Toxoplasma creates specific brain damage in rats and mice that sends them slightly insane
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ADELAIDE: Toxoplasmosis causes specific brain damage in rats and mice and makes them suicidally attracted to cat urine, according to a new study.
The mind-altering tricks of the Toxoplasma parasite, behind the disease, reverse their instinctive fear of cat urine - instead making them attracted to pheromones it contains. The findings may implicate how the parasite plays with human minds too.
Toxoplasma only reproduces in the gut of cats. It is spread when the parasite is passed out in cat faeces and contaminates soil, where rats and mice pick up the infection. The parasite's lifecycle is completed when the mind-altered rats, which have been shown to be less fearful of cats and have slower reaction times, are caught and eaten.
Now, a new study reveals that rats suffering from the affliction have remarkably specific brain damage that affects only their fear of cat urine - and not their instinctive fear of other dangers, their ability to learn to be afraid or their general anxiety.
"One would … assume that if something messes with the fear of cat pee, it will also mess up a variety of related behaviors," said lead author and neurobiologist Ajai Vyas at Stanford University in California. "We do not see that - Toxoplasma affects fear of cat odours with almost surgical precision."
Vyas and his team publish their findings this week in the U.S. journal Proceedings of the National Academy of Sciences.
Previously studies have implicated the effect of chemicals including the neurotransmitter dopamine, involved in the brain's reward system, and alterations of the secretion of the stress hormone corticosterone.
Vyas and his team conducted experiments which showed that other similar brain pathways involved in fear, such as rats' anxiety in open spaces or avoidance of novel food by mice, were not affected by Toxoplasma - suggesting that the mechanism is highly specific.
The parasite acts by forming cysts in the rodents' brains, and the researchers found the cysts are more abundant in the amygdala region than in other areas. This is important because the amygdala is involved in a variety of fear-related behaviors, said Vyas. "Both the location of these cysts and what is being secreted from these cysts could be important."
The researchers suggest the "extreme amount of specificity" Toxoplasma employs when altering rodent brains is down to the fact that it makes it more likely that cat will eat it and helps to guarantee the parasite's transmission and survival.
"It is a very nice study that clearly demonstrates the specificity of the effect [on rat] behaviour," commented parasitologist Jaroslav Flegr at Charles University in Prague, The Czech Republic. It would be intriguing to test the rat's reaction to other predators that are not Toxoplasma hosts, such as weasels or foxes, he said.
"Toxoplasma is known to influence the personality and behavior of infected men and women too," added Flegr.
Around 15 to 20 per cent of Americans are infected with the parasite, according to a study by the U.S. Centres for Disease Control and Infection (CDC). Many experience no symptoms and would not be aware of the infection, but toxoplasmosis has been linked to an increased risk of miscarriage if a woman becomes infected during pregnancy.
Research also suggests a mind-altering effect in people – one study found that sufferers have an increased tendency to be involved in car accidents due to slow reaction times, another has linked schizophrenia to the disease.
Other parasites are also known to effect the brains of their hosts, such as the parasitic Nematomorph hairworm that causes grasshoppers and crickets to plunge into water and die, allowing the mature hairworms to escape and reproduce in the water.
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