Credit U.S. Fish and Wildlife Service
One of ongoing debates in the world of avian influenza is over exactly how much of a role migratory birds play in the spread of highly pathogenic H5N1.
Bird enthusiasts tend to point to the poorly managed poultry trade as being the main source of the spread of bird flu while those in the poultry industry are often quick to blame migratory birds.
Over the years we’ve studies that either implicate migratory birds in the spread of the virus, or minimizes their role (see India: The Role Of Migratory Birds In Spreading Bird Flu).
Personally, I see no reason why these positions should be mutually exclusive. As far as I can tell, both are important factors in the spread of the virus.
But the debate goes on, often heatedly.
The recent outbreak of H5N1 in India (see Bangalore: More Poultry Culled Due To H5N1) at a government poultry farm is very near Hesaragatta lake - a man-make reservoir and winter nesting ground for migratory birds that has fallen into disrepair in recent years from a lack of water.
Reportedly, the number of birds that now visit every year is greatly reduced.
Nevertheless, several thousand birds continue to stop over at Hesaragatta lake each winter, and that has prompted local officials to suggest that migratory birds are to blame for the current outbreak.
A position that local ornithologists are taking serious umbrage over.
First a link to the article from DNA (Daily News & Analysis) that attempts to exonerate wild birds as carriers of the H5N1 virus, then I’ll return with some comments of my own.
Published: Thursday, Nov 1, 2012, 15:50 IST
By Aishhwariya Subramanian | Place: Bangalore | Agency: DNA
The hunt for the elusive cause of the avian influenza outbreak at Central Poultry Development Organisation & Training Institute (CPDOTI) on Tuesday saw officials of animal husbandry department blaming migratory birds for carrying the virus. But ornithologists have debunked the theory despite animal husbandry officials stating that the Hesaraghatta Lake close to CPDOTI facility which attracts migratory birds was the culprit for the outbreak.
The arguments being made by the experts interviewed in this article are basically two pronged.
- The first seems to be the old canard that `sick birds don’t fly’
- The second is the reduced number of migratory birds arriving at the reservoir in recent years.
While it’s true that wild birds that acquire the virus, sicken, and die are less likely to transfer the virus across long distances, the `sick birds don’t fly’ stance ignores the fact that in migratory waterfowl, the H5N1 is often an asymptomatic infection.
We’ve looked at asymptomatic waterfowl as reservoirs of the virus a number of times in the past, including:
We’ve also witnessed the geographic spread of new clades of the H5N1 virus in migratory waterfowl, and the apparent importation of H5N1 into Hong Kong, Japan, and South Korea by the arrival of migratory birds in the past.
The second argument, that the number of migratory birds arriving each year at Hesaragatta lake has recently dropped may reduce the odds that birds brought the virus to Bangalore, but it certainly doesn’t eliminate the possibility.
The bottom line - it just takes one infected bird to arrive and introduce the virus to the environment.
None of which tells us how the H5N1 virus made it to a government poultry operation in Bangalore, but it does help put migratory birds as carriers of avian flu into better perspective.
Lest bird lovers cry `fowl’, the role of poultry producers in the spread of avian flu – including, but not limited to: poor biosecurity, overcrowded conditions, covering up reports of illnesses or deaths among birds, and smuggling – are well documented in this blog, and elsewhere.
While migratory birds appear to have a role in introducing avian flu viruses to new regions, poorly run poultry operations obviously help to amplify and spread these viruses further.
The fact that this debate is unresolved nearly a decade after the re-emergence of H5N1 in 2003 underscores the need for better research and surveillance, both in migratory birds and in poultry operations.
Until we can clearly understand the various mechanisms by which this virus is spread, the prospects of halting outbreaks in the future remain slim.