Migratory Ducks Carry Bird Flu From Asia to Alaska

Wild migratory birds appear to be important carriers of avian influenza viruses from continent to continent, according to new research that scientists say has important implications for highly pathogenic avian influenza virus surveillance in North America.

Migratory bird species, including many waterfowl and shorebirds, that frequently carry low pathogenic avian influenza and migrate between continents may carry Asian strains of the virus along their migratory pathways to North America.

Scientists with the U.S. Geological Survey, in collaboration with the U.S. Fish and Wildlife Service in Alaska and the University of Tokyo, have found genetic evidence that northern pintail ducks carried Asian forms of avian influenza to Alaska.

"Although some previous research has led to speculation that intercontinental transfer of avian influenza viruses from Asia to North America via wild birds is rare, this study challenges that," said Chris Franson, a research wildlife biologist with the USGS National Wildlife Health Center and co-author of the study.

Most previous studies examined bird species that are not transcontinental migrants or were from mid-latitude locales in North America, regions far removed from sources of Asian strains of avian influenza, Franson said.

For this study, scientists with the USGS, in collaboration with the U.S. Fish and Wildlife Service, state agencies, and Alaska native communities, obtained samples from more than 1,400 northern pintails from locations throughout Alaska.

Samples containing viruses were analyzed and compared to virus samples taken from other birds in North America and Eastern Asia where northern pintails are known to winter.

The scientists observed that nearly half of the low pathogenic avian influenza viruses found in wild northern pintail ducks in Alaska contained at least one of eight gene segments that were more closely related to Asian than to North American strains of bird flu.

None of the samples were found to contain completely Asian-origin viruses and none were highly pathogenic.

The low pathogenic form of the disease commonly causes only mild symptoms such as ruffled feathers and a drop in egg production, and may easily go undetected.

The highly pathogenic form is far more dramatic. It spreads very rapidly through poultry flocks, causes disease affecting multiple internal organs, and has a mortality that can approach 100 percent, often within 48 hours.

Under the crowded conditions of intensive poultry farming, some variants of the H5 and H7 viral subtypes derived from wild birds can evolve into highly pathogenic avian influenza viruses.

It was a highly pathogenic form of the H5N1 bird flu virus that spread across Asia to Europe and Africa over the past decade, causing the culling of hundreds of millions of chickens and ducks, and the deaths of 245 people, raising concerns of a possible human pandemic.

Avian influenza viruses do not normally infect humans but there have been instances of certain highly pathogenic strains causing severe respiratory disease in humans. In most cases, the people infected had been in close contact with infected poultry or with objects contaminated by their feces.

Still, there is concern that the virus could mutate to become more easily transmissible between humans, raising the possibility of an influenza pandemic.

In June, the Centers for Disease Control and Prevention in Atlanta released results of a study suggesting that some North American avian influenza A H7 virus strains have properties that might enhance their potential to infect humans and their potential to spread from human to human.

"We know that influenza viruses are constantly changing and that is why it's so important to watch them carefully," explained Dr. Jessica Belser, CDC lead author on the project. "In this study, we discovered that some recently identified avian influenza A H7 viruses have some properties that could enhance their potential to infect people and possibly spread among people."

The role of migratory birds in moving the highly pathogenic virus to other geographic areas has been a subject of disagreement among scientists that focused on how likely it is for H5N1 to disperse across continents via wild birds.

For this study, the researchers chose northern pintails because they are fairly common in North America and Asia, they are frequently infected by low pathogenic avian influenza, and they are known to migrate between North America and Asia.

In addition to the samples from more than 1,400 ducks, the scientists utilized satellite telemetry in their research. In February 2007, biologists from the Alaska Science Center worked with Japanese scientists to mark 27 northern pintail ducks with satellite transmitters at Lakes Izunuma-Uchinuma in the Miyagi Prefecture of Japan. In February 2008, this international research team marked 52 pintails with satellite transmitters. Pintails were again marked at Lake Izunuma-Uchinuma as well as at Gosho Reservoir in the Iwate Prefecture. An additional sample of pintails will be marked in 2009, the final year of the study.

"This kind of genetic analysis - using the low pathogenic strains of avian influenza virus commonly found in wild birds - can answer questions not only about the migratory movements of wild birds, but the degree of virus exchange that takes place between continents, provided the right species and geographic locations are sampled," said John Pearce, a research wildlife biologist with the USGS Alaska Science Center and co-author of the study.

"This research validates our current surveillance sampling process for highly pathogenic avian influenza in Alaska and demonstrates that genetic analysis can be used as an effective tool to further refine surveillance plans across North America", said Pearce.

The study is published this week in the journal "Molecular Ecology."

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Migratory Ducks Carry Bird Flu From Asia to Alaska