杏吧原创

BLOODSUCKING flies common all over the world may have begun the HIV epidemic by spreading the virus from chimpanzees to humans. That鈥檚 the latest challenge to the established theory that game hunters became infected when HIV-laden chimpanzee blood got into open wounds.

Most bloodsucking insects pose no risk of passing on HIV. Mosquitoes, for example, inject saliva through one tube and suck up blood through another, so they don鈥檛 pass on blood-borne viruses.

But stable flies, Stomoxys calcitrans, could be an exception. They are known to transmit equine leukaemia virus between horses, and they also bite people. When feeding, they scrape skin to make a wound, suck up blood and regurgitate some on the skin next time they feed. Any viruses in the regurgitated blood can invade the body through the wounds made by the flies.

Gerhard Brandner of the University of Freiburg and Werner Kloft of the University of Bonn proved in 1992 that HIV survives in blood regurgitated by the flies. Unlike the few other bloodsucking insects that regurgitate blood, the stable fly doesn鈥檛 digest the blood it regurgitates. This difference is crucial, says Brandner.

鈥淭he anterior part of the mid-gut where the regurgitate is kept is just for storage and is free of digestive enzymes,鈥� he says. 鈥淭hat鈥檚 our key result, and is a precondition for transmission of HIV.鈥�

At the time this finding was largely overlooked. But it acquired a new significance in 1999 when a team led by Beatrice Hahn of the University of Alabama reported finding HIV infections in sub-Saharan chimps for the first time (Nature, vol 39, p 436). If these chimps were the source of HIV, perhaps the flies transmitted the virus from them to people.

Brandner, Kloft and Manfred Eigen of the Max Planck Institute for Biophysical Chemistry in G枚ttingen have now published their theory. 鈥淎pes are traditionally hunted in Africa and are offered for sale in open-air meat markets,鈥� they say. 鈥淭he bloody carcasses are regularly covered with blood-feeding flies, possibly including the stable fly.鈥� If the flies suck up HIV-infected blood from the chimps, they might transmit it when they feed on humans, say the investigators.

They speculate that sporadic cases of HIV transmission via stable flies may have occurred for years but gone unrecognised. If these rare infections still happen at all, they now pale into insignificance alongside the explosive spread of HIV through sex between people.

But Hahn, who discovered the crucial HIV-like virus called SIVcpz in Pan troglodytes troglodytes chimps in 1999, doesn鈥檛 think the German claim should be taken seriously until further research backs it up. 鈥淭here鈥檚 no shortage of hypotheses of how SIVcpz made the jump to humans, and this is just another one,鈥� she says. 鈥淭he task at hand is to find out for sure what happened and back it up by hard evidence.鈥