A widespread die-off of bottlenose dolphins off the Mid-Atlantic Coast — the worst of its kind in more than a quarter-century — almost certainly is the work of a virus that killed more than 740 dolphins in the same region in 1987 and 1988, marine scientists said Tuesday.

Since the beginning of July, 357 dead or dying dolphins have washed ashore from New York to North Carolina — 186 of them in Virginia. Authorities have received numerous additional reports of carcasses floating in the ocean, said Teri Rowles, director of the marine mammal health and stranding response program for the National Oceanic and Atmospheric Administration’s fisheries service. The actual number of deaths is certainly greater, she said.

The cause is thought to be cetacean morbillivirus, which has been confirmed or is suspected in 32 of 33 dolphins tested, she said. Marine officials are looking at the possibility of other factors, including high levels of polychlorinated biphenyls and other chemicals in the water, but have not linked the die-off to anything else.

From 2007 to 2012, the average number of yearly strandings — when dead or dying dolphins wash ashore — in the same states was 36, Rowles said.

“If, indeed, this plays out the way that die-off occurred, we’re looking at the die-off being higher and the morbillivirus spreading southward,” Rowles said. The 1987-88 episode affected 50 percent of the coastal migratory bottlenose dolphins, according to NOAA’s Web site, leading them to be classified as “depleted.”

The virus poses no threat to people, although it is related to the virus that causes measles in humans and distemper in canines. So far, there is no evidence of the virus jumping to other species, but other animals that have washed ashore are being tested, the scientists said in a telephone news conference Tuesday afternoon.

Secondary infections could be dangerous. Authorities urged people to stay away from stranded dolphins.

“For people not trained in working with these animals and who don’t understand the risk, it’s much better . . . to stay away from them, particularly if you have open wounds,” Rowles said.

It is not clear what started the most recent problem, but Jerry Saliki, a virologist at the University of Georgia, said enough time had probably passed since the last mass die-off that herds of dolphins now lack natural immunity to morbillivirus. The virus is spread by direct contact between the animals or inhalation of droplets exhaled by infected dolphins above the water’s surface.

“When the collective immunity drops below a certain, critical point, which we don’t really know for marine mammals, then the whole population becomes susceptible,” Saliki said. Generally, the virus causes death by suppressing the immune system, leaving the dolphin vulnerable to pneumonia and other lethal infections.

The large number of deaths in Virginia “is really not surprising if you understand how the population of dolphins works,” said W. Mark Swingle, director of research and conservation for the Virginia Aquarium and Marine Science Center, which is part of a network of agencies that responds to marine animal strandings along the East Coast.

“I think we have more suitable habitat, we have more coastline. The Chesapeake Bay is good habitat for dolphins, especially the lower bay. . . . We just have more dolphins,” he said.

This weekend, the aquarium’s staff and three pickup trucks collected 30 stranded animals, Swingle said. The aquarium has collected at least one for 35 consecutive days. Most were dead, but some had to be euthanized. Some were bitten by sharks, probably after they died in the surf, he said.

“When you get into an event like that, it just becomes a bit overwhelming, and it’s discouraging to say the least,” he said.

For Bob Schoelkopf, this has been a summer like none since 1987.

“We’re up to 74 animals since July 9,” said Schoelkopf, founding director of the Marine Mammal Stranding Center in Brigantine, N.J., referring to the number of dead or dying dolphins his group has collected.

The organization has picked up as many as five bottlenose dolphins in a single day, all with telltale signs of the virus, such as black lesions in their mouths, or with shark bites, he said.

Schoelkopf said that struggling dolphins, especially in shallow water, may attract sharks, so it is important for people to stay away from them in the surf.

Schoelkopf, who collected carcasses during the 1987-88 morbillivirus die-off, said he quickly notified NOAA and other authorities when the first dolphin washed ashore in Seaside Park, N.J., on July 9. Preliminary testing indicated the presence of the virus, which NOAA confirmed, he said.

In 1987, it took “two or three years to finally come up with a decision” on the cause of the die-off, he said. This time, “we knew to look for it, and we knew what to look for. Before this was like a search in the dark.”

Little can be done even now that the cause is nearly certain, marine scientists said.

“There isn’t anything we can do to stop the virus,” Rowles said. “In terrestrial populations, there are vaccines that are developed and [that] are easily usable. We don’t have a vaccine that can be easily deployed.”