Since mid-July, more than 300 bottlenose dolphins have been found dead on coasts between New York and North Carolina, nine times the historical average for the months of July and August in the region, Xinhua cited the US National Oceanic and Atmospheric Administration (NOAA) as saying.
On Aug 8, NOAA Fisheries officially declared the situation an "unusual mortality event".
Samples were collected and analysed to see if there was any relationship between all the strandings.
"After completing initial diagnostic tests on more than two dozen animals from all affected states and consulting with disease experts, we have determined that the likely primary cause of this event is a virus -- the cetacean morbillivirus, which is similar to measles in humans or canine distemper in dogs," the NOAA said in a statement.
It said 11 of the 32 dolphins tested confirmed positive for morbillivirus, with all of the remaining dolphins suspected of having the fatal infection.
The agency said morbillivirus usually spreads through the air or direct contact between animals, and affects the lungs, brain and immune system of dolphins and causes illness and death.
While this virus can easily spread among dolphin populations since the animals are highly social, it is not infectious to humans, the NOAA said.
But the NOAA also said there was little it can do to curb the outbreak.
"Unfortunately, there is no way to currently stop the spread of the virus. There are no vaccines or anti-viral medications available to administer to wild dolphin populations in an effective or practical manner," it said.
There was a similar outbreak of morbillivirus along the Mid-Atlantic, which killed more than 740 bottlenose dolphins between June 1987 and March 1988.