Since July 1, 333 dead dolphins have washed ashore from North Carolina to New York, 10 times normal levels. In Maryland, 18 have been found dead since July 1, with most of those in the past few weeks.
The virus could remain a threat to the dolphin population through next spring, the scientists said.
The most promising theory is that the dolphin population gradually lost resistance to the disease, known as morbillivirus, since it caused a similar epidemic in the late 1980s. It's also possible that pollution or other environmental factors are making the creatures more susceptible to illness, said Dr. Teri Rowles, head of the National Oceanic and Atmospheric Administration's marine mammal health and stranding response program.
It could take weeks or months for a better understanding of the outbreak to emerge from data obtained in necropsies of the animals. The deaths could meanwhile continue for months, if history is any guide. It took a year for the 1980s outbreak to run its course.
"We don't have a lot of insight into when it will stop," Rowles said Tuesday on a conference call with reporters. "Typically, outbreaks last as long as there are susceptible animals that can be infected."
Only a tenth of the reported dead dolphins have been confirmed cases of morbillivirus, Rowles said. It can take up to six weeks from the time a carcass washes ashore before the virus can be confirmed, she said. Many of the tests remain pending.
The number of dead dolphins reported in Maryland spiked this month from a total of seven reported as of Aug. 5, according to Dr. Cindy Driscoll, a veterinarian and the state Department of Natural Resources's fish and wildlife health program director. Twenty-six of the animals have been found dead along the state's shores so far this year, compared with eight to 10 in a normal year.
The state's first reported live dolphin stranding this year also was reported last week, and appeared related to the die-off.
National Aquarium scientists, who contract with NOAA to respond to reports of live dolphin strandings, euthanized a dolphin at Assateague Island National Seashore that was found to have symptoms similar to the others, said Jennifer Dittmar, the aquarium's stranding coordinator. They worked with Johns Hopkins Hospital's comparative pathology lab to perform a necropsy and found the animal had signs of pneumonia as well as an empty stomach and intestines.
While the bulk of cases have been reported this summer, 488 dolphin carcasses have washed ashore since Jan. 1 between New York and North Carolina, three times the normal levels, according to the NOAA's fisheries department. In recent years, 150 to 170 dead dolphins have been found annually along the same stretch of coastline.
Diseased dolphins typically exhibit lesions on the skin, mouth, lungs or brain, and often wash ashore thin and malnourished, officials with the Virginia Aquarium and Marine Science Center said. Dolphins contract the virus by breathing it in. It can be passed through direct contact with infected dolphins or through air.
The virus is not believed to be hazardous to humans, said Dr. Jerry Saliki, a virologist at the University of Georgia.
Varieties of morbillivirus can only be passed among similar species, Saliki explained. For example, one variety can spread between domesticated dogs and lions in Africa, and human measles has spread to primates, he said.
But there is some risk from other bacterial infections the dead dolphins exhibited, officials said, prompting advisories that those with open wounds should not swim in some areas where large numbers of dolphins have washed up.
Scientists believe some other species of ocean life may have introduced the virus to the dolphins. Given the 25 years since the last major outbreak, in which 742 dolphins died from New Jersey to Florida, it's possible the population's vulnerability reached a "tipping point" that allowed rapid spread of the disease, said Dr. Stephanie Venn-Watson, director of translational medicine and research for the National Marine Mammal Foundation.
"What we know about East Coast populations through other studies is that many of the dolphins younger than 26 have limited to no immunity to this virus," Rowles said. "If the virus is introduced, they don't have the initial antibodies to protect them from significant illness."
Dolphins have an average life span of 20 to 25 years, but some live as long as 40 years.
It's also possible that exposure to man-made toxins and pollutants may be making the creatures more vulnerable to disease, she said, but more data are required to determine any link. Dolphins along the East Coast are known to carry high levels of chemicals known as PCBs, which can compromise some immune function. Scientists plan to look closely at an area off the coast of Georgia where PCB levels are especially high in dolphins because of a Superfund site nearby, Rowles said.
Bottlenose dolphins live in pods and can be found along the East Coast from New Jersey to Florida. The mammals spend the winter in the temperate waters off the Southern states, then come north to the bays, sounds and open waters off the Mid-Atlantic coast from May to October.
Scientists said they hope that by gathering more data on any possible common threads between morbillivirus cases, they will better understand how the outbreak is affecting the population as a whole and the various migratory groups, or "stocks," within it.
It cannot be determined how many deaths are occurring in the wild and aren't being detected because the bodies never wash ashore.
"Once we understand how many of each stock are impacted we will then have the ability to understand what the effect is on the population," Lance said. "There is a concern it has a significant population-level impact."
In the meantime, there is little to be done to stop the spread of the virus. While vaccines exist for domesticated dolphin populations, there is no practical way to administer them in the wild, Rowles said.
Eventually, the assumption is that some animals who contract the virus will survive, and once that group reaches a "critical mass" of the remaining population, the outbreak will end, Saliki said.