AVIAN FLU is being incubated by “Trojan ducks” that do not fall ill with the virus but transmit it to other species, including human beings.

An analysis of H5N1 influenza in ducks in Asia has disclosed that since 2002 the strain has become much less dangerous to the birds, allowing them to become a reservoir for the disease.

The findings, from a team led by Robert Webster, of St Jude Children’s Research Hospital in Memphis, Tennessee, suggest that ducks could help to trigger a global pandemic.

Dr Webster said: “These results suggest that the duck has become the ‘Trojan horse’ of Asian H5N1 influenza viruses. The ducks that are unaffected by infection continue to circulate these viruses, presenting a pandemic threat.”

The deadly H5N1 strain of the virus has been circulating in South-East Asia since 2002 and is confirmed to have killed 38 people in Vietnam, 12 in Thailand and 4 in Cambodia. At least 97 people have been infected. Few cases of human-to-human transmission have been suspected — one of the key factors in a pandemic — but scientists are so alarmed that the World Health Organisation has declared it one of the gravest threats to global health.

In the study, published today in the Proceedings of the National Academy of Sciences, Dr Webster’s team investigated how the H5N1 virus has evolved in ducks. When it emerged in 2002 it killed many of them but it has evolved to a point when it no longer makes them ill.

Dr Webster’s team inoculated ducks with H5N1 strains isolated between 1997 and 2004 from human beings or poultry. The older strains killed the ducks or made them seriously ill but the more recent ones did not. Even so, the ducks shed the virus for between 11 and 17 days — a much longer period than the ducks that received pre-2002 strains. While the newer strains were not dangerous to ducks, they still caused disease in chickens.

The longer shedding period increases the potential for human beings to be exposed to the virus. The more people are exposed to the virus, the greater the chance that it will evolve the capacity to move from person to person.

The team said that the mechanism helped to explain the origin of new H5N1 viruses. “There is a real possibility that if these H5N1 viruses continue to circulate, further human infection will occur, increasing the potential for human-to-human transmission.”

• Oseltamivir, a drug used against influenza that is marketed as Tamiflu, could help to contain avian flu, researchers at St Jude Children’s Research Hospital reported in the Journal of Infectious Diseases.

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