A measles-like virus is behind the deaths of at least 333 dolphins on the US East Coast in less than two months, authorities say.
Morbillivirus is responsible for nine times the average number of bottlenose dolphins washing up - usually dead - along the shores of the mid-Atlantic region since July, according to National Oceanic and Atmospheric Administration (NOAA).
Most of the dolphins are washing up badly decomposed on beaches, mainly in Virginia.
"Along the Atlantic seaboard, this is extraordinary," said Teri Rowles from NOAA.
State and federal officials say there are untold numbers of other dolphins that have also died and have not washed ashore, likely making the death toll much higher.
The virus infects the lungs and the brain, causing pneumonia and abnormal behaviour, and is usually fatal. It also causes lesions on the skin, mouth, joints and lungs.
Earlier this month, NOAA declared an "unusual mortality event" so it could provide extra funds to investigate the deaths.
At the time, they suspected the cetacean morbillivirus was causing the deaths, just as it did 25 years ago in the last dolphin die-off.
In 1987 and 1988, the virus killed 740 dolphins between New Jersey and Florida.
Although research will continue, NOAA said it has collected enough evidence to declare the virus as the "tentative cause" in the most recent string of deaths as well.
Thirty-two of 33 dolphins tested have been confirmed positive or are suspected of dying from morbillivirus.
"We are now calling this a morbillivirus outbreak," said Ms Rowles.
But authorities say there is little they can do to stop the spread of the virus.
Based on what happened in the 1980s, officials believe the disease and strandings will spread south and last through the spring of 2014.
Officials say in time the dolphins will become more resistant to the disease, just as they have before.
Bottlenose dolphins typically live between 40 and 50 years, but a new generation of dolphins will also likely become susceptible to the disease again in the future.
It is unclear what sparked the latest outbreak, but scientists think that some sea creatures have natural immunity to morbillivirus, but others do not.
When the two populations come in contact, mass illness and death can occur in the weaker animals.
"The primary hypothesis is that East Coast dolphins simply don't have the immune response to effectively fight off this virus," said Stephanie Venn-Watson of the National Marine Mammal Foundation.
Viruses in the same family can cause measles in humans, canine distemper in dogs and wolves and rinderpest in cattle, the NOAA said.
But the illness is not likely to spread to humans.
"All morbilliviruses known to date infect a small number of closely related species," said Jerry Saliki, a virologist with the University of Georgia.
"So there is no indication that this virus could jump into humans given the species gap between marine mammals and humans."