Sea star wasting disease is caused by a virus
For nearly a year and a half, sea stars – in particular, those of the class Asteroidea have been suffering from mass die-offs. The cause of the widespread sea star mortality has been uncertain, so it has simply become known as “sea star wasting disease” (SSWD). But now a large group of researchers from sixteen different institutions thinks they may have found the cause: a tiny virus.
The asteroids that have been dying all come from the Pacific coast of North America, and while sea star wasting disease has been known at least since the 1970s, the current outbreak is notable for its extent, stretching from Baja California all the way to Southern Alaska, and also for the fact that some twenty asteroid species are susceptible. While there is some variation from species to species, the disease progresses in a predictable way. It starts with behavioral changes; sea stars become lethargic. Then their limbs begin to curl. Then they begin to show lesions, they go limp. The arms detach and walk away from the body. Then, finally, death. The skin becomes inflamed, necrotic, and ulcerated. Individuals who show symptoms of SSWD rarely regain their health in the wild and are only slightly more likely to do so in laboratory settings.
The origins and causes of SSWD have proven hard to uncover. Some have suggested that storms or anomalies in water temperature were to blame, while others have pointed towards starvation or infection. Others have suggested that anthropogenic pollution was the cause.
The manner in which SSWD has popped up in distinct communities along the Pacific coast of North America supports the idea that SSWD is contagious. It moves from site to site like an infectious disease would. In addition, of all the big aquariums on the coast, it was only those that did not sterilize inflowing seawater with UV light that were infected by SSWD. That those aquaria that filtered their incoming seawater through sand still wound up with SSWD suggested that the disease could be transferred via some microscopic agent, rather than requiring physical contact with an infected individual. “Overall,” write the researchers in this week’s issue of the Proceedings of the National Academy of Sciences, “these patterns suggest a microscopic, water-borne, infectious disease agent, rather than environmental pollutants.”
Analysis of infected individuals had not indicated microbial infection, so the researchers wondered if a virus could be to blame. They collected adult sea stars (Pycnopodia helianthoides) that didn’t yet display any symptoms from several sites in Washington’s Puget Sound, prior to any known SSWD occurrence at those sites. The animals were placed into aquaria containing seawater that had both been sand-filtered and UV-treated. Then half of them were given a treatment derived from diseased animals, others were given inert versions of the same treatment as a control. Then, the researchers watched to see what would happen.
The sea stars injected with what the researchers called “virus-sized material” showed signs of the disease after 10 to 17 days, while the control animals never showed any symptoms. The researchers conducted the experiment again, this time extracting the “virus-sized material” from one of the diseased individuals from the first experiment. Again, inoculated individuals showed symptoms after 8 to 17 days, while control animals never became ill. Together, this convinced the researchers that “the disease is transmissible from symptomatic to asymptomatic individuals and that the pathogenic agent is virus-sized,” or smaller than 200nm in diameter. Further analyses of infected individuals identified a virus that the researchers have named “sea star-associated densovirus” (SSaDV).
Turning back to the field, the researchers collected 286 affected sea stars and 49 asymptomatic asteroids from 14 different species. While the SSaDV virus was present in both symptomatic and asymptomatic sea stars, the number of viruses per milligram of tissue, or the viral load, was higher in affected than unaffected individuals. Why would the unaffected individuals have had SSaDV present? It could be that there were infected but not yet symptomatic, as the laboratory experiments showed that it took one to two weeks before symptoms would appear. It could also be that the virus was present on the exterior surfaces of the animal and had not yet actually infected its tissues. Still, individuals were more likely to be diseased if they had a high viral load, further implicating the virus. Further, the virus was detected in samples of seawater itself, as well as in sediments collected from aquarium tanks. “Water column SSaDV transport helps explain how SSWD spreads among” distinct groups of sea stars, they say.
Because SSaDV affects such a wide range of species in such a wide range of locations, the researchers wondered whether it could be responsible for prior SSWD outbreaks. To find out, they looked for the virus in preserved museum specimens collected in British Columbia, Washington, Oregon, and California as far back as 1923, as well as in asteroids from the North Atlantic coast and from Connecticut’s Mystic Aquarium. They found evidence of SSaDV in the museum specimens at least as long ago as 1942, suggesting that the virus could have been infecting Pacific coast sea stars for more than seventy years. They also found it in the Atlantic individuals, suggesting it is present in other ocean basins.
They also found the virus in other non-asteroid echinoderms, like basket stars (Ophiuroidae). It’s not known whether those animals suffer from the infection, or even whether they can become infected at all, but it is possible that they act as a reservoir, keeping the virus circulating through an ecosystem even after its hosts (the asteroids) are all but wiped out.
Given its presence in multiple oceans and for such a long time, why is the current outbreak so severe? The researchers propose that it could have become epidemic only recently due to environmental factors that make animals more susceptible or that make transmission more likely. It is also possible that the virus has evolved recently to become stronger.
“It remains to be seen how infection with SSaDV kills asteroids,” they warn. “What the role is for other microbial agents associated with dying asteroids,” they wonder. Is it the virus itself that does the damage, or does the virus somehow make it possible for other microbes to deliver the final blow? Hewson also wants to discover “what triggers outbreaks, and how asteroid mass mortalities will alter near-shore communities throughout the North American Pacific Coast.” As is often the case, every answer brings with it dozens of new questions. And it’s unlikely that this virus is the final word on SSWD. It’s as if we have found several of the corner and edge pieces, but still aren’t sure what the complete puzzle is supposed to look like. – Jason G. Goldman | 19 November 2014
Source: Hewson et al. (2014). Densovirus associated with sea-star wasting disease and mass mortality. PNAS.
Header image: ©NOAA photo library
*Note: This article has been updated to reflect that Asteroidea is a taxonomic class, not a family.
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