NORFOLK -- An outbreak of a disease that has killed about 275 bottlenose dolphins along the mid-Atlantic coast has ended as mysteriously as it began, according a researcher investigating the deaths.

"We're seeing the trailing edge of mortality, animals that have managed to hold on for the longest time," Joseph R. Geraci, a marine pathologist, said Thursday. "We're not seeing any more acute disease."

Geraci, a marine mammal expert from the Ontario Veterinary College, said in an interview with The Virginian-Pilot and The Ledger-Star that viruses are the prime suspects in the two-month-old epidemic of dolphin deaths.

The illnesses may have been triggered by changes in ocean currents, weather or food supply, but it is improbable that pollution played a role, he said.

"As the condition progressed, it became more and more obvious we were dealing with an infectious disease," Geraci said.

Geraci will return to Guelph, Ontario, this weekend to start analyzing information gathered in about 30 necropsies done here since the investigation began Aug. 6. Other researchers will analyze raw information about the dolphin population, weather, currents and water quality.

Three researchers will remain in Virginia Beach to collect dolphin carcasses, perform necropsies at the Little Creek Naval Amphibious Base and collect tissue samples. The Virginia Museum of Marine Science will continue to field reports of dolphin strandings, he said.

An average of two to three dead dolphins a day continues to come ashore in Virginia, said James Mead, marine mammal curator for the Smithsonian Institution. The death rate represents a decline from the five reported daily in late July and early August.

Geraci determined last month that dolphins were being killed by common ocean bacteria. But he said he is unsure why the animals were vulnerable to usually innocuous agents.

"A virus is likely because of the way it spread through the population," he said.