Something Weird Is Killing Great White Sharks In The North Atlantic

The Canadian sharks join four previously found dead in US waters – all under similarly perplexing circumstances. There were no signs of starvation; no injuries that would have killed them. But microscopic testing eventually revealed the culprit: meningoencephalitis, an inflammation of the brain tissue and surrounding membranes that can eventually impact cognitive function. In a shark, that could translate to an inability to feed or swim properly, causing them to get beached more easily.

But meningoencephalitis is a symptom, not a disease – and what caused the inflammation, experts don’t know.

“We know very little about what natural diseases affect wild sharks and rays compared to other marine species,” Harley Newton, Chief Scientist and Veterinarian at OCEARCH – the people behind Shark Tracker – told IFLScience via email – so, “while having multiple sharks of the same size/age class/species strand with signs of brain inflammation in clusters over a few years is concerning, it is hard to say how much concern there should be as we do not understand how this fits into the greater picture of wild shark health,” she said.

It was Newton who first identified meningoencephalitis in a great white, having found it back in 2022 in a sample of brain tissue taken from a shark that washed up in Long Island, New York. Other samples, from shark carcasses found as far apart as Prince Edward Island, Canada, and South Carolina, USA, have revealed the same mysterious inflammation – but as of yet, the cause is still unknown.

It’s bad news for a species that’s already in danger – and especially for this particular population, which has only just begun to recover from the consequences of overhunting and fishing in the area. 

“Overall, sharks are not doing great,” shark conservation scientist David Shiffman told IFLScience. “They are one of the most threatened groups of vertebrates in the world.” 

“About a third of all known species of sharks and their relatives, which are the skates, the rays, and the chimeras, are assessed as threatened with extinction by the IUCN Red List,” Shiffman pointed out. But “the United States, at least for now, is one of the bright spots of shark conservation,” he added, thanks in a large part to the rise of science-based conservation and management planning.

It’s for that reason, he explained, that North Atlantic great white sharks are something of an enigma – at least in comparison with other populations of the species. “They're relatively not that well studied,” he told IFLScience, but it’s because “they have only recently, relatively recently, returned to these areas.”

A mysterious and fatal disease spreading through the local population is, therefore, a worrying proposition – even if it has, so far, only been found in a handful of individuals. 

“It is, right now, not a population level threat,” Shiffman said – though cautioning that he was not familiar with the investigation. “But depending on what the cause is, it could certainly spread. And it's good to study warning signs of emerging things.”

Hopefully, some clues may soon be discovered: Newton has submitted brain tissue from the shark found in South Carolina to the Washington Animal Disease Diagnostic Laboratory for genetic sequencing, which may reveal any virus or bacteria that could be behind the meningoencephalitis.

“I don’t expect to know the cause soon,” Newton said, “but additional samples from fresh stranded would always be helpful. The best-case scenario […] is letting people know that stranded sharks may be dying of disease. We hope it will encourage people to expend some time and resources to evaluate and sample these animals, so we can become better informed about natural diseases in sharks and what impact they may have on populations.”

Until the results come back, however, the puzzle remains unsolved. The North Atlantic great white sharks stay in their precarious situation – and so too, therefore, does the entire ecosystem.

“They're an iconic species. They're really important to the ecosystem,” Shiffman told IFLScience. “Generally speaking, predators help keep the food chain in balance.” 

“If you want a healthy ecosystem, you want a healthy food chain,” he said, “and that means you need a healthy top of the food chain.”

This article was amended to include quotes from Harley Newton.