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Many cats unwittingly transmit Toxoplasma to their ownersImage: Bilderbox

Cat parasite

March 28, 2012

A Czech biologist is trying to show how Toxoplasma gondii, a parasite that normally affects only cats and rodents, also affects adult human behavior. He's now trying to prove a damaging effect on intelligence.


Since its discovery over a century ago, scientists and doctors have known for many years that the parasite Toxoplasma gondii, which is transmitted through cat excrement, causes cognitive problems in mice.

Research has shown that in cases where the immune system is weakened - such as in fetuses or those with immune deficiencies - the parasite can lead to birth defects, swollen lymph nodes or brain damage. However, approximately half of the world's population has already been infected from exposure to cats, usually to little effect.

In some European countries, such as France and Belgium, Toxoplasma screening for pregnant women is routine - but in the United Kingdom and the United States, the medical establishment does not recommend the practice.

However, for years, Jaroslav Flegr, a professor of biology at Charles University in Prague, has argued that this cat parasite can cause an array of cognitive and behavioral problems, including schizophrenia, in normal adults. He has catalogued the effects of the parasite Toxoplasma gondii through tests on thousands of students since 1992.

The proposition that a strange parasite reaps destructive changes in human behavior is a tough pill to swallow. So Flegr is now working to strengthen his case.

Having documented subtle personality changes he blames on the parasite, the professor is moving on to IQ. He predicts that in Toxoplasma-infected people, intelligence deteriorates more quickly than in non-infected subjects. This year, Flegr is continuing a study on Toxoplasma's effects on 300 students' cognitive ability.

"At first I was surprised because I expected much weaker effects,” he told DW. “Later on I got used to this idea, and I think it's not so surprising. For example, alcohol also changes behavior very strongly, and other drugs, too. Why not the parasite?"

Schlagworte: Jarovslav Flegr, toxoplasma Wer hat das Bild gemacht/Fotograf?: Shant Shahrigian Wann wurde das Bild gemacht?: 2012 Wo wurde das Bild aufgenommen?: Prag Bildbeschreibung: Jarovslav Flegr is a professor of biology at Charles University In welchem Zusammenhang soll das Bild/sollen die Bilder verwendet werden?: Artikelbild / Master Quer Artikel über Jarovslav Flegr - Es handelt sich um ein durch einen Verlag, ein Unternehmen oder eine Institution bereitgestelltes Bild (außer eine Bild-Agentur, mit der die DW einen Rahmenvertrag abgeschlossen hat): I authorize DW to publish these photos.
Jarovslav Flegr has been studying Toxoplasma for 20 yearsImage: Shant Shahrigian

The mouse connection

Studies by Flegr and a handful of others show Toxoplasma's apparent effects range from schizophrenia to personality changes. But there have been other noteworthy Toxoplasma-related studies in recent months.

A study from Canada published last year showed an increase in rates of brain cancer in countries that tend to be affected by the parasite. But not all of its effects have been shown to be negative. One study published last week from South Korea found that Toxoplasma in mice appears to reduce the effects of the mice equivalent of Alzheimer's disease.

This month, in the UK, the National Institute for Health Research is beginning work on a massive, 2.2 million-euro ($2.9 million) study on the effects of minocycline. That's an anti-acne antibiotic drug which may be able to alleviate schizophrenia, and kills the toxoplasma parasite.

Either way, for those of us that have the parasite inside us, there simply hasn't been enough work done on humans to know exactly what it does. Flegr has found infected men to be more suspicious and dogmatic, while infected women tend to show stronger extraversion.

To understand why the parasite is apparently targeting the human brain, it's helpful to look back at the mice and see similarities in infected subjects from both species. While Toxoplasma-infected humans can show a variety of symptoms, one that people share with infected mice is slowed reaction times.

"Toxoplasma is not able to recognize it is in human brain, not [in a] mouse's brain. So it does what evolution taught it to do,” Flegr said.

Flegr submitted that Toxoplasma thrives best in cats, and the parasite just uses mice as intermediaries. The parasite slows down mice reaction times -- and can even reduce their aversion to a cat's smell -- so the mice will make for easier prey.

Säugetiere/Katzen Norwegische Waldkatze Felis domestic cat Katzenklo Junges rot-weiß in Katzentoilette Schlagworte Säugetiere/Katzen, Säugetiere/Katzen, Norwegische, Waldkatze, Felis, domestic, cat, Katzenklo, Junges, rot-weiß, in, Katzentoilette
Many women are advised to stay away from cat litterboxes during pregnancyImage: J-L Klein & M-L Hubert/OKAPIA

Controversial results

In one of his most controversial results, Flegr found that Toxoplasma-infected people are more likely to get into traffic accidents than uninfected drivers. He concluded that the chances of an accident go up by as much as 16 percent in infected Czechs with Rh-type blood.

Flegr argued that could be because Toxoplasma wants to shorten its carriers' lifespans so the parasite can further spread.

But not everyone is convinced of Flegr's findings, saying that much more work needs to be done before concluding that Toxoplasma definitively affects adult human behavior.

"I would go to different parts of the world where people do not have cats, but where schizophrenia is found, which is a pretty worldwide phenomenon,” said Wulf Schiefenhoevel, a human behavior scientist at the Max Planck Institute in Andechs, in southern Germany. “That would be my very first thing to do. I would do that cross-culturally."

Author: Shant Shahrigian, Prague
Editor: Cyrus Farivar

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