Two gray seals swim near the shoreline of Duck Island at the Isle of Shoals on the Maine-New Hampshire border. Credit: BDN File

Researchers say an outbreak of distemper is to blame for a rash of seal deaths in Maine, New Hampshire, and Massachusetts.

The National Oceanic and Atmospheric Administration said Friday that the main pathogen found in the seals is phocine distemper virus. The agency said avian flu also was suspected based on early results but new test results do not point to it as a major cause.

In late August, NOAA declared the seal deaths to be an “unusual mortality event.”

All told, more than 660 dead seals have washed ashore along the coastline in Maine, New Hampshire and Massachusetts since July.

There’s no danger to humans because the virus that causes distemper in seals is not contagious, but the virus can affect pets.

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