Agents secrets de l'évolution : le cerveau parasité ?
Dans son dossier de janvier («Mind»), le mensuel Australasian Science revient sur une hypothèse intéressante: les parasites seraient les véritables agents secrets de l'évolution animale. Modifiant les comportements de leurs hôtes, il seraient les formes vivantes les plus ingénieuses et manipulatrices, superstructures des comportements sexuels, de la prise de risque ou encore de l'intelligence… Parmi ces petites bêtes, la toxoplasmose (Toxoplasma gondii), infecte selon ses formes latentes ou actives entre 30 et 60 % de la population humaine. Ce parasite, présent en général dans la viande crue ou les déjections félines, est bien connu pour son effet tératogène sur le foetus. Il peut aussi être mortel chez des personnes immunodéprimées. En dehors de ces deux cas, la toxoplasmose était considérée comme une affection relativement bénigne. Mais depuis quelques années, la neu rotoxicité et les modifications psychologiques et comportementales induites par le parasite sont plus précisément étudiées. Les premières études sur des rats ont ainsi montré que les mâles infectés devenaient plus téméraires, donc plus susceptibles d'être mangés par des chats ou d'autres prédateurs (perpétuant ainsi l'existence du parasite). Chez les mâles humains, une autre étude menée par la même équipe tchèque montre que la toxoplasmose augmenterait de 2.7 fois les risques dêtre impliqué dans un accident de voiture, abaisserait les scores de QI, le niveau d'éducation ainsi que l'attention. Les hommes infectés seraient plus indépendants, antisociaux, suspicieux, jaloux, déprimés... et moins attirés par les femmes que les autres. D'autres études soulignent même une corrélation entre toxoplasmose et schizophrénie. Chez les femmes, les effets semblent un peu inversés : le parasite les rend plus attirantes, ouvertes, amicales et plus enclines à la badinerie sexuelle. En résumé, pour Nick Boulter, chercheur à l'institut de biotechnologie des maladies infectieuses de l'Université de Sidney : «Le parasite transforme les hommes en chats de gouttière et les femmes en chattes en chaleur».
Références:
Flegr J. et al (2002), Increased risk of traffic accidents in subjects with latent toxoplasmosis : a retrospective case-control study, BMC Infect Dis., Jul 2, 2, 11
Flegr J. et al (2003), Decreased level of psychobiological factor novelty seeking and lower intelligence in men latently infected with the protozoan parasite Toxoplasma gondii Dopamine, a missing link between schizophrenia and toxoplasmosis?, Biol Psychol., Jul, 63, 3, 253-68
***
Parasite 'turns women into sex kittens'
· By Jane Bunce
· December 26, 2006
A COMMON parasite can increase a women's attractiveness to the opposite sex but also make men more stupid, an Australian researcher says.
About 40 per cent of the world's population is infected with Toxoplasma gondii, including about eight million Australians.
Human infection generally occurs when people eat raw or undercooked meat that has cysts containing the parasite, or accidentally ingest some of the parasite's eggs excreted by an infected cat.
The parasite is known to be dangerous to pregnant women as it can cause disability or abortion of the unborn child, and can also kill people whose immune systems are weakened.
Until recently it was thought to be an insignificant disease in healthy people, Sydney University of Technology infectious disease researcher Nicky Boulter said, but new research has revealed its mind-altering properties.
"Interestingly, the effect of infection is different between men and women,'' Dr Boulter writes in the latest issue of Australasian Science magazine.
"Infected men have lower IQs, achieve a lower level of education and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more anti-social, suspicious, jealous and morose, and are deemed less attractive to women.
"On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.
"In short, it can make men behave like alley cats and women behave like sex kittens''.
Dr Boulter said the recent Czech Republic research was not conclusive, but was backed up by animal studies that found infection also changes the behaviour of mice.
The mice were more likely to take risks that increased their chance of being eaten by cats, which would allow the parasite to continue its life cycle.
Rodents treated with drugs that killed the parasites reversed their behaviour, Dr Boulter said.
Another study showed people who were infected but not showing symptoms were 2.7 times more likely than uninfected people to be involved in a car accident as a driver or pedestrian, while other research has linked the parasite to higher incidences of schizophrenia.
"The increasing body of evidence connecting Toxoplasma infection with changes in personality and mental state, combined with the extremely high incidence of human infection in both developing and developed countries, warrants increased government funding and research, in particular to find safe and effective treatments or vaccines,'' Dr Boulter said.
Accueil 
