Dolphins around the world developed "fresh-water skin disease (FWSD)" when influxes of freshwater drastically reduced the salinity of coastal waters, causing the cetaceans' skin to take on water to the point of cells bursting, explained Pádraig Duignan, chief pathologist at The Marine Mammal Center (TMMC) in Sausalito, CA.

Duignan and his fellow researchers identified the novel skin disease by focusing on two separate dolphin die-offs that happened in 2007 and 2009.

"They were having these die-offs in dolphins, and we didn't know what they were," Duignan said. "We couldn't find a link to any disease that had been described in the literature before. Then we looked to all this data. Sure enough, we saw that it's part of a pattern."

Searching for underlying causes, Duignan and his co-authors examined the skin and lesions of dolphins who had died with this condition. They found that it was not a viral disease, one of the initial hypotheses. Access to long-term physico-chemical water quality data from permanent monitoring systems revealed the surprising culprit: freshwater.

It turns out that "sudden, dramatic and prolonged" exposure to freshwater causes devastating skin damage in dolphins that the researchers have since called FWSD, said Nahiid Stephens, second author on the study and veterinary pathology lecturer at Murdoch University in Perth, Western Australia. Because dolphins have evolved over millennia to live in marine environments, they cannot adapt to such drastic changes to salinity, Stephens explained.

When inundated with freshwater, the dolphins' skin cells become dull as they start to take on water through osmosis. The cells swell and inflame until some pop and create holes, in a few days at most, Duignan told EcoWatch. The holes turn into ulcers and lesions, which are a complete breach in the skin, within a few weeks. The skin no longer can serve as a healthy barrier against the outside world, exposing body tissues and allowing for a loss of fluids, salts, electrolytes and essential elements, he added. Loss of essential solutes and proteins can lead to organ dysfunction, shock and death. Badly damaged skin can also open the door for secondary opportunistic infectious organisms such as fungi, bacteria, and algae, Stephens said.

Both researchers compared the skin lesions to a severe third-degree burn.

"Some of these animals lose 70% of their skin. There's no way back from that in the wild," Duignan said.

The study, which was published in the journal Scientific Reports, also mentioned anecdotal reports from around the world of similar outbreaks. Dolphins in Louisiana, Mississippi, Alabama, Florida and Texas have suffered from what appears to have been FWSD in the wake of rainfall surges and flooding post-Hurricanes Katrina (2005) and Harvey (2017), Stephens said. Similarly, Duignan mentioned a humpback whale and her calf that got caught up the Sacramento River for 20 days.

"Their skin started showing these changes and sloughing to visible ulcers," the pathologist said. "After 20 days, the whales got back to the San Francisco bay and the ocean, and within a day of being back, their skin improved. They got back in saline water in time, so they survived."

Stephens explained to EcoWatch, "Common to all outbreaks in all locations was a preceding extreme weather event with extensive rainfall which causes a sudden, significant influx of freshwater into an enclosed to semi-enclosed body of water, which is normally brackish to marine in nature, causing a sudden, dramatic, and persistent drop in salinity to become freshwater. So finally we accepted that the cause was not infectious, but environmental."

The study also examined their data and hypothesis against the extensive studies and climate crisis modeling worldwide that predict that severe weather events will increase in frequency and intensity, Stephens said. These weather events are likely to trigger the sudden dramatic freshwater surges required to trigger the environmental fluctuations that cause FWSD, so the scientists made the link to climate change, she explained.

"We are concerned now about how this is being seen more frequently," Duignan said. "This year was a record hurricane season, and who knows about next year. More Katrinas and more Harveys might be on their way, and each time, this will be happening to the dolphins. I think it will get worse."

He concluded, "People are probably getting sick of hearing about climate change, but it's fundamental to everything right now. This is just another example of a disease happening to animals that never happened before. This is all because of the climate and ultimately we're to blame for it."