Voir la version complète : Des fourmis "zombies" contrôlées par des champignons

08/03/2011, 17h37
Quatre nouvelles espèces de champignons s'attaquant à des fourmis ont été découvertes dans la forêt humide du Brésil. Celles-ci infectent leurs victimes, prennent le contrôle de leurs esprits et s'en servent pour disséminer leurs spores.

Des champignons capables de décimer des colonies entières de fourmis. C'est l'impressionnante découverte qu'ont fait des chercheurs au cours d'une expédition dans la forêt de Minas Gerais au sud-est du Brésil. Appartenant à quatre espèces distinctes du genre Ophiocordyceps, ces champignons infectent les fourmis, les transforment en "zombies" et se servent de leurs victimes pour se propager. Un procédé assez redoutable dont les détails sont publiés en ligne dans la revue PLoS One.

Si cette famille de champignon parasite était déjà connue, les quatre nouvelles espèces semblent s'attaquer tout particulièrement aux fourmis charpentières, répandues dans la forêt humide amazonienne. De même, chaque espèce semble s'attaquer à une espèce précise de fourmis charpentières et selon des techniques différentes. "Ce sont des organismes fabuleusement complexes", a expliqué au Guardian le Dr David Hugues, entomologiste de l'Université de Pennsylvanie aux Etats-unis.

Leur cycle de vie est en effet assez impressionnant. Les champignons infectent les fourmis grâce à des spores qui usent d'enzymes pour pénétrer dans le corps de l'insecte avec lequel elles sont en contact et où le champignon peut croître. En libérant des substances chimiques, le parasite prend ensuite le contrôle de l'esprit de la fourmi qui se met à errer et finit par planter ses mandibules dans une feuille avant de mourir. "Le champignon manipule de manière précise les fourmis infectées pour les faire mourir à l’endroit où le parasite veut être, en leur faisant faire un long voyage dans les dernières heures de leur existence", a expliqué le Dr Hugues. Une fois la fourmi morte, le champignon germe pour s'extraire du cadavre et propager de nouveaux spores.

D'autres champignons parasites encore inconnus

Pour l'instant, les chercheurs ne savent pas encore comment le parasite parvient à transformer la fourmi en zombie. Mais ils ont quelques indices sur les substances chimiques libérées pendant le processus qui fascine les chercheurs : "Il y a de la beauté dans la chose, que ce soient les molécules en action qui prennent le contrôle de la fourmi ou que ce soient les spores qui tentent une stratégie puis une autre pour trouver un hôte dans la forêt", a précisé l'entomologiste américain avant d'indiquer qu'il devait exister bien d'autres espèces. Dans les environs de Cairns en Australie, les chercheurs ont en effet découvert six autres sortes de ces champignons parasites.


08/03/2011, 17h39
Half of the world's human population is infected with Toxoplasma, parasites in the body—and the brain. Remember that.
Toxoplasma gondii is a common parasite found in the guts of cats; it sheds eggs that are picked up by rats and other animals that are eaten by cats. Toxoplasma forms cysts in the bodies of the intermediate rat hosts, including in the brain.
Since cats don't want to eat dead, decaying prey, Toxoplasma takes the evolutionarily sound course of being a "good" parasite, leaving the rats perfectly healthy. Or are they?
Oxford scientists discovered that the minds of the infected rats have been subtly altered. In a series of experiments, they demonstrated that healthy rats will prudently avoid areas that have been doused with cat urine. In fact, when scientists test anti-anxiety drugs on rats, they use a whiff of cat urine to induce neurochemical panic.
However, it turns out that Toxoplasma-ridden rats show no such reaction. In fact, some of the infected rats actually seek out the cat urine-marked areas again and again. The parasite alters the mind (and thus the behavior) of the rat for its own benefit.
If the parasite can alter rat behavior, does it have any effect on humans?
Dr. E. Fuller Torrey (Associate Director for Laboratory Research at the Stanley Medical Research Institute) noticed links between Toxoplasma and schizophrenia (http://www.livescience.com/9330-fine-line-revealed-creativity-insanity.html) in human beings, approximately three billion of whom are infected with T. gondii:

Toxoplasma infection is associated with damage to astrocytes, glial cells which surround and support neurons. Schizophrenia is also associated with damage to astrocytes.
Pregnant women with high levels of antibodies to Toxoplasma are more likely to give birth to children who will develop schizophrenia.
Human cells raised in petri dishes, and infected with Toxoplasma, will respond to drugs like haloperidol; the growth of the parasite stops. Haloperidol is an antipsychotic, used to treat schizophrenia.

