Genetics Provides Evidence for the Movement of Avian Influenza Viruses from Asia to North America via Migratory Birds


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Genetics Provides Evidence for the Movement of Avian Influenza Viruses from Asia to North America via Migratory Birds

By Catherine Puckett
December 2008

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Wild migratory birds may be more important carriers of avian influenza viruses from continent to continent than previously thought, according to new scientific research that has important implications for highly pathogenic avian influenza virus surveillance in North America.

As part of a multipronged research effort to understand the role of migratory birds in the transfer of avian influenza viruses between Asia and North America, scientists with the U.S. Geological Survey (USGS), in collaboration with the U.S. Fish and Wildlife Service in Alaska and the University of Tokyo, have found genetic evidence for the movement of Asian forms of avian influenza to Alaska by northern pintail ducks, which spend time both inland and in coastal areas.

Above left: Northern pintails marked with satellite transmitters in February 2008 were distributed as shown in mid-August, when they were nesting and molting in Russia and on St. Lawrence Island, Alaska. One goal of the research project is to estimate the extent to which Asian and North American pintails use the same nesting and molting areas, where they might exchange influenza viruses. [larger version]

Above right: Locations during early November of northern pintails marked with satellite transmitters in February 2008. Red dot in the middle of the Sea of Okhotsk shows the position of a female during her southwestward flight from the nesting area to wintering sites in the Japanese Archipelago. Visit "Movements of Northern Pintail Ducks with Satellite Transmitters" for additional information. [larger version]

In an article published online in October 2008 in the journal Molecular Ecology, USGS 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 avian influenza.

Over the past decade, a highly pathogenic form of the H5N1 avian influenza virus spread across Asia to Europe and Africa, causing the deaths of 245 people and raising concerns of a possible human pandemic. The role of migratory birds in moving the highly pathogenic virus to other geographic areas has been a subject of debate among scientists. Disagreement has focused on how likely it is for H5N1 to disperse among continents via wild birds.

"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 coauthor of the study. Franson added that most of the previous studies examined bird species that are not transcontinental migrants or were from mid-latitude locales in North America, far removed from sources of Asian strains of avian influenza.

Above left: A male northern pintail duck in Japan. Photograph courtesy of USGS. [larger version]

Above right: Dr. Hiroyoshi Higuchi (left), Mr. Ken-ichi Tokita (right), and other cooperators from the University of Tokyo work with USGS scientists to attach satellite transmitters to the backs of northern pintails on wintering areas of northern Honshu, Japan. Transmitters are used to evaluate the ducks' movements, migration, and areas of overlap with North American northern pintails. Photograph courtesy of USGS. [larger version]

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 at sites throughout Alaska. Samples containing viruses were then analyzed and compared with virus samples from other birds in North America and eastern Asia, where northern pintails are known to winter. Researchers chose northern pintails as the focus of the study because they are fairly common in North America and Asia, they are commonly infected by low-pathogenic avian influenza, and they are known to migrate between North America and Asia. None of the samples were found to contain completely Asian-origin viruses, and none were highly pathogenic.

"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 also about 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 coauthor of the study. "Furthermore, 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," Pearce added.

Implications of this research include:

Migratory bird species, including many waterfowl and shorebirds, that commonly carry low-pathogenic avian influenza and migrate between continents may carry Asian strains of the virus along their migratory pathways to North America.
USGS researchers found that nearly half of influenza viruses isolated from northern pintail ducks in Alaska contained at least one of eight virus genes that were more closely related to Asian than North American strains. None of the samples contained completely Asian-origin viruses, and none were highly pathogenic forms that have caused deaths of domestic poultry and humans.
The central location of Alaska in relation to Asian and North American migratory flyways may explain the higher frequency of Asian lineages observed in this study relative to more southerly locations in North America. Thus, continued surveillance for highly pathogenic viruses by sampling of wild birds in Alaska is warranted.

Above left: A flock of wintering northern pintails in northern Honshu, Japan. Photograph courtesy of USGS. [larger version]

Above right: A resident of Iwate Prefecture feeds a wintering flock of northern pintail ducks and whooper swans in northern Honshu, Japan. In spring 2008, both of these species were observed on wetlands in Japan where the highly pathogenic H5N1 strain of avian influenza was detected, and several swans died from exposure to the virus. Photograph courtesy of USGS. [larger version]

Adding the type of genetic analyses used in this study to future surveillance for avian influenza in wild birds will lead to better understanding of patterns of migratory connectivity between Asia and North America and virus ecology.

For more information about USGS research on avian influenza and northern pintails, see "Movements of Northern Pintail Ducks with Satellite Transmitters."

The full reference for the new paper is:

Koehler, Anson V., Pearce, John M., Flint, Paul L., Franson, J. Christian, and Ip, Hon S., 2008, Genetic evidence of intercontinental movement of avian influenza in a migratory bird; the northern pintail (Anas acuta): Molecular Ecology, v. 17, no. 21, p. 4754-4762, doi:10.1111/j.1365-294X.2008.03953.x [URL http://dx.doi.org/10.1111/j.1365-294X.2008.03953.x].

Movements of Northern Pintail Ducks with Satellite Transmitters
USGS

Genetic evidence of intercontinental movement of avian influenza in a migratory bird: the northern pintail (Anas acuta)
Molecular Ecology