The mass mortality event that claimed 162 harbor seals from September to December 2011, was caused by a form of avian influenza according to research published in the July/August issue of the journal mBio.
Postmortem analyses revealed the presence of H3N8 influenza A virus – the same subtype that has been found in horses, birds, seals, and dogs, but with molecular differences that more closely resemble a virus that has been observed in North American waterfowl since 2002.
The harbor seal outbreak is significant, not only because of its detriment to seals, but because the virus has acquired some genetic mutations that are known to increase transmissibility and virulence in mammals, including humans.
At this time, there is no evidence to suggest that the “seal flu” has infected humans, but like all mutations that jump species, the virus will need to be closely monitored for additional changes that could potentially expand to infect domesticated animals or other wildlife species. The influenza A virus does not cause disease in fish.
The 2011 harbor seal outbreak occurred along the coasts of Maine, New Hampshire and northern Massachusetts, primarily affecting pups less than six months old. While all mortalities suffered from pneumonia and many exhibited skin lesions, they appeared to be well-nourished and otherwise in good condition.
Fatal outbreaks of influenza have been observed among harbor seals in the northeastern U.S. several times in recent years, including outbreaks of H7N7 in 1979-1980, H4N5 in 1982-1983, and H4N5 and H3N3 in 1991- 1992.
All marine mammals, including harbor seals, are protected by the Marine Mammal Protection Act. Anyone encountering an animal that appears ill is advised to not to approach, but to report it to the NOAA stranding hotline.