By Kat Kerlin

As migrating mallard ducks touch down on the wintering grounds of California’s Central Valley and mingle with resident mallard populations, they sometimes carry new avian influenza strains with them.

So concludes a new study conducted at the University of California, Davis, with researchers from the U.S. Geological Survey, Massachusetts Institute of Technology, Canadian Department of the Environment, and University of Minnesota. The study appears in the journal Molecular Ecology.

“This study confirms that mallards bring viral strains from their northern breeding grounds to California, while resident mallards appear to be serving as reservoirs for virus that likely persists year-round,” said Nichola Hill, who led the research while a postdoctoral student at UC Davis. Hill is now an ecologist with the USGS and MIT.

In North America, mallard ducks are the main host of “low pathogenic” avian influenza strains — not to be confused with highly pathogenic avian influenza strains such as H5N1, which has not been detected in North America. “Low path” avian influenza viruses rarely cause mortality in domestic poultry or wild waterfowl populations, and serve as important models to study how viruses are transmitted and spread to new geographic areas.

Researchers sampled mallards from the Sacramento National Wildlife Refuge, a major wintering ground for wild birds. The scientists analyzed swab samples for virus type and feathers to determine each bird’s geographic region.

Feathers, which reveal the birds’ geographic origins in their isotopic signatures, were analyzed at the UC Davis Stable Isotope Facility. Isotopes are different versions of chemical elements. An isotopic signature is used to determine a sample’s origin or age.

The tests revealed where each duck flew from and the influenza strains it brought to California.

“All mallards are not the same, and our results clearly show that pathogen transmission is strongly associated with migratory versus resident status of these mallards,” said Walter Boyce, a study coauthor and veterinarian in the UC Davis School of Veterinary Medicine’s Wildlife Health Center.

“We’re trying to learn about wildlife disease transmission,” said USGS ecologist John Takekawa, also a coauthor. “There is more to learn about how waterfowl migration relates to disease spread and about the environmental or human factors that shape these migration strategies.”

The study was funded by the National Institute of Allergy and Infection Diseases, the U.S. Fish and Wildlife Service, and the USGS.