Since 2013, millions of sea stars from at least 20 species have been suffering devastating losses up and down the Pacific Coast between Alaska and Baja California -- and in a very gruesome way. It starts with inflammation and tissue ulcers and decay, and eventually their limbs pull away from their bodies, organs ooze through their skin, and they disintegrate and die. The extensive range and the number of species infected by this sea star wasting disease makes it one of the largest marine wildlife diseases ever, yet the cause is a mystery. Now, researchers may have pinpointed the deadly culprit: a densovirus that’s been festering at low levels since at least 1942. The findings were published in Proceedings of the National Academy of Sciences this week.
“There are 10 million viruses in a drop of seawater, so discovering the virus associated with a marine disease can be like looking for a needle in a haystack,” Cornell's Ian Hewson says in a statement. “Not only is this an important discovery of a virus involved in a mass mortality of marine invertebrates, but this is also the first virus described in a sea star.”
Hewson and colleagues studied healthy and infected sea stars during field surveys, and they also conducted laboratory infection studies where healthy sea stars were exposed to material from diseased sea stars -- which resulted in the onset of disease. A metagenomic analysis revealed that a previously unknown virus was the most prevalent contagious element contained in the material. They call it the sea star associated densovirus (SSaDV).
Infected sea stars from natural environments show higher levels of SSaDV than healthy sea stars, and in the lab, sea stars with the disease display increasing levels of SSaDV as their symptoms progressed.
The team is now working on understanding how the virus turned into a mass killer. After all, it’s been present in sea stars for at least 72 years: They found it in preserved museum samples collected in 1942, 1980, 1987, and 1991. The virus may have risen to epidemic levels in the last few years due to sea star overpopulation, environmental changes, or maybe a viral mutation.
"The fact that it has occurred historically indicates that while this virus may be the agent that causes the disease, something may have happened recently that caused it to go rogue, because we've never seen anything like the current outbreak," Peter Raimondi of UC Santa Cruz says in a news release.
It’s likely transported by ocean currents, and sea water, marine sediments, plankton, sea urchins, and the closely related brittle stars harbor the virus too. "If sea urchins are a nonsymptomatic reservoir for the virus,” Raimondi adds, “this might be a prolonged outbreak unless sea stars are able to develop immunity.” The virus was found in water filters from public aquariums, though didn’t spread in seawater sterilized with UV light.
Here’s some good news! Huge numbers of baby sea stars have appeared in some of monitored sites. Earlier this year, juvenile sea star numbers in the Monterey Bay region were the highest in 15 years.
Images: Neil McDaniel (top), Phil Garner (middle), Maya George (bottom)