Accidents with boats and commercial fisheries leave telltale signs on dolphin corpses, Schoelkopf said, allowing experts to rule those out as possible explanations for the recent rash of deaths. Water quality and temperature also appear normal this year.
Members of the public have suggested plenty of other possibilities that experts are not taking seriously -- including fall-out from Hurricane Sandy and gas released by the Syrian government.
Instead, the sudden spike in deaths along with preliminary pathology reports and evidence against other explanations suggests that some kind of disease is to blame, and that disease is probably morbillivirus.
“We get a lot of strange calls telling us what people think might be the cause,” Schoelkopf said. “Usually, when dolphins come down with a disease that’s communicable, it spreads easily because they live in such tight-knit family groups of up to a couple hundred animals in a group.”
Among the remaining mysteries -- assuming morbillivirus is the culprit -- scientists still don’t know why or how the disease occasionally causes so much trouble. One possibility is that the virus mutated in a way that made it more virulent, much like the human influenza virus can change some years. Another possibility is that something changed in the dolphin population, making them more susceptible. Or maybe the dolphins caught the virus from a population of pilot whales that swam too close.
Once it infects dolphins, Habecker said, morbillivirus affects multiple body systems and lowers immunity, leading to secondary infections that can kill them. Pneumonia was the ultimate cause of death in some of the dolphins his group has looked at.
That kind of cascading series of events complicates the investigation into the current outbreak, and an official cause won’t be announced until more data comes in from the several labs around the country that are analyzing tissues and sequencing viruses from this year’s crop of dead dolphins.
Even though the mass die-off may seem alarming, Habecker added, it’s fairly common for diseases to strike wild animals, both in the ocean and on land.
“The big difference is that now we’re paying attention to these things,” he said. “It strikes many people as new and ominous, but the reality is that this is just a cycle of nature.”