The spread of bird flu from Asia to eastern Europe and now west Africa has increased the chance that the virus will mutate and cause a possible pandemic among humans, said UN bird flu chief David Nabarro.
Nabarro said on Friday there is no evidence yet of any change in the bird flu virus.
"Unfortunately, we cannot tell when the mutation might happen, or where it might happen, or how unpleasant the mutant virus will turn out to be," Nabarro said in an interview.
"Nevertheless, we must remain on high alert for the possibility of sustained human-to-human virus transmission and of a pandemic starting at any time."
Nabarro said the arrival of bird flu in Nigeria should be "a strong wake-up call" to countries to ensure that their veterinary services are on alert, and that health services quickly identify unexpected clusters of disease that could represent the start of a pandemic.
"We have got bird flu now in southeast Asia, central Asia, eastern Europe, and west Africa," he said. "Compared with eight months ago, this is a major extension of the avian influenza epidemic."
Nabarro said control measures have helped to contain the spread but bird flu is still expanding across the world "putting at risk the health of people who are living intimately with poultry."
He said the increase in the quantity of the virus in the world is increasing the overall chance of mutations that could then spread into humans.
``That's why we get so concerned about the spread of the virus, because we want to do everything we can to reduce the opportunity for mutation,'' Nabarro said.
He said one of the urgent needs is to establish how avian influenza reached west Africa.
``The likely means is by migrating wild birds traveling from north to south, and one of the main migratory routes passes from Siberia through the Black Sea area, including Crimea and on to west Africa,'' Nabarro said.
``The alternative is that the virus arrived in birds that are being traded -- and if that is the case, they would have been smuggled, as Nigeria had banned import of birds from avian influenza-affected areas during the last two years,'' he said.
UN experts have just received the genetic sequence of virus samples taken from the farm in Kaduna where the H5N1 strain of bird flu was discovered, he said.
Over the next few days, he said, the World Organization for Animal Health and the UN Food and Agriculture Organization will try to match that sequence with the genetic sequence of viruses from birds in other countries affected by bird flu, he said.
``If it turns out that H5N1 was carried to west Africa by migratory birds, we need to be prepared for the possibility that within the next six months it could be brought back to the northern hemisphere -- but perhaps along a different flyway,'' Nabarro said.
Nabarro said he was delighted that the Nigerian government will pay compensation for birds culled, "but unfortunately that never truly replaces the lost chicken."
"The sadness is that this will directly affect poor people for whom a chicken is a short-term savings account with an excellent rate of interest, and they depend on their birds for getting cash at times of need," he said.
Nabarro also praised the action being taken by Nigeria's Ministry of Agriculture, "which appears to be firm and rapid" but he also expressed concern that the sheer scale of the problem could overwhelm authorities.
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