Mysterious Disease Wreaks Havoc on Sea Stars
by Luisa Loi
The causes that lead to the disease are still unknown and recovery is inconsistent.
A mysterious illness that appeared eight years ago and nearly wiped out sea star populations along the Pacific Coast is continuing to leave researchers puzzled.
The disease is called sea star wasting disease and Benjamin Miner, Western Washington University’s Biology Department Chair and researcher, said there appears to be evidence showing that almost all species of sea stars are vulnerable to it.
Benjamin Miner said, among the severely impacted species, there is the giant sunflower star, which died in massive numbers when the disease had just appeared.
“I would probably say that only less than 1 percent of the [sunflower sea star] population remains in comparison to before the disease,” Benjamin Miner said.
The ochre sea stars were common in subtidal areas too and were depleted by the disease. “Just from probably those two species alone, I would say, probably, many hundreds of millions of sea stars died as a result of the disease along the coastline,” Benjamin Miner said.
Benjamin Miner does not know of any sea star species that have gone extinct. He said they can be “locally extirpated,” which means they are gone from some sites where they used to live.
Researchers initially saw mortality near Vancouver, B.C., and Seattle, which only appeared months later in the other areas of the Puget Sound and the Salish Sea, Benjamin Miner said. He said that when the disease had made its debut in the area, he showed a reporter some sites in Bellingham, where they saw a modest amount of dead sea stars. He then returned a week or two later.
Michael Kyte, a marine biologist with nearly 60 years of diving experience, said he has never seen anything like this before.
“There have been outbreaks of what people now recognize as a form of sea star wasting disease in the past, but nothing this widespread or this intense where we lost 90-plus percent of the sea star populations in general on the West Coast,” Kyte said.
Before the outbreak, Kyte said he would find mounds of live and healthy mottled sea stars, “the size of a dining room table, 2-3 meters diameters for instance, and the mound would contain upwards to 100 and 200 sea stars.” After the mortality wave in 2013-14, those mounds disappeared.
Casey Cook, the coordinator for the Marine Life Center in Bellingham, said when she began working at the center in 2010, sea stars were an abundantly seen species under our docks and on our pilings in the harbor. She said, “I remember counting over 30 stars in one small corner of the water under our docks here one morning. There were six different species.”
Sea stars would also be easily found at the local beaches, or caught by fishing nets, Cook wrote in an email. “Now, stars seem to have nearly gone from most of these places. I find maybe 10 stars a year under our dock, and there are only two species. Our beaches have some stars, but about half of them that we find exhibit evidence of wasting disease. And I no longer find the great sunflower stars anymore,” she said.
Rocky Road to Recovery
Melissa Miner is a researcher who has been observing the disease, like her husband, Benjamin Miner. Melissa Miner has been looking for potential for recovery. She said that the population has recovered when there is a large number of different specimens at different life stages.
Recovery has been erratic and inconsistent. There’s almost no recovery at all in southern California, while some areas in northern California, Oregon and the outer coast of Washington are showing signs of recovery. Melissa Miner said the Puget Sound and the Salish Sea are inconsistent as well, where they have not seen recovery to pre-wasting levels.
Covid-19 has made monitoring more difficult. Melissa Miner had to cancel fieldwork in the summer of 2020 and those who could work did so in small crews.
Melissa Miner said there have been some improvements compared to 2013 and 2014 when the sea stars were dying in masses. “The disease is still around, but it seems like it’s at a lower level than in the past,” she said.
“It’s just there,” Kyte said. In the sites he’s been monitoring, the disease is at “background levels,” where they see several healthy sea stars and a couple that are mildly or severely diseased.
According to the data he collected at Cama Beach on Camano Island this spring, there were 77 sea stars, all of which were healthy. In the winter of 2020/21, Kyte found 63 sea stars, again all healthy, while in fall he found one sea star with severe wasting disease out of a population of 226 healthy individuals.
The habitats or the animals are not disturbed in the process of data collecting, Kyte said. The process consists of first identifying the species, then assessing the appearance of the sea star by looking for lesions or missing limbs, and measuring the size, from the center of the disk to the tip of the longest arm. Then, they are rated according to a healthy-mild-severe scale, he said.
