Wild Birds: Vectors or Victims of Avian Flu?
JOHANNESBURG Do the wild birds that fly through cold winter skies to warmer lands silently carry deadly bird flu around the world? Or are they simply potential victims?
With the virulent H5N1 form of bird flu now killing people in Turkey, there is a growing debate about how it is spread.
Many scientists believe migrating wild fowl are responsible for carrying the virus from Asia and Siberia to Romania and Turkey. And although some argue there is not enough evidence yet for firm conclusions, the theory is gaining ground.
"Scientists are increasingly convinced that at least some migratory waterfowl are now carrying the H5N1 virus in its highly pathogenic form, sometimes over long distances, and introducing the virus to poultry flocks in areas that lie along their migratory routes," the World Health Organization said in its latest bird flu fact sheet last week.
It said scientists found that viruses from the most recently affected countries, all of which lie along migratory routes, were almost identical to viruses recovered from dead migratory birds at Qinghai Lake in China.
The viruses from Turkey's first human cases were also virtually identical to the Qinghai Lake strain, it added.
In Romania, the outbreak was first detected in and around the remote Danube Delta, Europe's largest wetlands which also happen to lie on a major migratory route for wild birds.
"We do know that avian influenza viruses are carried by migratory birds all over the world. But not all of them are highly pathogenic or H5N1," Juan Lubroth, the senior officer for infectious diseases with the United Nations Food and Agriculture Organization (FAO), told Reuters.
"I think that wild birds may introduce the virus but it is through man and man's marketing systems (the poultry trade) that the disease spreads. It is also possible that poultry can transmit the virus to wildlife when they share the same ecosystem," Lubroth said.
MORE QUESTIONS THAN ANSWERS
The H5N1 virus has killed at least 80 people since late 2003, mostly in Asia. Victims contract the virus through close contact with sick birds, but there are fears it could mutate into a form that can pass easily from person to person -- making the question of how it is spread across regions so important.
"Should this new role of migratory birds be scientifically confirmed, it will mark a change in a long-standing stable relationship between the H5N1 virus and its natural wild-bird reservoir," WHO said.
The FAO said this month that the virus could spread to Africa and Europe during the northern spring migration.
"The avian influenza virus could become entrenched in the Black Sea, Caucasus and Near East regions through trade ... and it could be further spread by migratory birds particularly coming from Africa in the spring," it said.
The H5N1 strain has not yet been detected in Africa -- not an easy task given already high rates of mortality among the continent's chickens. Tests on dead wild birds from Malawi and Ethiopia have been negative and hundreds of tests of migratory bird droppings in South Africa have found no trace either.
The growing popularity of the migratory bird theory has worried an increasingly vocal group of conservationists who fear unfounded claims could lead to indiscriminate slaughters.
"The pattern of outbreaks between Asia and eastern Europe do not follow any known pathway for migrant birds, which tend to fly on northerly-southerly routes. They don't go east-west," Dr Richard Thomas of BirdLife International told Reuters.
Andre Farrar, an ornithologist with Britain's Royal Society for the Protection of Birds (RSPB), said if migratory birds were spreading H5N1, it would have been spotted elsewhere.
"Go back a stage and start off in southeast Asia. If migration was the primary route you would have expected it in Australasia but it hasn't shown up there," he said.
However, Farrar said it was prudent to keep looking.
"There is clearly a theoretical risk that migrant birds can carry bird flu. There is published work showing that ducks in captivity can survive H5N1 infection and can shed the virus and we'd be foolish to ignore this," he said.
He said the focus on migratory birds detracted from other, more useful policies to fight the virus like public education, biosecurity measures and curbs on the movement of poultry.
Conservationists say tens of thousands of healthy wild migrant birds in infected countries have been tested over the last decade, but not one has had the virus.
Wild birds that have been found to have the H5 virus, such as swans found in Croatia in October, were already dead -- suggesting they were victims rather than vectors.
BirdLife says the poultry trade is a more likely vector.
"South Korea and Japan are two countries to have suffered outbreaks of H5N1 in poultry and wild birds following importation of infected duck meat," it said in a statement.
"Both countries stamped the virus out by culling infected poultry around disease areas, and imposed strict controls on poultry and poultry meat imports. Neither country has suffered a recurrence of the virus despite the influx each autumn of hundreds of thousands of wild migrant birds," it said.
Conservationists are also concerned about reports of wild birds being killed because of fears of avian flu in several countries from Madagascar to Vietnam.