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White lesions on a sea star's arms are the first sign of sea star wasting disease. Credit: Melissa Miner.

Science

Sea star wasting disease (SSWD) is currently one of the biggest epidemics in the animal kingdom and researchers at the University of Vermont, USA, has discovered that the balance of different microbes within the body of a sea star can alter the progression of the disease.

Sea star wasting disease affects more than 20 species of sea star and its symptoms include deflating, body lesions, and disintegrating tissue, all of which often lead to death. "It's really quite sad to see. They become limp and liquidy, with holes straight to their insides and no tissue integrity. If an arm starts walking in one direction, it's likely to leave the rest of its body behind," said Melissa Pespeni, a marine biologist who co-led the new study published in Scientific Reports, an online journal from the publishers of Nature.

Relatively small-scale SSWD events have been observed on the west coast of North America since the 1980s but in 2013 SSWD was observed at an unprecedented scale in terms of the geographic range and number of species affected. "The disease has crossed into multiple different genera of sea stars, and suddenly appeared in many far-apart places," says Pespeni. "Because of the widespread nature of the disease, it seems increasingly less likely that it's a single pathogen."

In order to understand which bacteria are involved in the disease, the researchers investigated the microbiome of the keystone species Pisaster ochraceus, otherwise known as the Ochre sea star. Individuals that had contracted SSWD were sampled periodically throughout each stage of the disease and their microbial make-up was compared to that of healthy individuals.

Their results discovered that a whole soup of microbes increase in abundance as the disease begins to progress. The main culprits were bacteria from the family Tenacibaculum, whilst at the same time, known beneficial bacteria, from the group Pseudoalteromonas, greatly decreased in abundance at the onset of the illness. Unfortunately, by the end of the study, twenty-nine of the thirty-seven sea stars had died as a result of the rampant disease. However, the eight animals that didn't get the disease had a higher overall richness of microbes than the sick ones. And, the healthy animals hosted an abundance of helpful microbes that can detect disease-causing microbes, inhibit their growth, and clean up harmful substances that might damage the sea star- like environmental pollutants.

For years many scientists believed that disease was often caused by one pathogen infecting a host, but now studies are showing that it isn’t always that simple. "We were thinking we might find a smoking gun of one microbe but no, there's a whole suite of known pathogens that increase in abundance with the onset of the disease," Pespeni said. "Even if we find a single pathogen at the root of sea star wasting disease, it could be doing its work by throwing off the microbiome of the host”.

"It's not uncommon at all to observe a sea star healthy one day and in pieces and near death the next morning," said Melanie Lloyd, a co-author on the study. "If this disease was happening in humans, it would be the making of a Stephen King novel."

As large-scale epidemics increase in frequency in humans and other species, particularly in marine populations, it will be essential that we gain a greater understanding of how microbial communities interact within the body, and how this determines the effectiveness of a disease.

By Ellis Moloney

Lloyd, M., & Pespeni, M. (2018). Microbiome shifts with onset and progression of Sea Star Wasting Disease revealed through time course sampling. Scientific Reports, 8(1). doi: 10.1038/s41598-018-34697-w

This material is based upon work supported by the National Science Foundation RAPID grant IOS-1555058 (to M.H.P.).