Migratory Birds as Scapegoats: The Role of Birds in Spreading Avian Flu
Bennun, Leon, Multinational Monitor
THERE ARE SEVERAL WAYS in which H5N1 can be spread within and between countries. Three major potential routes are the movements of infected poultry (and poultry products), movements of caged wild birds in trade and movements of wild birds. Effective responses need to focus on all of these possible means of spread.
Outbreaks among wild birds in Europe and Iran during 2006 show that wild birds are capable of carrying the virus to new sites after infection, possibly in a "leap-frog" fashion by travelling for a short time and passing on infection to another group of birds before dying. Many questions remain concerning the effects of the virus on wild birds and the efficiency with which they can spread it to other wild birds or to domestic poultry.
By contrast, recent outbreaks in Cameroon, Egypt, India, Israel, Jordan, Niger, Nigeria and Pakistan originated within the poultry industry. Here, as in most other H5N1 outbreaks, there is strong circumstantial evidence that movements of poultry and poultry products are responsible. The timing and location of these outbreaks do not match the movements of migratory birds. Moreover, in many of these countries, poultry outbreaks occurred almost simultaneously in multiple large-scale poultry operations, indicating that migratory birds were an unlikely agent of the transmission.
For Southeast Asia, recent comprehensive analysis of viral lineages concludes that poultry movements were responsible for multiple reintroductions, both within and between countries, and that transmission within poultry is the major mechanism for sustaining H5N1 in the region.
In some parts of the world, authorities have proposed attempting to control the spread of HSN1 by culling wild birds, or destroying their habitats, or displacing them from breeding and roosting grounds. These approaches are unlikely to prevent the transmission of the disease and may in fact spread it to non-infected areas by forcing already-infected birds to disperse. They are at best ineffective, probably counterproductive, and distract from more suitable interventions. They could also add to the stresses already imposed on some species through habitat loss.
THE ROLE OF WILD BIRDS
In 2006, wild bird outbreaks have occurred across Europe, and sporadic new incidents continue to be reported. Nearly all incidents involve just one or a few individual birds (usually less than 10) apart from the wild bird outbreak on Rugen Island (Germany) which killed over 100 birds.
Outbreaks among wild birds in Europe during 2006 show that wild birds are capable of carrying the virus to new sites after infection--at least during the early stages of infection. It is possible that the initial outbreaks in Europe in February related to movements of birds away from the Black and Caspian Sea regions in response to unusually cold weather. These areas are known to have widespread H5N1 infection in poultry and limited biosecurity measures in place.
In contrast to the recent European incidents, the movement of wild birds is not to date the main cause of the spread of H5N1 in Asia over the decade since the virus was first discovered there. Prior to April 2005, wild birds found dead or dying with H5N1 in Asia were largely sedentary species that scavenge near poultry, live markets or captive bird populations.
From May 2005 to July 2005, there were significant die-offs of migratory wild birds in Qinghai Lake (1,500-6,300 birds) and Mongolia (126 birds). The majority of the birds affected in May 2005 in Qinghai Lake were Bar-headed Geese. Deaths from H5N1 in the geese occurred weeks after their arrival from wintering grounds in India, suggesting that the source of the H5N1 infection was local to Lake Qinghai. Furthermore, no Bar-headed Geese or other wild birds were found dead in other wetlands near to Qinghai Lake.
In Mongolia, at Lake Ethel, the main species found dead or dying with H5N1 in July 2005 were Bar-headed Geese and Whooper Swans. …