Baby harbor seals were dying in droves on the New England coast last autumn, and researchers now think they know why: a new strain of avian flu.

A total of 162 dead or dying seals washed up from Maine to Massachusetts starting last September. That was four times the usual number of dead seals that wildlife officials were used to seeing, and they didn't seem like normal cases of malnutrition. Many bore skin lesions and showed signs of pneumonia.

In a paper published Tuesday in the journal mBio, researchers led by Columbia University scientist Simon Anthony say they've traced the seal deaths to a new strain of influenza related to a strain that's been passed among water fowl since 2002. They isolated the virus from five of the dead seals.

The specific type of flu virus the scientists found belongs to a subtype of influenza A called H3N8 that is also found in horses and dogs. The strain of influenza A that we're more familiar with as "bird flu" is a different strain of another subtype, H5N1.

"When initial tests revealed an avian influenza virus, we asked the obvious question: How did this virus jump from birds to seals?" Anthony said in a statement.

Unlike previous flu outbreaks in seals, this virus has mutations that show it's adapted for life inside mammal hosts. It also has other mutations that are known to increase virulence and transmissibility in H5N1 bird flu. The new seal flu virus can also target a specific protein in the human respiratory tract, the researchers found.

Scientists are particularly concerned by the strain's apparent ability to readily jump from birds to seals, meaning that it could possibly make the leap to humans.

Anthony and his colleagues also found that the seal H3N8 virus shares three small mutations with human H3N2 influenza viruses. They aren't sure exactly what these mutations do yet, but they are not found in H3N8 flu viruses previously taken from animals or pandemic H1N1, also known as "swine flu."

More research needs to be done, but the emergence "of a pathogenic virus that can transmit between mammals, found in a species that can become infected with multiple influenza virus subtypes, must be considered a significant threat to both wildlife and public health," the authors wrote.

SOURCE: Anthony et al. "Emergence of Fatal Avian Influenza in New England Harbor Seals." mBio 3: e00166-12, 31 July 2012.

Swine flu Avian influenza Bird flu H5n1