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. Secondary infections
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. The deaths “may very well be exacerbated by human activity,” says Susan Barco, research coordinator and senior scientist at the Virginia Aquarium and Marine Science Center Foundation. “We may not be able to completely understand how the scope of the event may be impacted by human activities,” she shrugs with resignation. Until all the data is pooled it’s hard to pick up trends, says David Rotstein, a veterinary pathologist who studies the dolphin samples from his home in Silver Spring, Md. Moreover, penning the reports that identify the probable cause of death is part science, part art. “It’s like going to a museum, looking at a painting on the wall and someone saying describing that painting. That’s really what we do,” he says. There’s also little data to flesh out the big picture: Scientists do not track the incidence of nonlethal morbillivirus in healthy dolphins, largely because of the logistical challenges and cost of tracking live dolphins. NOAA has launched an emergency response and investigation, employing the same approach adopted for other federal emergencies like massive wildfires—complete with an incident command system spearheaded by federal personnel who coordinate a response across state lines. Instead of sending helicopters laden with fire retardant, they dictate where dolphin remains should be sent for analysis. NOAA also has set up a researcher loaner program that helps find scientists willing to help out with the deluge of necropsies. At the local level, Virginia Aquarium personnel have been facing an onslaught of hotline calls reporting dead dolphins that have washed ashore. That’s on top of the normal calls they respond to, which include picking up hurt or dead sea turtles. And each call triggers a flurry of action that sends volunteers to retrieve the animal and bring the injured or deceased back to the Virginia Aquarium Marine Animal Care Center. Dolphins on the table
The center is a nondescript warehouselike building replete with mats immersed in bleach solution for disinfecting shoes every time anyone exits one area and enters another. One wing features live marine creatures being nursed back to health. In another researchers are spending an increasing amount of time processing data about the deceased dolphins. A white, screened-in tent next to the building is set up for necropsies, but recently the area just outside the tent is often adorned with spillover marine life mortalities, waiting for their turn on the table. Flies swarm inside the tent, providing a buzzing soundtrack. Inside, several researchers, decked out in protective gear including N95 respirator masks, try to ignore the bugs and the pungent smell of decaying marine flesh as they take measurements of dead dolphins and empty the bodies of organs, divvying up the pieces into labeled bags or jars of solution before depositing the remaining bodily parts into a large dumpster outside. From these necropsies, pathologists map out the probable cause of death by looking at slides from various organs under the microscope, hunting for telltale fused cells that would indicate morbillivirus—akin to sunnyside-up eggs with a bunch of extra yolks. Labs across the country, including one at the University of Georgia in Athens employ genetic analysis to confirm and expand on the pathologists’ findings. “We are strictly testing what we are asked for,” says Jeremiah Saliki, a virologist and director of the Athens lab. So right now no one is teasing apart the constellation of symptoms that could provide more insights, including detecting infections and biotoxins. And that’s a problem because more could be happening than we know with potential lasting consequences for oceans. It’s not unprecedented for humans to get it wrong when it comes to dolphin mortalities. During the dolphin die-off of 1987–88 scientists fingered brevetoxin, a killer of fish, shellfish and birds related to red tides. But something didn’t add up, because red tide was reportedly absent at the time and that toxin would not explain some of the lesions found on the dolphins. An analysis of their lungs and lymph nodes some six years later found that the real killer had been morbillivirus. Testing techniques have improved since then—increasing the chances that scientists are right this time around—but the investigation is continuously evolving, and revisions in the analysis may be made. An ecosystem threat?
Although the charismatic nature of dolphins is enough to inspire some people to action, scientists’ chief concern arises from the dolphin’s pecking order in the ocean. As apex predators they may be amplifying negative effects lower in the food chain; if tiny prey organisms are getting sickened and fish eat them, and the dolphins consume the fish, the bioaccumulation of toxins might be serious. Dolphins are “sentinels of ocean health” Barco says. The very fabric of dolphin’s lives may set the stage for the die-off to worsen. Dolphins are highly social creatures; they even breathe together. As a group, dolphins may come to the surface, issue a forceful exhalation through their blowholes and then all inhale, sharing aerosolized particles expelled from adjacent dolphins. Sex, nursing and playing also lead to direct contact. Moreover, small groups are continually striking off on their own, forming new groups, and latching onto other groups, thereby repeatedly introducing diseases into new communities. Without additional information about the underlying causes of the dolphin die-offs, some researchers worry the body count may only rise. Follow Scientific American on Twitter @SciAm and @SciamBlogs. Visit ScientificAmerican.com for the latest in science, health and technology news.
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