Back in September, unusually high numbers of dead or dying harbor seals began to be reported from Massachusetts up through Maine. Over the next two months, 162 dead seals were tallied, qualifying the die-off as an official Unusual Mortality Event (UME). The majority of the affected animals were the young of the year. In a normal year, young seals found dead have usually depleted all their body fat, succumbing to malnutrition. During this mortality event however, the dead seals were in good body condition. Many of the animals also showed ulcerations of the skin, and in the five individuals submitted for full autopsy, bacterial pneumonia and other secondary infections were identified. But secondary to what?
Virology tests performed at two independent laboratories identified an influenza A virus known as H3N8 in the five seals sampled. The flu virus damaged the seals’ respiratory tracts and left them susceptible to the opportunistic bacterial infections that ultimately killed them. This particular flu virus is generally found in wild birds, and it is unclear how the seals contracted the disease. A related, though molecularly distinct form of H3N8 has been found in dogs and horses, but both of those species typically recover from the infection. No human cases of H3N8 have ever been reported.
While influenza viruses have been responsible for seal die-offs in the 80s and early 90s, this is the first time H3N8 has been implicated. H3N8 was detected in a harp seal retrieved dead from a fishing net several years ago, but no mass die-off was detected in association with that finding. The National Oceanic and Atmospheric Administration (NOAA) has been coordinating this investigation, and they are quick to point out that diagnosing H3N8 in 5 seals does not necessarily diagnose the entire die-off, and tests continue on samples collected from several more seals during the event. Many of the carcasses were well decomposed and not suitable for testing, and the same held for many seabird carcasses collected during the same time period. It may be impossible to determine whether H3N8 was present in a number of shearwaters found in Rye, NH for instance, as many of these were nearly mummified when found.
For those of you walking the beaches, the usual precautions should apply when encountering seals or other marine mammals: do not approach them, and for God’s sake, don’t let your dog approach them either! Should you encounter a stranded marine mammal, or an unusual number of dead ones, please call NOAA’s marine stranding hotline at 1-866-755-NOAA (6622) and let the professionals handle it.