Parasites who control mind Paula Bronstein/Getty Images

Comment les parasites tirent les ficelles

LIVERPOOL – La science fiction a depuis longtemps envisagé la possibilité terrifiante que nous soyons dépourvus de libre-arbitre et qu’une créature peu sympathique contrôle notre cerveau ou nous transforme en zombies laborieux. Mais il se trouve que le contrôle de la pensée n’est pas qu’un classique littéraire. C’est également une méthode communément utilisée par les parasites pour avoir accès à un environnement où ils peuvent grandir, se reproduire et compléter leur cycle de vie.

Prenons par exemple le champignon Cordyceps : après avoir infecté des fourmis des forêts tropicales, il interfère avec leur comportement en les incitant à grimper au sommet des arbres et à s’accrocher à une feuille avant de mourir. Le champignon se reproduit ensuite en faisant retomber ses spores dans le sous-bois où elles infectent d’autres fourmis. De même, un virus qui infecte les chenilles du bombyx disparate les empêche de redescendre du sommet des arbres selon leur cycle habituel, et elles en meurent. Le virus se multiplie alors et fait pleuvoir des particules virales sur le sol de la forêt.

Ces parasites incitent leurs hôtes à rechercher une élévation qui accroît la dispersion de leurs spores ou particules infectieuses. Mais d’autres espèces sont capables de provoquer des comportements autrement plus complexes. Les nématomorphes, par exemple, sont des vers qui infectent les grillons et les poussent à se suicider en se jetant dans un plan d’eau, que ce soit une flaque ou une piscine. Alors que le grillon se noie, le vers s’en échappe et complète son cycle biologique.

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