She said it could have important implications for humans, especially patients with schizophrenia.

"It is very useful to know that these fatal feline attraction behavioural changes do appear to be hard-wired," Prof Webster said.

Poppy Lamberton, also at in Imperial College London, explained that the findings conflicted with hypotheses that certain drugs successfully used to treat schizophrenia, were thought to act in part by reducing T. gondii infection levels.

"If some of the behavioural alterations have already occurred during the early, acute stage of the infection, then the fact that anti-T. gondii drugs may help change these behaviours in chronic infections, leads to many more interesting research questions," she told BBC News.