A major die-off of bottlenose dolphins on mid-Atlantic coast has now been attributed to a measles-like virus called morbillivirus, NBC News reported.

A similar outbreak has been seen before, from June 1987 to March 1988, the virus killed more than 700 bottlenose dolphins. So far, 357 dolphins either dead or dying have washed onto coasts between New York and North Carolina.

"We are now calling it a morbillivirus outbreak," Teri Rowles of the National Oceanic and Atmospheric Administration's (NOAA) Marine Mammal Health and Stranding Response Program said.

Rowles said 32 of the 33 dolphins tested were confirmed or suspected to have the virus, while additional genome sequencing showed the virus present in 11 other animals.

"There's no doubt in my mind that these kinds of disease have been [recurring] for millennia," said Perry Habecker, staff veterinary chief at the University of Pennsylvania, adding that infections have "always been happening in cycles."

Jerry Saliki, a virologist at the University of Georgia, told the Washington Post the virus had resurfaced because enough time had passed since the last outbreak for dolphins to lose their natural immunity to it.

"When the collective immunity drops below a certain, critical point, which we don't really know for marine mammals, then the whole population becomes susceptible," Saliki said.

The Marine Mammal Stranding Center said they find as many as five dolphins in a single day with defining signs of morbillivirus, like shark bites and black lesions around their mouth.

Experts said it took about two years in 1987 to determine what the cause of the massive dolphin die-off. This time, Rowles said they are near certain what the cause is because they knew what to look for. However, not much can be done because the vaccine cannot be easily distributed.

"There isn't anything we can do to stop the virus," Rowles said. "In terrestrial populations, there are vaccines that are developed and [that] are easily usable. We don't have a vaccine that can be easily deployed."