More than 300 bottlenose dolphins have stranded themselves along the US East Coast this summer, and now researchers think they know why: the dolphins are sick.

Preliminary tests suggest that cetacean morbillivirus, a cousin of the virus that causes measles in humans, is killing dolphins from New York to North Carolina, officials with the US National Oceanic and Atmospheric Administration (NOAA) said today.

Since 1 July, 333 dolphins have beached themselves, more than twice the normal yearly average for that stretch of coastline, prompting the NOAA to declare an ‘unusual mortality event’ on 8 August. Many of the dolphins that have washed up have borne signs of disease, including severe weight loss and skin and mouth lesions.

Initial tests for antibodies in 33 of the animals have confirmed that 32 are positive or ‘suspected positive’ for cetacean morbillivirus, says Teri Rowles, who coordinates the agency’s Marine Mammal Health and Stranding Response Program. Further genetic tests have confirmed the presence of the virus in some of those dolphins, while necropsies have found lesions consistent with morbillivirus.

Researchers are not sure how long the outbreak will last. A similar event in 1987 lasted for 11 months, killing so many animals that two East Coast dolphin stocks were subsequently listed as ‘depleted’ by the US government. “If indeed this plays out the way that die-off occurred, then we are looking at mortality being high and morbillivirus spreading southward — likely continuing through the spring of 2014,” Rowles says.

Lance Garrison, a biologist at the NOAA’s Southeast Fisheries Science Center in Miami, Florida, says that researchers are trying to determine where the morbillivirus epidemic started. “One of the real challenges is determining which populations of bottlenose dolphins are affected,” he says.

Scientists are trying to determine whether environmental effects, such as pollution, could be weakening the dolphins’ immune systems. They also want to know how the age and sex of a dolphin affects its ability to survive morbillivirus infection, and whether other dolphin species are vulnerable to the current outbreak.