Natural wild bird flus are mild; it's evolutionary

letter I've just had published in the Independent (UK):

Don't blame deadly flu on wild birds

Sir: I am an ornithologist based in Hong Kong, and since 2003 have followed the spread of H5N1 and the purported role of wild birds. I was surprised to read Sir Hugh Pennington's assertion (letter, 19 March) that "the evidence that these viruses [highly pathogenic avian influenzas] evolve in wild birds and are spread by them is overwhelmingly strong," as this contradicts all evidence I am aware of.

Natural wild bird flus are mild. However, in poultry farms these influenzas can evolve to higher virulence: as the Food and Agriculture Organisation notes, "Outbreaks of HPAI originating from low pathogenic viruses carried from wild birds have occurred relatively frequently in domestic poultry in the last decade." An FAO table lists 28 highly pathogenic avian influenza outbreaks from 1959 to 2004. All were first recorded in poultry.

There are sound evolutionary reasons for wild bird flus being mild. As noted by the evolutionary biologist Paul Ewald, flu must be directly transmitted from carriers. With wild birds, it is useless to the virus if birds become very sick or die. But in poultry farms, even very sick or dying birds can transmit virus, leading to virulent forms evolving.

These evolutionary principles continue to apply with H5N1. No bird species has been shown to survive and sustain and spread this virus; instead, the vast majority of wild birds confirmed to have been infected with H5N1 were dead. Here in east Asia - the epicentre of H5N1 - there has been no report of H5N1 in a migratory wild bird since last summer.

DR MARTIN WILLIAMS

HONG KONG

Online at: Letters

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