A viral outbreak that's killing bottlenose dolphins is moving down the U.S. East Coast as the animals migrate south for the winter. Between July 1 and November 3, at least 753 animals have died.

The outbreak began along the coast between New York and Virginia this summer. Now, carcasses are washing ashore in the Carolinas and Florida. Researchers have identified the cause as dolphin morbillivirus, a pathogen that’s related to human measles and canine distemper. Morbillivirus infects dolphins’ lungs and brains, causing weird behaviors and skin lesions and pneumonia (but the marine mammals can't pass it on to humans).

In a normal year, during this same timeframe and in the same geographic area, the average number of dolphins recovered from the beaches would be 74.

So far, Virginia has been the hardest hit by the outbreak, with more than 330 dolphins retrieved from its mid-Atlantic shores. New Jersey takes the dubious honor of second place, with 131 dolphins. In the last month, the Carolinas saw their total spike to more than 120.

>'There is no vaccine that is developed that can be deployed for a large, wild population of bottlenose dolphins.'

And last week, Florida had its first confirmed fatality. That makes marine biologist Megan Stolen nervous. She and her team at the Hubbs-SeaWorld Research Institute respond to marine mammal strandings along miles of northern Florida coastline – and from her lab in Melbourne Beach, she’s been tracking the outbreak and its slow march south.

It’s kind of like preparing for a hurricane: You know something bad is heading your way, but you don’t when it will arrive, or where, or how bad it could be. “We don’t know if we’ll get one a week or 10 a day,” Stolen said.

The die-off has already been classified as an Unusual Mortality Event by the federal government – a designation that frees up resources and sends investigators and responders to the hardest-hit areas. It’s already exceeded the pace set by the last major morbillivirus outbreak on the East Coast, an event that lasted for 11 months, between June 1987 and May 1988, and ultimately claimed 742 dolphins.

“We are less than halfway through that time frame, and we have surpassed the number of cetacean strandings reported in the 87-88 die-off,” said Teri Rowles, the coordinator of NOAA’s Marine Mammal Health and Stranding Response Program. “There is no vaccine that is developed that can be deployed for a large, wild population of bottlenose dolphins. Or any cetacean species.”

NOAA )

Indeed, there’s something in the mix this time around that could be even more worrying. Other species have been showing up dead with dolphin morbillivirus in their tissues. Since July, three out of four dead humpback whales (in Massachusetts, Virginia, and North Carolina), and a two out of three dead pygmy sperm whales (in Georgia and Massachusetts) have tested positive for the pathogen.

Dolphin morbillivirus isn’t often reported in these species. Whether the whales are dead because of a morbillivirus infection – or simply exposed to it – is still unknown.

“We don’t yet know if we do have, indeed, an outbreak of morbillivirus in those species,” Rowles said. “We know we have the presence of virus, but we’re waiting on other tests to confirm that it was causing clinical disease or death.”

Marine mammals on beaches are usually in very bad shape – that’s why trained teams of responders come to deal with them. With death rates at least 10 times higher than normal this summer, local response teams have had to call in reinforcements: Veterinarians and scientists have been coming to help from all over the country, from as far away as California and Hawaii. When the morbillivirus outbreak was peaking in Virginia in August, teams there responded to as many as 18 animals in a single day.

“It was very unreal dealing with the numbers of animals that were coming in at the peak in August,” said Mark Swingle, director of research and conservation at the Virginia Aquarium & Marine Science Center. “But we are still dealing with very high mortality levels.”

Roughly 5 percent of the dolphins infected with morbillivirus strand alive. They either die shortly after arriving on the beach, or are euthanized. ( Outer Banks Marine Mammal Stranding Network, North Carolina)

It’s not a trivial task to learn everything you can from an animal on the beach as the decomposition clock rapidly ticks. Teams process the dolphins as fast as they can, recording observations, investigating what’s inside, and taking out brains. Samples are collected, labeled, and stored quickly, filling freezers in multiple states with bags of tissues or tubes full of fluids. Some of these are sent to labs, like the one at the University of Georgia, where virologists helped determine that morbillivirus was the culprit behind the recent deaths.

In Florida, Stolen is busy getting her team ready for what could be the worst holiday season in years. For the last couple of weeks, they’ve been ordering supplies and getting protocols in order, in case they’re dealt the worst of it. It’s not the first time this year they’ve been dealt a tough hand: Since March, they’ve retrieved 74 dolphins from the Indian River Lagoon, a besieged and beautiful estuary that laps at the lab’s backyard. Unlike the die-off in the water to their east, though, the lagoon deaths remain unsolved.

Stolen doesn’t know if that event is over yet. She doesn’t know if the Atlantic Ocean will begin depositing dolphins on her other doorstep – or if those dolphins will mix with the lagoon population and infect them. But if the Atlantic outbreak does arrive, and if the dolphins in her backyard continue dying, she does know she’ll be overwhelmed with carcasses.

“We’ve done what we can do,” she said. “Now we just wait and see.”