WHO: Changes in H5N1 virus may promote spread in birds

Feb 21, 2006 (CIDRAP News) – The World Health Organization (WHO) says H5N1 avian influenza has infected birds in 14 more countries since the beginning of this month, and recent genetic changes in the virus may have something to do with its rapid spread in birds.

The agency said countries that have reported their first cases of H5N1 infection in birds this month, in chronological order, are Iraq, Nigeria, Azerbaijan, Bulgaria, Greece, Italy, Slovenia, Iran, Austria, Germany, Egypt, India, France, and Hungary. Authorities in Hungary confirmed today that three dead swans were infected with the virus.

In a separate statement yesterday, the WHO said, "Some recent evolutionary changes in the H5N1 virus appear to have made control efforts more difficult and further international spread of the virus in birds more likely."

Among other things, the agency said H5N1 viruses now may be able to infect some wild birds without harming them, making it possible for migratory species to carry the virus for long distances. In addition, H5N1 viruses have grown tougher and more lethal to laboratory chickens.

Outbreak situations vary
The outbreak situations in the 14 countries have varied nearly as widely as their geography.

Most of the affected European countries have good veterinary surveillance and have found the virus only in a few wild birds, with no evidence of spread to domestic birds, the WHO said. At the other end of the spectrum, Iraq's bird outbreak was identified only after a fatal human infection was found. In countries such as Azerbaijan and Egypt, die-offs of domestic poultry heralded the spread of the virus. Nigeria and India's bellwether cases were found on commercial farms.

Other than Iraq, which has had two human cases, none of the newly affected countries has reported any human cases. However, the WHO said in another statement today that 15 patients with possible signs of bird flu are under observation in hospitals in the area where India's outbreak occurred. The patients are being tested for the virus as well.

Qinghai Lake virus persists
In reporting on the H5N1 virus's evolution, the WHO said viruses from recent avian outbreaks have shown "remarkable similarity" to those found in migratory birds that died at China's Qinghai Lake wildlife sanctuary starting in April 2005. "Evidence is mounting that this event, which resulted in the deaths of more than 6,000 wild birds, signaled an important change in the way the virus interacts with its natural reservoir host," the agency said.

Before the Qinghai Lake die-off, the virus caused only a few scattered deaths among migratory waterfowl, and the latter were not known to carry the pathogen long distances, the agency said.

Viruses from Qinghai Lake had a distinctive point mutation that has been linked experimentally with greater mortality in birds and mice, the WHO said. Viruses from the recent outbreaks in Nigeria, Iraq, and Turkey, as well as from earlier outbreaks in Russia, Kazakhstan, and Mongolia, are "virtually identical to Qinghai Lake viruses."

"It is considered unusual for an avian influenza virus causing outbreaks in birds to remain this genetically stable over so many months," the statement continues. "This finding raises the possibility that the virus – in its highly pathogenic form – has now adapted to at least some species of migratory waterfowl and is co-existing with these birds in evolutionary equilibrium, causing no apparent harm, and travelling with these birds along their migratory routes.

"If further research verifies this hypothesis, re-introduction of the virus or spread to new geographical areas can be anticipated when migratory birds begin returning to their breeding areas."

Are migratory birds spreading the virus?
The WHO stopped well short of assigning to migratory birds the major blame for the virus's recent spread, a notion that has been controversial. David Halvorson, DVM, a veterinarian in avian health at the University of Minnesota in S. Paul, said today that H5N1 is probably being spread both by the movement of poultry and by the movement of wild birds, but no one is absolutely certain.

"The fact is we don't really know why it's being found in so many places so suddenly," he told CIDRAP News.

Halvorson suggested that trains may play a role in the spread of H5N1, as they have in past outbreaks. In the United States in 1925, "People were shipping poultry to New York live bird markets. Then dirty, contaminated crates were being shipped back." This contributed to the spread of a highly pathogenic avian influenza (HPAI) outbreak.

"I think that the trans-Asian railway system fits the temporal and spatial pattern of virus distribution starting in July of last summer," Halvorson commented. "For us in the Western Hemisphere, it would be extremely unusual for water birds to be migrating thousands of miles in July and August, a time when they are ordinarily taking care of their young."

Even today it is normal to ship chickens by rail in many places. Birds also can be found on buses and trucks under circumstances that could contribute to spreading the virus, Halvorson said.

Most of the bird cases found in Western Europe this month have been in wild swans, which has raised puzzling questions. Halvorson listed some in an e-mail message: "Are swans an indicator species? In other words are they detecting (and dying from) AI [avian influenza] in their environment? Or are they spreading it around? Are they flying north or south? Are they both an indicator species and also spreading it around?"

Jean Hars, a French veterinary epidemiologist quoted in an Agence France-Presse (AFP) report yesterday, said the mute swan, the hardest-hit species in western Europe, is not migratory.

Hars said some whooper swans, a migratory species, on a German island in the Baltic Sea were infected, but how they were exposed is a mystery because they stay in far northerly regions where no H5N1 outbreaks have been reported.

Human cases still rare
In reporting on the evolution of the virus, the WHO said the recent changes have not had any noticeable effect on the disease in humans. "Human infections remain a rare event," the agency said. "The virus does not spread easily from birds to humans or readily from person to person.

As reported here yesterday, the agency said its investigation of human cases in Turkey has yielded no evidence that viral mutations have changed the epidemiology of the disease in humans.

In today's statement, the WHO expressed concern about the virus becoming established in backyard flocks, a known risk factor human for H5N1 infections.

Halvorson echoed the concern.

"Clearly, it's in wild birds, whether they're picking it up from poultry or spreading it themselves," he said. "We have to separate our poultry from wild birds. It's essential. Some places, it's going to be easy. In some places it will be virtually impossible."

See also:

Feb 21 WHO statement on situation in India

Feb 21 WHO statement on the spread of avian flu to new countries

Feb 20 WHO statement on mutations in H5N1 virus

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