Mysterious strandings of dolphins and whales could be explained by a previously unknown cause of death that has come to light through a 20-year study of beached dolphins and whales. A chronic, progressive disease caused by a combination of a pre-existing or concomitant liver vascular disorder and gas bubbles, created during diving, finally provides an explanation for gas-filled cysts in the livers of dolphins.

Veterinary pathologist Antonio Fernandez and his colleagues at the University of Las Palmas de Gran Canaria in Spain discovered the condition by examining the carcasses of some 1,200 marine mammals that had washed up along the shores of the Canary Islands, an archipelago off northwest Africa. During the decades-long study, which was recently published in Scientific Reports, the team identified four striped dolphins with harmful gas bubble lesions in their livers.

The sleek sea-dwellers were also in poor general health, with parasites in their digestive tracts. But the most distinctive feature of their disease was blockages in the veins of parts of their livers. Meshes of platelets, proteins and blood cells, these blockages were full of cavities containing trapped gas.

The liver anomalies resembled those found in people with Budd–Chiari syndrome, a rare, life-threatening condition that results when clots impede blood from draining out of the liver. But Fernandez suspects that the dolphin’s diving life — which involves diving up and down, over and over — contributes to the liver dolphin disease by producing gas bubbles like in decompression sickness, akin to the dreaded ‘bends’ in human divers. The bubbles then become trapped in the vein clots and form cysts.

Notably, the newly described liver disease differs from other deadly causes of gas-bubble formation that can afflict stranded beaked whales — the most notorious being sonar activity from naval ships, which can prompt behavioral changes that lead to decompression sickness.

Sonar exposure also typically triggers the buildup of gas in all tissues, something not seen in the four striped dolphins with the liver condition. And since the four dolphins had no link in space or time to naval exercises, Fernandez doubts that their liver disorder was connected to sonar. Instead, he contends that the dolphins died from natural causes.

“Gas bubbles participate in the liver cyst formation,” Fernandez says, “but those dolphins did not strand due to military sonar activities.”

Scientists who study cetacean strandings say they will now be on the lookout for signs of the same liver disease in other beached whales and dolphins.

“This paper is showing us what the differential diagnosis list should include,” says veterinarian Michael Moore, who directs the Woods Hole Oceanographic Institution’s Marine Mammal Center in Cape Cod, Massachusetts, and was not involved in the study. “It’s pathological due diligence. They’re filling in the blanks.”

Although in this study, only striped dolphins showed this condition, in the past, researchers had described similar types of liver lesions among harbor porpoises, common dolphins, Risso’s dolphins and at least one beaked whale.

Thanks in part to the new study, researchers in the field will ask whether those deaths could also have been caused by the liver condition, and whether many more cases might exist?

To read more about the research, explore the paper in Scientific Reports.

Antonio Fernandez is a professor of veterinary pathology at the University of Las Palmas de Gran Canaria in Spain. As director of the university’s Institute of Animal Health and Food Safety (OIE Collaborating Center for Marine Mammal Health), he and his group oversees all research on cetacean strandings in the Canary Islands.