Why are rats afraid of cat urine

A protozoan takes the fear away from rats, and not just from cats

Does toxoplasmosis change character in humans too? At least it doesn't seem to specifically encourage feline love.

A protozoan called Toxoplasma gondii lives in an estimated 30 percent of people worldwide. But for him they are only intermediate hosts, his main hosts are cats. It does them little harm, they are usually even immune to it. In contrast to rats and other rodents, whose behavior he influences in creepy ways: they lose their shyness of cats and the loathing of the smell of cat urine. This increases your risk of becoming infected with it and being eaten by the cats. Which naturally helps the unicellular organism to spread.

Most researchers believe that toxoplasmosis is usually symptom-free in humans. But there are studies that see a connection with mental illness. The US psychiatrist E. Fuller Torrey even connects the cat mania of the Parisian and London bohemians of the late 19th century with the diagnoses of schizophrenia that were common then and there. Other studies claim that people infected with Toxoplasma are more risk-taking and more trusting. This is what the Prague researcher Jaroslav Flegr, himself a cat lover and Toxoplasma-positive, believes: “The parasite cannot know that it is in our brain and not in that of a rat,” he says.

The animals become more curious

But how can a protozoan find its way around the - but highly complicated - brain of a mammal so well that it prefers to attack precisely those centers that are responsible for the aversion to cats or their urine? A publication in Cell Reports (January 14th) could contribute to demystification: Biologists working with Dominique Soldati-Favre (University of Geneva) report that rats infected with Toxoplasma display no specific “fatal feline attraction” at all. They are generally more curious and spend more time in a freely accessible maze. They show less fear, for example of the hands of the experimenters; they run over the body of an anesthetized conspecific, which uninfected rats do not. And while they are less afraid of cat urine, they also avoid the smells of foxes and guinea pigs less.

These findings fit well with the examination of the rats after their death. The cysts filled with the pathogen, which are typical of toxoplasmosis, are widely scattered in the brains of the infected rats - and above all the distribution pattern differs from rat to rat, which clearly speaks against a targeted infestation of certain brain regions. The decisive factor for the symptoms is probably less a direct interaction of the unicellular organism with the brain cells than the chronic inflammation with which the immune system reacts to the infestation.

In any case, Soldati-Favre warns against an unreflective transfer of his results to people. They generally show fewer symptoms of Toxoplasma infection than rodents, he says: "We hope people understand that they don't get Crazy Cat Lady Syndrome when they are infected with Toxoplasma."

("Die Presse", print edition, January 15, 2020)