Outbreaks of two viruses have killed hundreds of harbor and gray seals off the coast of the northeastern US, according to officials from the National Oceanic and Atmospheric Administration (NOAA).

In a press call on August 31, experts from the agency announced that seals are perishing in high numbers from phocine distemper, a virus closely related to canine distemper, and a form of avian flu. Although it is common for a few seals to perish from circulating diseases, particularly at this time of year when there's a new generation of susceptible pups, the current spate of infections is serious enough to be declared an "unusual mortality event".

Between July 1 and August 29, 462 dead seals were found washed up on beaches from southern Maine to northern Massachusetts. During the same period, 137 live seals stricken with one or both of the infections were found struggling on the shore. NOAA estimates that hundreds more have likely perished, but the bodies haven't been found.

“There’s two things that we know. These outbreaks occur very commonly in the Atlantic, in Europe, and also along the US east coast, and their timing seems to be associated with after the pupping season, when there are more animals in close contact,” said Tracey Goldstein, a researcher at the Wildlife Health Center at the University of California, Davis. Avian influenza and phocine distemper, which is also a relative of the measles virus, are highly contagious and have triggered dramatic die-offs in the past.

“In terms of population effects, in Europe, during the ’88 and 2002 outbreaks, which are some of the largest that affected seal populations in Europe, [populations descreased] to about 50 percent of the numbers before the outbreak. So it does have a severe effect, or it can, on the population numbers.”

Teri Rowles, a veterinarian and the coordinator of NOAA Fisheries' Marine Mammal Health and Stranding Response Program, explained that laboratory analyses of samples collected from both live seals and carcasses have revealed that the distemper and flu viruses are behind the infections, but “it’s too soon to determine what each of those viruses are contributing to the mortality event, or whether the two of them together are expanding the mortality.”

The necropsies that have been performed thus far reveal that the diseases are present in the seals' lungs and brains. Seals fighting infection have displayed unusual behavior, and those found on the beach are often lethargic and coughing. The investigation is still ongoing, but NOAA researchers believe they've pinpointed the Isles of Shoals as the outbreaks' epicenter. This small group of islands off the coast of Maine and New Hampshire is a popular hangout for both species during the summer months, and harbor seals are known to pup on one of the islands in spring.

According to The New York Times, this event marks the first time the two infections have been detected together in seals from American waters, though dramatic outbreaks of each have separately claimed many seals in the past. Avian flu cases arise when birds pass the infection to seals, and phocine distemper is continually present in the ecosystem at various levels. NOAA scientists currently aren't concerned about the viruses spreading to other species, though they do note that dogs should be kept away from seal carcasses and distressed seals as they are also susceptible to the phocine virus (and because you should leave the poor seals alone and call an expert).