Starfish-killing bacteria revealed as cause of biggest undersea disease outbreak

Content warning: This article contains a graphic image of a starfish in the late stages of sea star wasting disease.

Severed limbs, protruding organs and melting bodies. While it may sound like the aftermath of a battle, this is what happens to starfish infected with sea star wasting disease (SSWD).

Also known as sea star wasting syndrome, this illness rapidly rose from obscurity to become the biggest marine epidemic on record. It’s estimated that billions of starfish have been killed by SSWD as their arms drop off and their bodies fall apart.

The sunflower sea star has been the disease’s greatest victim, losing over 90% of its population and becoming Critically Endangered in less than a decade. However, there may finally be some hope for these beleaguered starfish populations.

Researchers in the USA and Canada have now been able to identify the cause of the disease for the first time. They found that Vibrio pectenicida, a bacterium best known as a scallop pest, was able to cause the full range of symptoms of SSWD.

Dr Hugh Carter, one of our marine invertebrate curators and an expert in starfish, has welcomed the team’s research as a first step to tackling the disease.

“There have been lots of competing theories over what causes sea star wasting disease, so it’s great that we know what is actually causing it,” Hugh says. “It’s a massive step forward, but we’re still a long way from a solution.”

“While knowing the bacteria that cause it might allow us to detect new outbreaks earlier, it’s still hard to quarantine or treat the disease in the 3D dynamic environment of the sea. It’ll take a great deal more research until sea star wasting disease can be managed effectively.”

The findings of the study were published in the journal Nature Ecology & Evolution.

What is the impact of sea star wasting disease?

The epidemic began in 2013, when researchers noticed that starfish along North America’s Pacific coast were falling victim to a new kind of disease. More than 20 different species were affected, from those in the cold waters off Alaska to the warmer seas around Mexico.

“Sea star wasting disease is pretty horrid,” Hugh explains. “At first, it causes the starfish to stop feeding and forms white lesions across the body as the soft tissue starts breaking down.”

“When it hits the water vascular system, which starfish use to move, the animals go all limp. Their arms can detach and crawl around by themselves, almost as if they’re pulling themselves apart. Eventually, the animal dies as it disintegrates into a mush.”

The disease’s impact hasn’t just affected starfish, but the wider northern Pacific coastal ecosystem. Dr Melanie Prentice, the study’s lead author, explains that the loss of starfish means that they are eating fewer sea urchins, allowing these grazers to run rampant.

“When we lose billions of sea stars, that really shifts the ecological dynamics,” says Melanie. “In the absence of sunflower stars, sea urchin populations increase.”

“This means the loss of kelp forests, which has broad implications for all the other marine species and humans that rely on them. So, losing a sea star goes far beyond the loss of that single species.”

But understanding exactly what causes SSWD has been difficult. The disease is so rapid, with some starfish dying within a week of exposure, that scientists are often too late to investigate. Starfish also respond to a variety of stresses and diseases in the same way, making it hard to untangle SSWD from other illnesses.

While there were early suggestions that a virus might be responsible for SSWD, subsequent tests ruled it out. Now, after four years of experiments, the researchers believe they have the definitive cause.

Vibrio pectenicida: the cause of sea star wasting disease

To find out the causes of SSWD, the team raised sunflower sea stars in their lab.

They then began exposing the animals to infected parts of sea stars which had been treated in different ways. Some starfish were simply brought into contact with the body parts, while others were injected with a cocktail of its contents. Overall, 92% of the starfish exposed to the infection died.

However, if the injections were first filtered then the individuals didn’t get the disease. The starfish also weren’t infected by heat-treated injections, suggesting that whatever was causing SSWD was probably alive.

The team then took samples of coelomic fluid, which is essentially the starfish’s equivalent of blood, from both healthy and diseased individuals. Dr Alyssa Gehman, a co-author of the study, says the researchers were struck by the difference.

“When we compared the coelomic fluid of exposed and healthy sea stars, there was basically one thing different: Vibrio,” says Alyssa. “We all had chills. We thought, ‘That’s it. We have it. That’s what causes wasting’.”

While this was telling, it wasn’t enough on its own to prove that Vibrio pectenicida caused SSWD. So, the team extracted the bacteria from diseased starfish, grew a pure strain and then injected this into healthy starfish. The starfish quickly succumbed to SSWD.

Additional confirmation came during research in the waters of British Columbia, Canada. Analysis of sea star samples taken from before and after an outbreak showed that the bacteria was only present after the disease had arrived.

With the cause of SSWD now identified, the team now want to find out how Vibrio pectenicida causes the disease. The bacterium is known to secrete a toxin when infecting scallops, and it’s possible that this could cause the symptoms of the illness in starfish.

The disease is also known to spread more quickly in warmer waters. With the oceans getting warmer due to climate change, understanding why this happens will be crucial to prevent future mass outbreaks occurring elsewhere in the world.