Scientists have identified the likely culprit in a disease that has devastated sea stars along the west coast of North America. Genomic detective work and lab experiments show that the wasting disorder is associated with a previously unknown virus. With the discovery comes a deeper mystery, however. The sea star–killing virus is far from new. The authors of the study found it in museum samples up to 72 years old, so scientists are puzzled about why the current outbreak has been so severe.
"This is probably the most extensive and devastating disease of marine invertebrates that has happened," says ecologist Bruce Menge of Oregon State University, Corvallis, who was not involved in the new research. “It’s a major concern.”
The enigmatic disease came to broad attention in June 2013, when recreational divers near Vancouver, British Columbia, and Seattle, Washington, began noticing legions of dying sea stars. The sea stars first developed lesions, then began to lose their arms, and finally decayed into piles of skeletal ossicles (bits of calcium carbonate such as a star’s plates and spines). As the year progressed, the disease was seen in more and more locations in the waters off California.
Although there have been minor outbreaks in previous decades, this one is much more widespread, and more than 20 species of sea stars have been afflicted; other kinds of echinoderms, the animal group to which sea stars and sea urchins belong, have not. Researchers have raced to collect samples and conduct laboratory experiments to investigate any pathogens that might be involved. A feature in Science earlier this year, now available for free, examined the mystery.
Scientists sent hundreds of tissue samples to Ian Hewson, a microbial oceanographer at Cornell University. When he sequenced the DNA in the samples, he discovered that a densovirus was more common in the sick stars than in ones that looked healthy. (Densoviruses are known to infect insects, crustaceans, and some sea urchins.) Additional evidence came from experiments conducted by marine ecologist Drew Harvell of Cornell and other researchers, who took tissue from sick sea stars, filtered out everything larger than viruses, and injected the tissue into apparently healthy sea stars. They developed symptoms—and, concurrently, the amount of densovirus in their bodies increased. Other sea stars injected with sterilized tissue did not develop symptoms of the wasting disorder.
“We have very good evidence that this is a densovirus,” Hewson says. But because the virus cannot be grown in culture, scientists cannot satisfy the classic tests for identifying the culprit of a disease: four criteria collectively referred to as Koch’s postulates. The researchers published their results online today in the Proceedings of the National Academy of Sciences.
Looking for some historical perspective, Hewson tested museum samples of sea stars collected between 1923 and 2010 along the U.S. west coast. The virus existed in healthy looking specimens from five different years, suggesting it has persisted in the environment. Hewson speculates that the virus may have mutated as it wiped out various species of sea stars, allowing it to infect others. He is also trying to figure out the source of the virus, by analyzing sea stars from around the world, and whether it can infect other kinds of echinoderms.
The biggest question is why the current epidemic has been so bad. A likely situation, Hewson and his colleagues say, is that an overabundance of sea stars increased the transmission of the virus, especially if they were stressed by competition for food, which could make them more vulnerable to infection.
That theory makes sense to marine pathologist Marta Gomez-Chiarri of the University of Rhode Island, Kingston, who was not involved in the new paper. She and her students have been studying an earlier densovirus outbreak on the east coast; populations of sea stars in Rhode Island’s Narragansett Bay abounded before a crash in 2011. It’s not clear whether the same densovirus that caused the west coast die-off is also involved in the eastern declines. Hewson found some densovirus genes in sea stars from Connecticut but did not have enough samples for firm conclusions.
Menge doesn’t think overabundance played a role in the current outbreak among 13 sea star populations that he follows on the coast of Oregon. Instead, he wonders whether ocean acidification, which may also be a source of stress that weakens sea stars, is a possible contributing factor. So far, the evidence is mixed for the role of acidification, Menge admits.
Whatever the cause of the epidemic, Menge says, the demise of purple stars has already led to greater survival of its prey, including barnacles and mussels. As a result, he predicts, the mussels will eventually take over the rocky shore, crowding out many other species of invertebrates. In a way, he adds, the epidemic is a once-in-a-lifetime opportunity for ecologists to study these predator-prey relationships. But that doesn’t dull the pain of losing familiar and charismatic species. “From a personal standpoint, it’s really disheartening.”