ADVERTISEMENT

Massive Dolphin Die-Off Eludes Final Explanation

The morbillivirus is the prime suspect in the unprecedented and ongoing die-offs, but other culprits are possible



NOAA/Virginia Aquarium

Almost every day the bodies wash ashore. Sleek, once-powerful swimmers now lie in the surf, wasted by disease and pocked by lesions. Sometimes fishermen spot the creatures in their final throes of illness, swimming erratically before stranding themselves on the beach. The death toll has now climbed to 1,441.
 
But more than a year after the die-offs started to climb upward, scientists are still grasping for answers about the cause of the bottlenose dolphin deaths that have piled up since last summer along coastlines from New York State to Florida. The leading theory is that a pathogen called cetacean morbillivirus, an RNA virus related to measles, is fueling the deaths. And many of the creatures have tested positive for the disease. But the epidemic among the beleaguered creatures has shown only slight signs of slowing: 65 dolphin deaths occurred last month, nine more than in June, but more promising than the 114 body count in July 2013.
 
On the Virginia coast, where dolphins have been hardest hit, the average yearly toll for strandings is 64. But since last July nearly 400 dolphins have washed ashore just in that one state. By all measures, the death toll appears to be worse than past epidemics, lasting longer and taking a greater toll than the one caused by morbillivirus infection some 25 years ago. From the summer of 1987 through May of the next year, 742 dolphins died from that virus.
 
The good news is the current die-offs appear to be limited to dolphins—researchers have not noticed any mortality increase in other sea creatures—which suggests, alongside other evidence, that the killer is not something like algal blooms. Yet dolphins’ place at the apex of the food chain also raises the specter of bioaccumulation of toxins playing a role in the dolphin deaths.
 
But unlike the 1987–88 epidemic, the National Oceanic and Atmospheric Administration (NOAA) now has a network of experts and volunteers who report and collect the carcasses of the stranded dolphins and attempt to determine the cause of death. Unfortunately, answers have still remained elusive. Indeed, researchers feared last fall that whatever was triggering the deaths would trickle southward with the migrating dolphins and affect dolphin stocks living there year-round, and at least some of those fears were realized. Even while the deaths stalled in Virginia this winter, the numbers climbed in North Carolina and Florida waters, where dolphins had migrated for the wintertime. In their travels they apparently mixed with new dolphin pods—creating ideal breeding grounds for infection to jump from one dolphin to another as the mammals played, fed and breathed together.
 
Now, scientists and volunteers will be watching their phones in the coming weeks, hoping for a reduction in reports of dead bottlenose dolphins. A slowdown soon would suggest the population of remaining animals will not be further decimated when the cycle of migration repeats this fall. But only time will tell.
 
Investigating the deaths has come with a steep price tag. A combination of federal, state and private funds has been spent on the investigation so far, with almost all of it earmarked for the increases in staff needed to perform necropsies or process the dead samples (including about $200,000 in federal emergency grant funds). But almost none of the money supported forthcoming testing work to glean more details about the ages and identities of the dolphins.
 
More potent threat
Although morbillivirus remains the prime suspect—it has been found in 240 of 250 tested dolphins—a barrage of other microbes has been found in the creatures as well. For example, 20 out of 64 dolphins harbored brucella, a bacteria that causes miscarriages, brain infection, blubber abscesses and pneumonia in marine mammals. That presents a complex chicken-or-egg question: Were morbillivirus-weakened dolphins prone to contracting other sicknesses? Or, is some combination of different pathogens striking the initial blows against the dolphins?
 
Moreover, if more dolphins are dying this time around, does that mean that this strain of morbillivirus is somehow more lethal than the outbreak that tore through the dolphin communities a quarter century ago? Researchers do not expect to have definitive answers anytime soon. “If the mortalities don’t pick up again this summer, we hope to have preliminary conclusions in winter—probably January or February 2015—and a final comprehensive report would be available, hopefully near the end of 2015 or early 2016,” says Deborah Fauquier, who is heading up the day-to-day operations of this investigation for NOAA’s National Marine Fisheries Service. Although the virus is related to the microbe that causes measles in humans and distemper in canines, this dolphin-killing pathogen poses no threat to humans. And so far there has been no evidence that the virus can jump to other species, but still NOAA advises humans not to approach sick or dead creatures.
 
This fall scientists plans to melt down thin slices of the quarter-century-old wax that contain DNA samples from the virus and compare its sequence to that of the current viral strain, and other strains, too—hoping to find signs of mutations that may explain the current outbreak’s potency. NOAA also plans to award contracts to scientists who will genetically test the dead dolphins to confirm what types have died. Normally, a lot of that identification would take place by comparing the animals’ dorsal fins, but because the carcasses often wash ashore in an unidentifiable state (read: thoroughly decomposed or chewed on by sea life) their exact identities have often remained mysterious. Other researchers will conduct testing on the dolphins to look for signs of biotoxins that may have also played some role in the dolphins’ deaths.
 
Still, with no vaccine or treatment for dolphins suffering from morbillivirus in the wild, this new data will likely only equip responders to better save the existing population—safeguarding the habitat of certain bottlenose dolphins that have been hit the hardest.
 
 

Rights & Permissions
Share this Article:

Comments

You must sign in or register as a ScientificAmerican.com member to submit a comment.
Scientific American Holiday Sale

Black Friday/Cyber Monday Blow-Out Sale

Enter code:
HOLIDAY 2014
at checkout

Get 20% off now! >

X

Email this Article

X