What’s Causing the Illness?
Melissa Miner said there’s the hypothesis that the disease might be caused by warmer oceans. “There have been previous wasting events, mostly in southern California, and those wasting events were always timed with warm water. And then, as soon as the water would get cold, the disease would disappear.” However, she said it’s hard to determine whether temperature is the reason.
Kyte warily thinks it could have a connection with water temperatures, as he explained that warmer water contains less oxygen, and less oxygen on the surface of the sea star encourages the formation of bacteria that could cause an infection.
Melissa Miner said that she believes it’s more likely that it’s not water temperature in itself, but a deviation from what the normal temperature should be during that time of the year. It is still uncertain. “There seems to be a potential link in some areas, but in other areas it doesn’t seem as clear,” she said.
Melissa Miner also said there is a hypothesis that the activity of bacteria might cause low levels of oxygen that could contribute to the development of lesions and tissue decay in sea stars. Miner said she’s hesitant to embrace these findings, because the hypothesis was tested in a controlled and artificial setting, and not in a natural one.
Initially Benjamin Miner worked on a study (https://www.pnas.org/content/111/48/17278) that found a particular virus — called densovirus — to be a possible primary cause of the disease, but that possibility is currently almost certainly nonexistent, he said.
Benjamin Miner said in 2018 he experimented on microbes obtained from swabbing the surface of sea stars. He said they grew the microbes in different conditions to determine whether they were able to grow on collagen, which is what sea stars’ skin is made of, and if those microbes were able to degrade cells and create lesions. They identified a particular bacteria that causes similar diseases in sea cucumbers.
They were unable to continue the experiments as the bacteria was found to be able to grow in conditions similar to the human body temperature. Benjamin believes that whatever is causing the disease is not a great concern for humans, as he recalled that he would personally dive to collect the degrading sea stars, where the water could have potentially been infected as well.
Benjamin Miner said there is evidence that treating the water with antibiotics could help in part, but it “certainly didn’t protect individuals over the long term,” which could be because their immune systems are compromised. He said he doesn’t expect great efforts to treat the sea stars in their habitat, because dumping antibiotics into the sea would be risky for the ecosystem and potentially unproductive.
Cook said that the Marine Life Center invested in a UV filtration system to reduce the impacts of the wasting disease.
“We are able to keep our stars healthier with this kind of filter, which aids in reducing bacteria that causes secondary infections in stars that have the disease,” Cook said. “The sea water runs through the filtration unit, where the water is subjected to ultraviolet radiation, which is proven to reduce bacteria and viruses.”
When asked about secondary infections, Cook said she saw sea stars melt entirely, and sea stars whose legs had detached from their central disc, which is their body.
Cook said UV light exposure can prevent spreading wasting disease to healthy stars in the exhibit.
“Our light is rated for wasting disease specifically,” she wrote. “This will help ease symptoms of infection in stars that are already infected and hopefully prevent our healthy stars from exposure. I am no expert in wasting disease, and we do not study the disease itself. However, it is hard not to be a part of the evolution of the understanding of this terrible disease when we work so closely with these animals every day.”
With less sea stars around, the environment has been going through drastic changes.
Kyte said that the sunflower sea star is a keystone species — one that controls its habitat and the populations it shares the habitat with.
“Our local ochre star [Pisaster ochraceus] predates animals like mussels, and, without the stars, we have seen mussel numbers dramatically increase,” wrote Cook, explaining how this might have led to bacteria killing the mussels.
The areas that have less oxygen, Cook said, need to maintain their predator-prey balance to avoid the formation of bacterial blooms, as they have the potential to infect other shellfish and impact the fishing economy. “When one species fails to thrive, it allows for others to prevail,” Cook said.
Luisa Loi is a journalism student at Western Washington University — her work mostly focuses on the environment. You can find more about Luisa and her work at luisaloi.journoportfolio.com, or follow her on LinkedIn.
This article was first published in Western Washington University’s The Front: https://www.westernfrontonline.com/article/2021/05/a-mysterious-disease-is-wreaking-havoc-on-pacific-coast-sea-stars