Joanne Webster, professor of parasite epidemiology at Imperial College London, explains that many parasites favor the brain, “because it shelters them from the full fury of the immune system”. But, she says, “it also gives them direct access to the machinery to alter the host’s behavior”.
“There is a single-celled parasite called Toxoplasma gondii, which is found in domestic cats, and is estimated to infect 350,000 people a year in Britain. Its effect on humans became the obsession of Jaroslav Flegr, professor of evolutionary biology at Charles University in Prague, who linked it with disturbed behaviours such as reckless driving and a greater risk of suicide. Rats infected with Toxo, as scientists at Imperial College discovered, actually like the smell of cat urine, instead of being terrified by it. And studies at Stanford University in California have revealed the neural changes that lay behind this transformation.
Toxo – which comes in the form of tiny single-celled cysts – was clustered in two areas of the brain: those controlling fear and pleasure. Pathways that normally responded to the smell of cat urine with alarm had been damped down, while the pleasure hormone dopamine, normally released in response to female rodent urine, was now triggered by the whiff of cat. Most recently, researchers have shown that Toxo’s DNA includes two genes that boost dopamine production. Human brains have plenty of similarities with those of rats and mice, suggesting that the greater number of car crashes among those with a Toxo infection could be due to it damping fear responses.”
The scientists that reported these findings are based out of the UK and they estimate that about 40% of the population in their area are infected with this parasite. The estimate that they give only applies directly to the population in the UK, but with these parasites so abundant in various different parts of the planet, it is likely that this 40% figure is just as relevant many other places world wide.
Another study, which is published in the scientific journal PLoS Pathogens, was led by Dr Barragan and conducted together with researchers at Uppsala University. Dr Barragan explained in an interview afew months ago that this is no new problem for the human species:
“We believe that this knowledge may be important for the further understanding of complex interactions in some major public health issues, that modern science still hasn’t been able to explain fully….At the same time, it’s important to emphasize that humans have lived with this parasite for many millennia, so today’s carriers of Toxoplasma need not be particularly worried.”