Eliot Barford in Nature:
A parasite that infects up to one-third of people around the world may have the ability to permanently alter a specific brain function in mice, according to a study published in PLoS ONE today1. Toxoplasma gondii is known to remove rodents’ innate fear of cats. The new research shows that even months after infection, when parasites are no longer detectable, the effect remains. This raises the possibility that the microbe causes a permanent structural change in the brain. The microbe is a single-celled pathogen that infects most types of mammal and bird, causing a disease called toxoplasmosis. But its effects on rodents are unique; most flee cat odour, but infected ones are mildly attracted to it. This is thought to be an evolutionary adaptation to help the parasite complete its life cycle: Toxoplasma can sexually reproduce only in the cat gut, and for it to get there, the pathogen's rodent host must be eaten.
In humans, studies have linked Toxoplasma infection with behavioural changes and schizophrenia. One work found an increased risk of traffic accidents in people infected with the parasite2; another found changes in responses to cat odour3. People with schizophrenia are more likely than the general population to have been infected with Toxoplasma, and medications used to treat schizophrenia may work in part by inhibiting the pathogen's replication. Schizophrenia is thought to involve excess activity of the neurotransmitter dopamine in the brain. This has bolstered one possible explanation for Toxoplasma’s behavioural effect: the parasite establishes persistent infections by means of microscopic cysts that grow slowly in brain cells. It can increase those cells’ production of dopamine, which could significantly alter their function. Most other suggested mechanisms also rely on the presence of cysts.