A massive string of dead dolphins along the East Coast is most likely due to cetacean morbillivirus, a marine strain of a virus similar to measles, according to the National Oceanic and Atmospheric Administration.

At least 333 bottlenose dolphins have been found dead from New York to North Carolina, officials said during a Tuesday press conference. The strandings peak in Virginia, where 174 carcasses have washed ashore. The agency declared the die-off an "unusual mortality event" earlier this month.

Looking at a chart of past years, death rates are nine-times higher than usual.

noaa

A die-off this large hasn't happened for more than 25 years -- between the summer of 1987 and the spring of 1988, more than 700 bottlenose dolphins washed up along the coast. NOAA officials say they don't know when the strandings will stop, but the similarities between the events could mean they likely won't end until next spring.

Although morbillivirus is similar to measles in humans, animals affected by the outbreak aren't vaccinated in the same way we are, according to Dr. Stephanie Venn-Watson, director of translational medicine and research at the National Marine Mammal Foundation.

"East Coast animals simply don't have the immune response to effectively fight off this virus," she said.

To report a live or dead stranded dolphin, contact your local marine mammal stranding network.

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  • A seahorse swims in an aquarium in the zoo of Frankfurt, Germany, Tuesday, Oct. 16, 2012.(AP Photo/Michael Probst)

  • A Cownose Ray swims in a tank during a preview of the newly renovated Suzanne and Walter Scott Aquarium at Henry Doorly Zoo in Omaha, Neb., Wednesday, April 4, 2012.The aquarium opens to the general public on Thursday, April 5. (AP Photo/Nati Harnik)

  • King penguins stand in an enclosure at the Hakkeijima Sea Paradise aquarium-amusement park complex in Yokohama, near Tokyo, Sunday, Sept. 30, 2012.(AP Photo/Itsuo Inouye)

  • In this photo taken Thursday Aug. 2, 2012 and released by the Monterey Bay Aquarium, a male weedy sea dragon at the Monterey Bay Aquarium swims with some of his newly hatched babies on in a sea dragon display that’s part of the aquarium’s special exhibition, “The Secret Lives of Seahorses.” in Monterey, Calif. The inch-long fish, Australian relatives of the seahorse, were carried as eggs on a brood pouch under the father sea dragon’s tail. (AP Photo/Monterey Bay Aquarium, Randy Wilder)

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  • Pterophyllum scalare fish are displayed at the 2012 Taiwan International Aquarium Expo in Taipei on November 9, 2012. More than one hundred fish tanks from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images

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  • Phenacogrammus interruptus fish are displayed at the 2012 Taiwan International Aquarium Expo in Taipei on November 9, 2012. More than one hundred fish tanks from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images

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  • A Pacific white-sided dolphin calf swims along with its mother Piquet, Tuesday, June 12, 2012, at Chicago's Shedd Aquarium. The baby male dolphin, which does not have a name, was born on Memorial Day. (AP Photo/Kiichiro Sato)

  • A seahorse (Hippocampus Reidi) is displayed during a news conference before the 2012 Taiwan International Aquarium Expo in Taipei November 5, 2012. More then one hundred tanks of fish from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images

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  • Georgia Aquarium's resident pregnant beluga whale Maris swims in the aquarium's tank in Atlanta, Wednesday, April 11, 2012. Maris, is expected to give birth by June. She the first mammal to conceive at the downtown Atlanta attraction since it opened in 2005. (AP Photo/John Bazemore)

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