A measles-like virus is being cited as a likely cause for the mass dolphin die-off that’s been plaguing the U.S. East Coast this summer.

Since July 1, 333 carcasses have littered shores from New York to North Carolina – a number that's roughly 10 times more than normal for this time of year. Scientists don’t yet know how many dolphins have died offshore without reaching mid-Atlantic beaches, but it could be thousands. In July, NOAA declared the die-off an Unusual Mortality Event, which frees up federal funding and investigators to address the crisis.

Now, a NOAA team in charge of investigating the event is pointing to a type of morbillivirus as the culprit behind the bottlenose dolphins deaths. Morbilliviruses are responsible for measles in humans, rinderpest in cattle, and canine distemper in dogs, coyotes, wolves and seals. There is no easy way to identify morbillivirus infection just by looking at a carcass, so identifying the pathogen as the cause of the die-off involved a feat of molecular detective work using tissue collected from the dead animals.

Virginia Aquarium )

“I don’t know of anything that screams morbillivirus when you look at the inside of the animal,” said Charley Potter, a scientist at Smithsonian’s National Museum of Natural History. Potter, along with his colleagues, has been helping to examine the carcasses of animals arriving in Virginia. “Some of the animals seem to be emaciated and debilitated, and others seem to be rather robust and healthy,” he said.

While there are no unifying anatomical findings that point toward the pathogen, many of the animals washing ashore have suggestive lesions in their mouths, lymph nodes, brain, or lungs. Potter, and the others who conduct necropsies (animal autopsies), collect bits of these damaged tissues, as well as other organs.

The tissues are then sent to several pathology labs, including those at the University of Georgia, the University of Florida, and the University of California, Davis.

There, scientists use a variety of techniques to search for morbillivirus within the tissues. One method involves looking closely at lesioned tissue for microscopic signs of the infection. Another uses antibodies to detect morbillivirus in the lesioned tissues. And the third looks for viral genes by amplifying genetic sequences within the animals’ tissues.

So far, nearly all of the carcasses – 32 out of 33 – fresh enough to be analyzed by these methods have tested positive for, or are strongly suspected of having, morbillivirus. Of those, genetic sequencing confirmed that 11 of the carcasses carry the cetacean form of the virus, which affects dolphins and porpoises.

“We feel very comfortable that this is a morbillivirus outbreak,” said Teri Rowles, NOAA’s Marine Mammal Health and Stranding Response Program coordinator. “We are going to continue to look at potential secondary and contributing factors.”

The hunt for the source of the die-off has been slowed by the condition of many of the carcasses. Most are either too far decomposed or scavenged to preserve evidence of what killed them. Some pathogens, such as morbillivirus, can only be detected in freshly dead animals, so teams responding to stranded animals have been prioritizing collecting tissues from fresher carcasses.

“When we get animals that show significant skin fluffing and they’re starting to gas up and things like that, we really ratchet back on what is collected,” Potter said.

Marine Mammal Stranding Center )

The bulk of the dead dolphins have stranded in Virginia, where teams have processed 174 carcasses since July 1. The next-hardest hit is New Jersey, with a total of 71 dead dolphins. There, teams are finding about a dolphin a day, said Bob Schoelkopf, founding director of the Marine Mammal Stranding Center in Brigantine, NJ.

“I just picked one up this morning – a youngster, about 150 pounds,” he said on Saturday. “But it was heavily scavenged by sharks, not salvageable for sampling.”

Initially, the event’s southern boundary ended at Virginia; but with North Carolina now reporting 42 dolphin strandings since July (about four times more than normal), the UME has been expanded to include that state.

The southward spread of the strandings is reminiscent of the morbillivirus outbreak of 1987-88, which ultimately led nearly 800 dead dolphins to wash ashore in the mid-Atlantic between June 1987 and May of the next year. Now, as it was a quarter century ago, the strandings began with a spike in the north before spreading to the south, a pattern scientists attribute to the southward migration of Atlantic ocean bottlenose dolphins.

“We’re expecting that if indeed this plays out the way that that die-off occurred, that we’re looking at mortalities being higher and morbillivirus spreading southward, and likely continuing to the spring of 2014,” Rowles said.

Scientists don’t know for sure what sparked the outbreak, but are guessing that affected dolphin populations have lost their ability to fight the virus, a protection they gained 25 years ago when it last struck the area.

“The picture that's playing out so far is a classic introduction of a virus to a universally naïve population,” said Stephanie Venn-Watson, a veterinarian with the National Marine Mammal Foundation. As populations grow and reproduce, antibodies developed to combat morbillivirus years ago are diluted within the populations; now, it appears those levels have dipped below a tipping point, Venn-Watson said.

And, though we know it’s spreading, there’s no way to vaccinate dolphins against the spread of the virus. “[The die-off] would probably end when the number of susceptible animals still out there to infect dwindles,” said Jerry Saliki, a virologist at the University of Georgia.

The good news? “Not all infected animals will die,” he said.