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, The Independent reported.
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.
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," Barragan said.
The study, published in the 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 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.
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.