Dr. Torrey got together with the Oxford scientists, to see if anything could be done about those parasite-controlled rats that were driven to hang around cat urine-soaked corners (waiting for cats). According to a recent press release, haloperidol restores the rat's healthy fear of cat urine. In fact, antipsychotic drugs were as effective as pyrimethamine, a drug that specifically eliminates Toxoplasma. Are parasites like Toxoplasma subtly altering human behavior? As it turns out, science fiction writers have been thinking about whether or not parasites could alter a human being's behavior, or even take control of a person. In his 1951 novel The Puppet Masters (http://www.technovelgy.com/ct/AuthorSpecAlphaList.asp?BkNum=193AuthorSpecAlphaLi st.asp?BkNum=3), Robert Heinlein (http://www.technovelgy.com/ct/AuthorSpecAlphaList.asp?BkNum=193AuthorTotalAlphaL ist.asp?AuNum=2) wrote about alien parasites the size of dinner plates that took control of the minds of their hosts, flooding their brains with neurochemicals. In this excerpt, a volunteer strapped to a chair allows a parasite to be introduced; the parasite rides him, taking over his mind. Under these conditions, it is possible to interview the parasite; however, it refuses to answer until zapped with a cattle prod.
He reached past my shoulders with a rod. I felt a shocking, unbearable pain. The room blacked out as if a switch had been thrown.. I was split apart by it; for the moment I was masterless. The pain left, leaving only its searing memory behind. Before I could speak, or even think coherently for myself, the splitting away had ended and I was again safe in the arms of my master...
The panic that possessed me washed away; I was again filled with an unworried sense of well being...
"What are you?"
"We are the people... We have studied you and we know your ways... We come," I went on, "to bring you peace.. and contentment-and the joy of-of surrender." I hesitated again; "surrender" was not the right word. I struggled with it the way one struggles with a poorly grasped foreign language.
"The joy," I repeated, "-the joy of . . .nirvana." That was it; the word fitted. I felt like a dog being patted for fetching a stick; I wriggled with pleasure.
Still not sure that parasites can manipulate the behavior of host organisms? Consider these other cases:

The lancet fluke Dicrocoelium dendriticum forces its ant host to attach to the tips of grass blades, the easier to be eaten. The fluke needs to get into the gut of a grazing animal to complete its life cycle.
The fluke Euhaplorchis californiensis causes fish to shimmy and jump so wading birds will grab them and eat them, for the same reason.
Hairworms, which live inside grasshoppers, sabotage the grasshopper's central nervous system, forcing them to jump into pools of water, drowning themselves. Hairworms then swim away from their hapless hosts to continue their life cycle.

Not all science-fictional parasites are harmful; read about the Crosswell tapeworm (http://technovelgy.com/ct/content.asp?Bnum=899) from Brian Aldiss' 1969 story Super-Toys Last All Summer Long (http://www.technovelgy.com/ct/AuthorSpecAlphaList.asp?BkNum=193) (the basis for the Kubrick/Spielberg film AI), which keeps people who overeat from becoming obese. Not to mention robots based on parasites (http://www.technovelgy.com/ct/Science-Fiction-News.asp?NewsNum=214). Read press release on evidence for link between Toxoplasma and schizophrenia (http://www.imperial.ac.uk/P7349.htm), Suicidal grasshoppers (http://news.nationalgeographic.com/news/2005/09/0901_050901_wormparasite.html). Story via ******* Carl Zimmer and his readers (http://loom.corante.com/archives/2006/01/17/the_return_of_the_puppet_masters.php).


08/03/2011, 17h49
Les fourmis n'ont pas d'esprit.....

08/03/2011, 17h51
Il y a une nuance entre esprit et âme... Du moins en terme scientifique.