A report in the respected British medical journal Lancet has raised the possibility that an insect might be able to transmit the Aids-causing human immunodeficiency virus (HIV) - a finding likely to cause scientists to re-examine the origin of the current HIV pandemic and how HIV got into the human population in the first place.

The journal Lancet Infectious Diseases quotes and comments on work by German researchers showing that the stable fly (Stomoxys calcitrans) is capable of feeding on HIV-infected blood and regurgitating an infectious virus.

The German researchers are from prestigious institutions: the Universities of Freiburg and Bonn, and the renowned Max Planck Institute for Biophysical Chemistry.

It has been generally held that biting insects are incapable of transmitting HIV because they ingest blood from their victims and inject saliva into their victims via different routes, said Dr Andrew Jamieson, medical director of Netcare Travel Clinics, in a statement.

But the stable fly is different as it uses the first part of its digestive tract to store ingested blood. Regurgitated material from stable flies fed on HIV-containing blood has been shown by two of the researchers, Brandner and Kloft, to contain intact HIV.

It is known that stable flies can transmit other viruses and retroviruses. These findings raise the possibility that stable flies feeding on blood-covered bush meat (a euphemism for chimpanzee meat) may have transmitted the chimpanzee form of HIV, known as SIV or simian immunodeficiency virus, to humans.

Biting a human after feeding on infected bush meat, the stable fly may have given the chimpanzee virus the bridge it needed to cross the species barrier and infect humans. Jamieson said: "The German theory is highly speculative, and it would be premature to blame the stable fly for starting the HIV pandemic."

He added that travellers into Africa could take reassurance from the fact that mosquitoes are unable to transmit HIV - a question frequently directed to Netcare Travel Clinics.

"Even if it transpires that the stable fly can cause HIV in humans, the same would not be true of mosquitoes."