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Genetic tests have confirmed that an outbreak of a measles-like virus known as morbillivirus is playing a major role in the massive dolphin die-off on the Mid-Atlantic coast.

This is a second big strike for the virus, which was the chief agent behind a wave of infections that struck bottlenose dolphins between June 1987 and March 1988, killing more than 700 animals before retreating into the blue. 

"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 during a telephone press briefing on Tuesday. As of Aug. 26, 333 animals have died on coasts between New York and North Carolina. 

Among 33 dolphins tested this summer, 32 dolphins have turned up with a suspected or confirmed case of the virus, Rowles said. Additional genome sequencing tests have confirmed that the cetacean morbillivirus was present in 11 animals. "Thus far, all indications from the histopathology is that this is a morbillivirus outbreak," Rowles said. 

Morbillivirus belongs to a family of RNA viruses that cause rinderpest in cattle, distemper in canine species, and measles in humans. In dolphins, the virus suppresses the immune system, so researchers are seeing "animals that are very thin, animals that have a lot of other diseases and infections," Rowles said. 

The resurgence of the infection in bottlenoses could merely be natural forces at work. Infections have "always been happening in cycles," Perry Habecker, staff veterinary chief at the University of Pennsylvania, told NBC News. 

The genetic makeup of bacteria and viruses changes slightly over time, Habecker said, and populations of animals can also lose their immunity. "There's no doubt in my mind that these kinds of disease have been [recurring] for millennia."

Researchers suspect that the virus is hosted by off-shore marine mammal populations, and was transmitted to more vulnerable coastal bottlenoses. In the spring or early summer, the virus seems to have entered a population that "simply [doesn't] have an immune response to effectively fight off the virus," Stephanie Venn-Watson of the National Marine Mammal Foundation said during Tuesday's press briefing. 

Analyses are being planned for the first cases, which might have been as early as February or March this year, to check if an environmental factor contributed as a trigger. 

In the last weeks, dead dolphins have been washing up on the North Carolina coasts, which indicates that the outbreak is moving south. 

Aside from the morbillivirus, a second prime suspect is the Brucella bacterium, which has been found in tissue samples from four dolphins from Virginia, NOAA says. Marine mammals are common hosts for the bacterium.

Nidhi Subbarman writes about science and technology. Follow her on Facebook, Twitter and Google+.