A total of 753 bottlenose dolphins have died along the US Atlantic coast due to a measles-like virus, surpassing the previous deadly outbreak 25 years ago, officials said Friday.

The strandings began in July and have spanned the shoreline from New York to Florida, amounting to more than 10 times the usual number of such dolphins washing up dead in an entire year.

The record number has been documented in just half the time of the 1987-1988 event that killed more than 740, raising concerns that this die-off could be far worse, said the US National Oceanic and Atmospheric Administration.

"The stranding networks are working very hard to deal with the overwhelming number," said Teri Rowles of the NOAA Fisheries Marine Mammal Health and Stranding Response Program.

There are also some early indications that the outbreak of morbillivirus may be spreading, since a handful of washed up humpback whales and pygmy sperm whales have tested positive.

However, scientists have not been able to confirm that morbillivirus was the cause of death since the animals were too decomposed by the time tests could be done.

"Currently there is nothing that can be done to prevent the infection from spreading or to prevent animals that get infected from having severe clinical disease," said Rowles.

She also said it is unclear what proportion of the population of wild bottlenose dolphins is affected, or what is causing this unusually severe outbreak.

"There are still a lot of unanswered questions," she told reporters.

Humans are not at risk of catching the morbillivirus but they can be susceptible to bacteria and other pathogens found in the carcasses, and so are advised to call experts for help if they see washed-up dolphins.

[Dead dolphin via AFP]