Genetic tests have confirmed that a measles-like virus known as morbillivirus is likely what's killing hundreds of dolphins off the East Coast.
Nearly 300 dead bottlenose dolphins have washed up on beaches from New York to North Carolina as of Aug. 20, almost seven times more than normal in some places.
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It's the second big die-off caused by the virus.
Morbillivirus was also behind a wave of infections that killed more than 700 bottlenose dolphins between June 1987 and March 1988.
So far, 11 of the 33 dolphins tested from the latest die-off have come back positive for morbillivirus, with all but one of the remaining dolphins suspected of having the fatal infection.
"We are now calling it a morbillivirus outbreak," Teri Rowles of the National Oceanic and Atmospheric Administration's Fisheries Marine Mammal Health and Stranding Response Program said of the latest die-off during a telephone press briefing Tuesday.
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The virus is often transmitted through direct contact among animals or from inhaling respiratory particles.
It affects the lungs and brain, resulting in pneumonia with accompanying respiratory problems, according to Newsday.
A second prime suspect is the Brucella bacterium, found in four dolphins from Virginia. Marine mammals are common hosts for the bacterium.