VIRGINIA BEACH—Strolling along private beaches nearby, where waves lap against shores dotted with summer homes and volleyball nets, it’s easy to forget that the ocean has given up more than 250 dead bottlenose dolphins this summer. Sometimes dolphins wash up alive, emaciated and laboring with their final breaths. Some are missing small chunks of their fins—evidence that a shark took an exploratory bite when the animal was slowed by disease or after its organs gave out. Often the bodies are not in a state for scientists to know the difference.
The Virginia coastline hasn’t been the only place where dolphins are washing ashore, but it’s been hit the hardest. From New York State to North Carolina, reports of hundreds of dolphin deaths have accrued this past summer. So alarming was the toll that the National Oceanic and Atmospheric Administration (NOAA) declared the die-offs to be an “unusual mortality event” and released federal funds to shore up an investigation. The likely culprit, NOAA says, is an RNA virus related to measles. But the high death count and presence of myriad secondary infections have led some researchers to suspect a wider problem—namely, a coastal ecosystem possibly sickened by human activity.
Clues in the past
Twenty-five years ago morbillivirus ravaged the coastal bottlenose dolphins, wiping out some 50 percent of their estimated population by spring 1988. Scientists already fear what the loss may be this time, because 553 dolphins have stranded just in the past three months. The death toll for the same period in the 1987 die-off was only between 300 and 400 dolphins, climbing to 742 by the spring of the next year.
Scientists have no treatment for dolphins infected with the morbillivirus, an RNA virus similar to those that cause measles in humans, distemper in dogs and rinderpest in cattle. Nevertheless, they want to track the deaths and identify their causes because dolphins, atop the food chain, help serve as a barometer of ocean health.
In years past dolphins had apparently built up some level of antibody defense against the virus so that contracting it did not always lead to death. That was evident in the low mortality counts: In the past six years the average number of yearly strandings from New York to North Carolina was less than 160. Since July, however, more than 500 coastal dolphins have died in that same area. In Virginia, where the annual death toll for dolphins hovered below 80 for the past decade, the surge has reached 286 deaths. Researchers anticipate that, like the last die-off, this number will continue to rise through next spring.
What makes the disease so lethal this year is an enduring mystery. Researchers suspect coastal dolphins caught the virus from offshore populations this year and the former were unable to mount a strong response to this viral strain. Viruses continually mutate, swapping and rearranging genes, so the explosion of the virus could just be nature at work.
On the other hand, pathologists have noticed another factor that could be a handmaiden in the killings: a plethora of secondary infections by fungi, bacteria and parasites. So far, it’s unclear whether these infections could have been potent enough to kill the dolphins on their own. Their presence, however, has left some researchers wondering if humans are to blame—specifically, have poor environmental conditions fueled by agricultural runoff and other human activities made dolphins unable to weather the diseases?
The theory goes that some dolphins encountering morbillivirus would have been able to recover from the infection, but those secondary infections preyed on the vulnerable ones, finishing them off. Biotoxins that dolphins had accumulated in their blubber may also have been released as the weakened mammals broke down their fat for sustenance, flooding their systems with toxins that hamper an immune response.