Avian flu experts fear unidentified species moving H5N1 virus around Eurasia

Helen Branswell
Canadian Press

Tuesday, October 18, 2005

(CP) – The vast new geographical expansion of the dangerous H5N1 virus has avian influenza experts worried a bird version of the Stealth Bomber may be at play.

And they readily admit that finding the asymptomatic culprit or culprits from among thousands of species of birds may be a Herculean challenge.

“If this is a . . . virus that seems to have fixed itself in some species and we don’t know which species it is – but maybe it’s not showing any clinical sign in this particular species – how do we find this guy?” Michael Perdue, avian influenza expert with the World Health Organization, asks with evident anxiety in his voice.

The realization that some mystery migratory birds are actually spreading the Asian virus suggests future unwanted appearances in Europe cannot be ruled out.

But experts insist wild waterfowl are unlikely to bring the Asian H5N1 virus to North America. Genetic analysis of avian influenza viruses shows little mingling between those that circulate in North and South America and those found in Eurasia, they say.

“Basically the viruses that are circulating in the migratory birds in the flyways that are associated with Asia, that’s a separate and distinct group of viruses from what would traditionally be found in North and South America,” says Dr. Jim Clark, acting director of the animal health and production program at the Canadian Food Inspection Agency.

An expert who has been collecting avian influenza viruses from birds in the wild for decades agrees.

“The Asian and Euroasian lineage of viruses – is different from the Western Hemisphere lineage. So our viruses aren’t crossing back and forth that readily,” says Dr. Richard Slemons of Ohio State University.

Wild bird surveillance, once the domain of a small cadre of nearly self-funded diehards like Slemons, has taken on a whole new level of importance with the discovery of the nasty Asian H5N1 avian influenza strain in Romania, Turkey and possibly Greece.

That discovery has turned their science on its ear.

The researchers who study the bird part of the avian influenza equation have long insisted migratory birds were victims, not vectors, in the spread of avian flu.

True, avian influenza outbreaks in poultry are initially ignited when a virus passes from a wild waterfowl to a domestic bird. But they argue that once that fire is lit, people – through commerce, smuggling or incompetence – fuelled its spread.

It seemed they had science on their side. In wild birds that carry them, avian viruses live in the gut, causing no illness. Once they pass to poultry, they adapt to become a virus that causes respiratory infection, as it does in humans.

Until H5N1 reared its very ugly head, it was thought these adapted viruses didn’t really pass back into wild water birds.

That’s been shown to be untrue. Wild bird die-offs involving the H5N1 virus have been documented in a number of species, including whooper swans, bar-headed geese and black-and brown-headed gulls and ruddy shelducks.

Many of those still rare events occurred near domestic poultry, leaving the possibility the wild birds were victims of their domestic cousins, not the other way around.

That argument sprang a leak when the Asian H5N1 virus was found to have killed an array of wild birds in Mongolia – far from any infected domestic poultry flocks – last June. When the same strain was found thousands of kilometres away, in Romania and Turkey, the wild bird defenders admitted defeat.

“I mean, 2,000 mile jumps are a little hard to explain – across mountain ranges and huge seas,” Perdue admits.

But dead birds don’t fly, the bird experts insist. And no one believes a bird sick with H5N1 influenza could make the arduous trek from Southeast Asia to Europe either.

So they are now considering the likelihood that some other wild bird they haven’t identified is carrying the virus from place to place. The wild bird die-offs, they think, occur when the carrier birds come in contact with other wild birds that are susceptible to the virus.

“One thing we know about highly pathogenic viruses” – this H5N1 is a high path virus – “is they’re not highly pathogenic to all species,” says Dr. David Halvorson, a veterinarian who specializes in avian health at the University of Minnesota.

“So . . . the birds that are dying may not be responsible for transmitting it from one place to another. They might be victims of some other cousin bird that’s transmitting it that’s less affected by it.”

And it may not be just one species of carrier birds.

“It could be several different bird species that can be infected and possibly fly far distances and shed virus and transmit the virus,” says Dr. David Swayne, director of the U.S. Department of Agriculture’s Southeast Poultry Research Laboratory in Athens, Ga.

“It’s just a matter that no one has found that species yet. The only thing they’ve found has been the dead birds when an outbreak has been found in wild birds.”

Complicating the matter is the fact that even among birds that carry these viruses, only a portion of a species will be infected at any one time. A negative test wouldn’t rule out the species, only that individual bird at that specific time.

Swayne suggests if the culprit or culprits are discovered, it could be a chance finding.

“It may be kind of difficult,” he admits.