London - 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 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 barrier protecting the brain.
Figures released in September by the Food Standards Agency show that 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 percent 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. 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 the parasite's complex life-cycle.
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 Karolinska 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. It means that the parasite had the capacity potentially to manipulate the central nervous system,” Dr Barragan said.
The study, published in the online 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. “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.”
Scientists emphasised it is still not known whether toxoplasma is capable of influencing the behaviour or mental state of infected people given the preliminary nature of the studies. - The Independent