Kat King, left, and Katie Gilbert, interns from Marine Mammals of Maine, examine the body of a seal that washed up on Aug. 17 at Scarborough Beach State Park. The interns took the carcass back to their headquarters in Harpswell for testing.

Many of the dead seals that washed ashore in northern New England in the past few weeks tested positive for either avian influenza or phocine distemper virus, but it is still too soon to say if those viruses are the primary causes of the unusual die-off, the National Oceanic and Atmospheric Administration said.

Researchers have been looking for the reasons behind a surge in seal deaths that began around Aug. 12, when an unusually high number of dead or sick seals began showing up on beaches in southern Maine and New Hampshire. More than 400 dead or stranded seals have been reported along the Maine coast so far this year, including more than 100 found on beaches south of Portland in the past two weeks.

And rescuers say reports of stranded or dead seals are still flowing in.

“We have many more samples to process and analyze, so it is still too soon to determine if either or both of those viruses are the primary cause of the mortality event,” said Jennifer Goebel, a NOAA spokeswoman.

The preliminary test results showed that four of the seals tested positive for both viruses, NOAA reported.

In 2011, more than 160 seals were found dead on the coast from northern Massachusetts to Maine between September and December. Tests conducted on five animals showed they suffered from a bacterial pneumonia caused by a strain of avian influenza – H3N8 – that can be transmitted to seals through contact with seabird guano.

Influenza also was detected in seals in New England during mortality events 1979, 1982, 1991 and 2006, NOAA said.

A dead seal lies on the shore at Bayview Beach in Saco on Aug. 12.

Marine animal rehabilitation clinics across the Northeast – including Harpswell-based Marine Mammals of Maine – stopped taking stranded live seals last week, concerned that healthy animals in recovery could be exposed to whatever is causing the die-off.

Since the beginning of the year, more than 400 dead and stranded seals, the vast majority harbor seals, have been reported in Maine, more than twice the annual average in the last seven years, NOAA said. Rescuers in Maine have reported finding 65 dead seals in July and 179 dead seals in August. They also found a total of 84 live seals since the beginning of July.

In New Hampshire and Massachusetts, rescuers reported 30 dead seals in July and another 58 in August. During that time, 31 live seals were rescued, NOAA said. There have not been reports of large numbers of seals being found sick or dead along the Maine coast north of the Portland area.

Lynda Doughty, executive director of Marine Mammals of Maine, which responds to strandings, said she is relieved that researchers are starting to get some answers about what is happening to the seals. She was not surprised that preliminary tests showed influenza and phocine distemper because those viruses are known to be present in the seal population and seals were showing symptoms consistent with the viruses.

“I’m glad it’s not something new,” Doughty said. “We’re committed to keep sampling these animals to learn more about what else might be going on.”

While experts have said viruses are a likely cause of the deaths, researchers also have said the seal population is at higher risk of disease because decades of chemical pollution have weakened the animals’ immune systems.

Researchers say live seals have shown symptoms such as lethargy, sneezing, coughing, discharge from the nose and eyes, seizures and skin abscesses. In dead seals, necropsy findings show signs consistent with pneumonia, NOAA said.

Some types of flu can be shared between animals and people, but phocine distemper virus cannot be transmitted to humans. The U.S. Centers for Disease Control and Prevention, the National Wildlife Health Center and other organizations are closely monitoring this mortality event and other animal influenza viruses because influenza viruses constantly change and it is possible for a virus to change so it could infect humans, NOAA said.

The agency is warning people not to touch stranded seals and to keep their dogs away from marine mammals. Doughty said her staff and volunteers wear protective gear when handling seals or taking samples because it is not clear what viruses they are dealing with.

The viruses spread between seals through inhalation of respiratory particles or through direct contact, including between mothers and their pups. They also can be exposed to the viruses through their eyes, mouths, stomachs, skin wounds and the urogenital tract.

While the sudden die-off has shocked volunteers and the public, the effect on the overall harbor seal population is unclear. Harbor seals are some of the most common marine mammals in the U.S., and while legally protected from hunting, they are not endangered or threatened.

The harbor seals commonly seen in Maine are part of a population that lives in coastal waters from the Bay of Fundy in Nova Scotia to New York and New Jersey. There are about 76,000 harbor seals in the western North Atlantic and the population had not significantly changed since the last survey in 2012, NOAA reported in its 2017 stock assessment. The number of gray seals believed to be in U.S. waters is much lower, around 23,160, according to NOAA’s most recent stock assessment, but scientists believe the population is growing.

Seal strandings in Maine can be reported to Marine Mammals of Maine at 800-532-9551.

Gillian Graham can be contacted at 791-6315 or at:

ggraham@pressherald.com

Twitter: @grahamgillian