September
6, 2005Parasitic Hairworm Charms Grasshopper Into Taking It for a SwimBy NICHOLAS WADEOnly in science fiction do people's minds get possessed by alien beings. For grasshoppers, zombification is an everyday hazard, and it obliges them to end their lives in a bizarre manner. Biologists have discovered and hope to decipher a deadly cross talk between the genomes of a grasshopper and a parasitic worm that infects it. The interaction occurs as the worm induces the grasshopper to seek out a large body of water and then leap into it. The parasite, known as a hairworm, lives and breeds in fresh water. But it spends the early part of its life cycle eating away the innards of the grasshoppers and crickets it infects. When it is fully grown, it faces a difficult problem, that of returning to water. So it has evolved a clever way of influencing its host to deliver just one further service - the stricken grasshopper looks for water and dives in. The suicidal behavior of the infected grasshoppers has been studied by a team of biologists from the French National Center for Scientific Research in Montpellier, France, led by Frédéric Thomas and David Biron. They did their fieldwork around a swimming pool on the border of a forest near Avène les Bains in southern France. Hordes of infected grasshoppers - more than 100 a night - arrive at the pool during summer nights at the behest of the parasites. The biologists captured grasshoppers before their suicidal plunge and removed the worms. The worms grow to several times the length of the grasshopper's body before they emerge. Because of their unusual size, it is easy to extract and analyze the different sets of proteins that they produce before, during and after they compel their hosts to drown themselves. "We found the parasite produces and injects proteins into the brain of its host," Dr. Thomas said. Two of the proteins belonged to a well-known family of signaling agents known as the Wnt family that are deployed in developing the cells of the nervous system. Though produced by the worm, the two proteins seemed similar to insect-type proteins and perhaps developed so as to mimic them, the French biologists report in an article in the current issue of Proceedings of the Royal Society. Parasites have long been known to influence their hosts' behavior in ways beneficial to the parasite. The rabies virus, for instance, makes animals rabid so that they bite others and transmit the virus. An unusually specific instance of behavioral manipulation was discovered recently in a wasp that parasitizes an orb-weaving spider in Costa Rica. The night before the wasp larva kills its host, it somehow reprograms the spider's web-building activity so that instead of its usual temporary web, the spider constructs a durable platform ideal for the larva to pupate on. Somehow the larva reprograms the spider into executing, over and over again, just the first two steps in a five-step subroutine from the early phase of web-building. If the larva is removed just before it can kill its host, the orb weaver will spin a platform-style web that and the following night, but revert to its usual web on the third night, as if it has shaken off some mesmerizing chemical the wasp has injected into its nervous system. The hairworm seems to have perfected an equally intimate manipulation of its host by inducing a fantastical desire to swim, of which the grasshopper is scarcely more capable than the worm is of flying. This is not the parasite's only trick. No one knows how, from its aquatic home, the hairworm manages to infect a terrestrial species. Dr. Thomas said he suspects that the larvae, minuscule on hatching, first infect aquatic insects like mosquito larvae and hide as cysts in their tissues. When the adult mosquito flies away and when it dies, its body may be eaten by a grasshopper or cricket. The hairworm "will then develop, eating absolutely everything not essential to keep its host alive," Dr. Thomas said. The zombified grasshopper is reduced to just its head, legs and outer skeleton by the time it goes for its final swim. There are some 300 species of hairworm found around the world. Their billions of larvae "will infect everything - frogs, fish, snails," Dr. Thomas said. But it is only in grasshoppers, crickets and katydids that these uninvited guests are able to usurp both the body and mind of their hosts. |