Guilty as charged? A mysterious epidemic ravaging sea stars on North America's Pacific coast has had scientists scratching their heads over the cause. Now they have a prime suspect. The discovery is the first step in helping researchers learn what triggered the outbreak, and whether it is likely to happen again.
When sea stars began dying in great numbers last year, all the way from Mexico to Alaska, some researchers called it the biggest marine disease outbreak ever recorded. Sea stars are important predators, and their loss means big changes to marine ecosystems.
Now a team led by Ian Hewson at Cornell University in Ithaca, New York, is building a case against type of parvovirus called a densovirus.
First, their experiments in aquaria showed that infection was transmitted by virus-sized particles, and that heat sterilisation of those particles – which kills viruses – prevented infection. Second, the team sequenced viral DNA and RNA from infected and healthy sea stars. Only one virus, which they call sea star-associated densovirus, or SSaDV, turned up much more frequently in diseased sea stars than in healthy ones. Third, lab experiments showed that the amount of SSaDV in sea stars increased as individuals became more diseased.
Reasonable doubt?
Other experts are cautiously optimistic that Hewson's team has found the culprit.
"The evidence is pretty conclusive that it's viral," says Curtis Suttle, a marine virologist at the University of British Columbia in Vancouver, Canada. "It's a little less convincing that this densovirus is actually the cause of the disease, but this is the one that stands out."
One reason for Suttle's concern is that Hewson's team did not find SSaDV in every infected sea star. Hewson thinks this is perhaps because some tissue samples they analysed happened not to contain infected tissues. He is now developing sea star cell cultures in which to grow the virus for direct-infection studies, which would provide conclusive proof.
Hewson's team also found SSaDV in museum specimens dating back more than 70 years, which suggests that the virus is normally present in the environment. If so, then some other factor must have triggered the present outbreak. Hewson suspects one factor might be unusually high sea star populations just prior to the outbreak, which could have increased viral transmission rates.
Climate change or ocean acidification could also be impairing the sea stars' immune systems or stressing them in other ways, says marine ecologist Carol Blanchette at the University of California, Santa Barbara. Knowing which virus to look for may help biologists find the trigger and possibly predict or prevent future epidemics. "Having something to look for is pretty helpful," she says.
Journal reference: PNAS, DOI: 10.1073/pnas.1416625111
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