At least 124 bottlenose dolphins have washed up along the Atlantic coast since July, a startling number of deaths that has prompted US officials to launch an investigation.
Scientists are working to find out if an infectious pathogen may be to blame since several of the dolphins appeared to have lesions in their lungs.
An "unusual mortality event" has been declared due to the "unexpected and significant die-off" that has spanned the coasts of New York, New Jersey, Delaware, Maryland and Virginia since early July, said the National Oceanic and Atmospheric Administration's Fisheries unit.
Eighty-nine were found in July and another 35 have washed up so far this month, a NOAA spokeswoman told AFP on Friday.
A NOAA statement released Thursday said "preliminary testing of tissues from one dolphin indicates possible morbillivirus infection, although it is too early to say whether or not morbillivirus may be causing this event."
However, NOAA scientists say an "infectious pathogen is at the top of the list of potential causes."
Morbillivirus is similar to a marine mammal form of distemper, which in dogs attacks the central nervous system and causes breathing problems, vomiting, diarrhea, brain swelling and often death.
Most of the dolphins were already dead when they were found, scientists said. A small number have been stranded alive, only to die shortly after.
Typically an average of seven dolphins wash up in Virginia in July, so the 47 animals found last month is a significant increase.
"It is an important issue," said NOAA spokeswoman Connie Barclay.
The last time morbillivirus was implicated in a mass dolphin die-off was in 1987 and 1988, when more than 740 bottlenose dolphins died from New Jersey to Florida.
According to the Virginia Aquarium and Marine Science Center, which has been collecting and studying the dolphin carcasses -- 67 of which have come ashore in Virginia -- most have been male.
Spokeswoman Linda Candler told AFP they have been found in "all conditions from very fresh to badly decomposed, all ages and sizes."
She added that a dolphin that had recently died came in for analysis on Thursday and samples were sent off for analysis, which may take up to two weeks.
Other potential causes for mass dolphin deaths could include stormy weather, ship strikes or pollution.
Candler said scientists "don't want to speculate, but the researchers feel comfortable in saying they do not believe that this is a result of human interaction, that it is some sort of biological event."
If it is a virus, there is not much that wildlife officials can do, she added.
"It has to run its course," Candler said. "You can't immunize a wild population, unfortunately."
Recent studies have shown that dolphins have an unparalleled ability among animals to remember each other for decades, and that they call each other by name using whistles that are distinct among close friends and family.
There are four distinct populations of bottlenose dolphins off the Atlantic coast, including about 22,000 living near the shore and nearly 82,000 in deeper waters.
According to NOAA expert Fionna Matheson, little can be done to help the dolphins that are already sick.
"However, we can work to minimize the stress caused to the dolphins by human activity," she told AFP by email.
"If there is a link to things humans are doing that may make the dolphins more susceptible to disease (e.g., exposure to pollution, malnutrition from lack of prey, stress from disturbance), then we can certainly strive to mitigate those threats."