From New York to Virginia, dead dolphins have been washing ashore in unusually large numbers this summer. As of Aug. 20, nearly 300 stranded bottlenose dolphins had been reported in the region, according to the National Oceanic and Atmospheric Administration, nearly seven times more than normal in some places.

As experts continue to investigate the cause, the leading contender is an infection called morbillivirus. Related to human measles and canine distemper, the virus seems to cause sporadic epidemics among dolphins. Many years, there are no detected cases, but when the virus hits, it can hit hard. The last epidemic struck off the Atlantic coast in the winter of 1987-88, killing more than 740 animals from New Jersey to Florida.

For now, there is no official announcement to confirm that morbillivirus is the culprit in the current outbreak, though experts involved in the investigation say that the virus has been confirmed in at least some of this year’s stranded dolphins.

PHOTOS: Sharks, Marine Mammals Hang in Paradise

“It’s no secret at this point,” said Perry Habecker, a large animal pathologist at the University of Pennsylvania’s School of Veterinary Medicine in New Bolton. His team has been analyzing tissues from dolphins that are washing up in New Jersey. “Morbillivirus is accounting for some of these deaths.”

Every year, at least some dead dolphins wash up onto beaches. Some get hit by boat propellers. Others get caught in fishing nets. Still others succumb to viruses, bacteria, lungworms, fungal infections and other diseases.

From 2007 to 2012, according to NOAA data, between 88 and 113 dolphins washed ashore in the area of the current East Coast die-off. As of last week, the current tally is 299, with a large spike since early July.

In New Jersey, there are usually between half a dozen and dozen strandings each year, said Bob Schoelkopf, founding director of the Marine Mammal Stranding Center in Brigantine, N.J. So far this year, there have been 74 dead dolphins found on the state’s beaches. In Virginia, there were 45 strandings last month, compared to the state’s annual average of just seven in July.

Getty Images

Accidents with boats and commercial fisheries leave telltale signs on dolphin corpses, Schoelkopf said, allowing experts to rule those out as possible explanations for the recent rash of deaths. Water quality and temperature also appear normal this year.

Members of the public have suggested plenty of other possibilities that experts are not taking seriously -- including fall-out from Hurricane Sandy and gas released by the Syrian government.

Instead, the sudden spike in deaths along with preliminary pathology reports and evidence against other explanations suggests that some kind of disease is to blame, and that disease is probably morbillivirus.

NEWS: Many Stranded Bottlenose Dolphins May Be Deaf

“We get a lot of strange calls telling us what people think might be the cause,” Schoelkopf said. “Usually, when dolphins come down with a disease that’s communicable, it spreads easily because they live in such tight-knit family groups of up to a couple hundred animals in a group.”

Among the remaining mysteries -- assuming morbillivirus is the culprit -- scientists still don’t know why or how the disease occasionally causes so much trouble. One possibility is that the virus mutated in a way that made it more virulent, much like the human influenza virus can change some years. Another possibility is that something changed in the dolphin population, making them more susceptible. Or maybe the dolphins caught the virus from a population of pilot whales that swam too close.

Once it infects dolphins, Habecker said, morbillivirus affects multiple body systems and lowers immunity, leading to secondary infections that can kill them. Pneumonia was the ultimate cause of death in some of the dolphins his group has looked at.

That kind of cascading series of events complicates the investigation into the current outbreak, and an official cause won’t be announced until more data comes in from the several labs around the country that are analyzing tissues and sequencing viruses from this year’s crop of dead dolphins.

Even though the mass die-off may seem alarming, Habecker added, it’s fairly common for diseases to strike wild animals, both in the ocean and on land.

“The big difference is that now we’re paying attention to these things,” he said. “It strikes many people as new and ominous, but the reality is that this is just a cycle of nature.”