March 3, 2015

Prof. Ian Hewson Examines Virus Killing Hundreds of Thousands of Sea Stars

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The mystery behind massive sea star deaths on the East and West Coast may finally be solved. Sea stars, commonly known as starfish, have been dying by the hundreds of thousands since June 2013, according to Prof. Ian Hewson, microbiology. The disease, which causes them to grow lesions and lose their limbs, is called Sea Star Wasting Disease.

After a year of investigations, Hewson and his colleagues have found a likely cause: the Sea Star Associated Densovirus. The findings, which were published in the December issue of the Proceedings of the National Academy of Sciences, came as a surprise to some.

“I think that some people were a little disappointed that it was not an environmental contamination issue directly causing the deaths,” Hewson said. “We received a number of emails from folks who were convinced that it was caused by radioactivity from Fukushima Daiichi [Nuclear Power Plant], but we just have no evidence for that whatsoever.”

While other scientists investigated possible environmental causes, Hewson’s research focused on what microbiological causes, such as bacteria or viruses, might make healthy sea stars become diseased.

From a microbiological perspective, sea stars are unique to study because numerous bacteria live—quite peacefully— inside and outside their bodies. According to Hewson, up to three thousand species of bacteria may inhabit one sea star.

Hewson likened the abundance of bacteria co-existing with sea stars, to the bacteria that exist in and on humans—in our guts, on our skin and so forth.

“Marine animals are absolutely coated in bacteria [and other microorganisms],” Hewson said. “Yet we do not know what role they play in host health, unlike humans, because there has not been a lot of research on aquatic animals.”

In order to find out if a bacteria or virus might be behind the disease, Hewson and his team compared several diseased and healthy sea stars. They found that the diseased sea stars harbored more of one particular virus than the healthy sea stars.

“Really the only type of virus that was more [abundant] in the diseased than the healthy was the sea star associated densovirus,” Hewsons said.

They also injected the virus into healthy sea stars and saw the same symptoms that the diseased sea stars exhibited.

Hewson cautioned that the virus, while a likely cause, was not necessarily the definite cause.

“We cannot say for sure that this is the cause of the disease because we have not satisfied the criteria to distinguish between a pathogen and an opportunist,” Hewson said, “but it is definitely our top candidate.”

The challenge in proving that the densovirus is not just coincidental is that there are currently no ways to test the virus’s effect on sea stars that do not already contain numerous microorganisms.

Scientists normally test such effects by using “cell lines,” or a group of cells that contain the exact same genes. While Hewson would theoretically use echinoderm cell lines, as sea stars are a type of echinoderm, such cell lines have not yet been developed.

According to Hewson, the closest available cell lines scientists have to test the densovirus’ effects are human cell lines, though they are dissimilar due to evolution.

“Once we have [echinoderm] cell lines then we can start to propagate the virus and do experiments in isolation of all the other bacteria and viruses that are there,” Hewson said.

Although much of the evidence points to the densovirus as the likeliest cause of sea star deaths, environmental contributions have not been ruled out.

“We definitely have not excluded effects like acidification or temperature change,” Hewson said. “But the purpose of our study was to identify the proximal cause of the disease, and we found a virus that was associated with it.”

According to Hewson, the virus was first discovered in the 1940s, and was present in aquariums in the Pacific Northwest. The sudden virulent outbreak, after nearly 72 years, he said, could be the result of a biological or environmental change. It is possible that the virus has mutated to affect more species, or an environmental factor has made the sea stars more vulnerable.

While the long lasting environmental effects of decimated sea star populations remain to be seen, it is likely that the disease will significantly impact the ecosystem. According to Hewson, sea stars are keystone predators, meaning their loss will result in more abundant prey.

“Diseases in the marine environment play a huge role in ecology, generally by knocking out important predators like sea stars,” Hewson said.

Indeed the bad rap that viruses get may not be completely warranted. According to Hewson, viruses maintain biodiversity by targeting the most dominant predators, allowing less common species to exist, and might even play a role in combating cancer.

“There is as yet a really unstudied role of viruses in being beneficial to organisms,” Hewson said.

As for the sea star associated densovirus, Hewson said his next step is finding where the disease spread ends.

“This is definitely one of the biggest, most high profile studies I’ve been involved with,” Hewson said. “There was just an incredible outpouring of public interest in science, which was fantastic.”