Virus implicated in starfish wasting disease

A team of scientists have figured out what’s killing West Coast sea stars, but bigger mysteries remain.

 

Last fall, after millions of West Coast starfish were found dead and dying, a team of 25 microbiologists, epidemiologists, marine biologists and other scientists from around the country set out to determine what was killing them. Now they have an answer — and even more questions. 

Here’s what they know: The culprit responsible for one of the most deadly marine diseases ever recorded is a type of densovirus, a microbe that usually attacks crickets and other insects. While scientists had never associated densovirus with marine invertebrates before, it’s been in the ocean for at least 72 years: The researchers found traces of the virus in ethanol-preserved starfish specimens from the 1940s and in healthy sea urchins alive today in Hawaii. 

But here’s what they don’t yet know: If the virus has been around for so long, why did it go from benign microbe to purveyor of an epidemic that, since June 2013, has caused up to 95 percent of local sea star populations from Baja California to Alaska’s Kenai Peninsula to dissolve into puddles of goo, altering the makeup of intertidal ecosystems for years to come? 

A starfish in the early throes of sea star wasting disease.
Katie Campbell/EarthFix

Ian Hewson, a Cornell microbial ecologist and one of 25 co-authors of a breakthrough paper released yesterday in the Proceedings of the National Academy of Sciences, has a few ideas. Before the epidemic emerged last year, sea star populations in the Pacific Northwest were “absolutely enormous,” Hewson says. “And when you get a large concentration of the same species in any small area, the contact rates between virus and host is extraordinary.” More potential hosts means more chances for the virus to mutate, and Hewson thinks that at some point, a random mutation caused the virus to become exponentially more lethal. 

But why did sea stars become so abundant to begin with? Again, no one’s sure, but nutrient pollution could be to blame. Most sea stars die because they run out of food when they’re still tiny, plankton-eating babies. But both urban and industrial runoff and natural cycles have caused more plankton blooms in recent years, which could have fueled a surge in the number of baby starfish that survived to adulthood.

Yet Hewson points out that densovirus isn’t just a killer — viruses are an important part of natural population cycles. “They’re a predator,” he says. “And they help increase diversity. When you have abundant populations of a single type of organism, viruses will (knock them back), permitting less abundant species to compete for space and resources.” So not only will starfish recover, but other animals might thrive as well. 

It’s also important to note that while densovirus has been fingered in this outbreak of sea star wasting disease, it’s not necessarily responsible for smaller die-offs during El Niño years in the 1980s and ‘90s, when the deaths were linked to warmer ocean temperatures. “There are only so many ways a starfish can die,” Hewson explains. “You can get the same sort of symptoms from putting one in an aquarium where the salinity isn't ideal.” In other words, starfish that are dying — either from increased salinity, warmer temperatures or microbial pathogens — all look the same, so what’s collectively been called starfish wasting disease might not actually be a single phenomenon. 

The new densovirus isn’t the only development on the sea star wasting front: There are also signs that the current epidemic is subsiding. Scores of juvenile sea stars are maturing into adults — which is a good thing this time around, given that adult populations have become locally extinct in some places. Even more encouraging, some juveniles with high concentrations of densovirus appear perfectly healthy, suggesting that the animals might be developing resistance to the disease. 

But that doesn’t mean the scientists’ work is done. If the past year’s race to solve the sea star mystery has taught Hewson anything, it’s that we know shockingly little about the ocean’s microbial communities. Learning more about them — including how widespread the densovirus is, what other animals it can infect and what caused it to suddenly turn deadly — will only become more important as warming and acidifying oceans impact marine ecosystems in ways we haven’t yet imagined.

 Krista Langlois is an editorial fellow at High Country News. She tweets @KristaLanglois2.