野鴨遷徙 將亞洲禽流感傳播到阿拉斯加

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.

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, said Franson.

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.

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.

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.

"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.

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