The first great white shark was found dead in August 2023 on a beach in a national park on Prince Edward Island, Canada: a young male, 500 pounds, 8 feet 9 inches from snout to tail. Park workers soon arrived with a pickup truck, loaded the carcass and drove it to a cooler at the Atlantic Veterinary College at the University of Prince Edward Island. Aside from some scrapes acquired en route, the shark showed no signs of injury.

Dr. Megan Jones, a veterinary pathologist at the college and regional director of the Canadian Wildlife Health Cooperative, or C.W.H.C., began a necropsy early the next morning, while the body “was really, really fresh,” she said. “When we look through the microscope at the tissues, they’re very well preserved.”

The C.W.H.C., a network affiliated with Canada’s veterinary schools, studies wildlife health issues. In 30 years, however, the group had never come across a great white, and it was not at all obvious how this one had died. Starvation was ruled out from the very first incision, when the shark’s 76-pound liver, where the animal stores fat, spilled onto the examination table. Other organs showed no sign of trauma. Only later, after microscopic testing, did the cause of death become apparent: meningoencephalitis, an inflammation of brain tissues.

At first Dr. Jones found the diagnosis more interesting than alarming. Then came the other sharks. Over the next few months the C.W.H.C. received either whole animals or tissue samples from four more white sharks found beached in eastern Canada. “Three of these five seem to have the same potentially infectious disease affecting their brain,” Dr. Jones said. “We need to know more about what that is.”

Those five white sharks are among nine known deaths dating from a shark found on July 4, 2022, in Massachusetts; most of those had brain inflammation. Such inflammation has been seen in other shark species, but the cause in those cases — bacterial infection, for instance — was obvious, unlike in white sharks. Dr. Jones is now part of a small group of scientists in the United States and Canada who are trying to untangle the mystery — and determine whether white sharks are facing a broader threat.

“I feel very strongly that there’s something significant going on,” said Dr. Alisa Newton, the chief veterinarian for OCEARCH, a shark research organization based in Florida that developed Shark Tracker, a popular app that monitors the movements of sharks. But Dr. Newton’s alarm is tempered by the fact that so little is known about the base-line incidence of shark deaths along the Atlantic Coast.

As research subjects, the sharks of the western North Atlantic population, which ranges from southern Florida to Newfoundland, are less understood than white sharks in other areas, and far less understood than marine mammals such as whales and dolphins. Shark science is relatively underfunded, and there are few protocols to connect local officials with scientists when a white shark is found beached on the eastern coast of North America. As a result, information — including tissue samples — tends to move slowly.

Dr. Newton was the first scientist to observe meningoencephalitis in white sharks on the Atlantic Coast, in 2022. She spotted it in a sample of brain tissue that she received at her lab in Jacksonville, Fla.: reddish cubes taken from a shark found on Long Island, N.Y., on July 20 of that year.

She also found swelling in the brain of the shark found on July 4, 2022 — although the tissue samples didn’t reach her lab until early 2023 — and in another recovered in South Carolina in April 2023. Six other earlier cases of beached sharks are being evaluated for possible meningoencephalitis. She wonders whether there are still more samples out there, sitting on shelves in jars of formalin, perhaps collected by state wildlife officials who don’t know that she’d like a piece of their brain.

The brains of white sharks are big, by fish standards, although considerably smaller than those of dolphins. They are smooth on the surface and knobby, roughly the size and shape of three Ping-Pong balls in a row.

Meningoencephalitis, an inflammation of the brain and surrounding tissues, is a symptom of an underlying issue. With nowhere for swollen tissue to go in the hard skull, the brain is squeezed and its normal functioning disrupted. In a shark, that may mean it is unable to feed, Dr. Jones said, or it loses its balance while swimming and gets stuck in shallow water, becoming beached when the tide goes out. But given the paucity of knowledge, it could be normal for white sharks to live with some amount of brain swelling.

“We know lots of animals that live with parasites or bacteria and they’re good, they’re fine, they always have sort of a natural load,” said Tonya Wimmer, executive director of the Marine Animal Response Society, or MARS, the organization that is called when animals are discovered beached in eastern Canada. “You should see the lungs of harbor porpoises: They’re chock-full of really icky worms, but it’s natural for them.”

The organization performed necropsies on three of the five beached sharks, including two that could not be moved to a lab and had to be dissected on sand that quickly soaked with blood. The group also enlisted a dive team to recover the head of the most recent casualty, a white shark found dead in November of 2023 in 30 feet of water near Halifax. (Its time in the water degraded the brain tissues too much for the C.W.H.C. to make any diagnosis about swelling.)

One juvenile male had eaten just before it died, and there were “big chunks of porpoise” in its stomach, including a flipper and part of the head, Ms. Wimmer said. And it showed no sign of meningoencephalitis under the microscope. Another white shark, which made Canadian headlines in October 2023 for its death throes, swimming erratically around a harbor and bumping into the wharf before beaching, seemed like a clear case of brain inflammation. But testing again showed none.

To further the investigation, Dr. Newton has submitted brain tissue from the South Carolina shark to the Washington Animal Disease Diagnostic Laboratory for genetic sequencing. The procedure catalogs all the DNA in the tissue to establish whether there is evidence of another organism, such as a virus or bacteria, inside the shark that could be causing the meningoencephalitis. That sequencing has not been completed yet, leaving the mystery open.

Ms. Wimmer is optimistic that the “baffling” wave of deaths could actually be a positive sign, the natural result of a population upswing for an animal that is listed as an endangered species in Canada. More white sharks might be turning up on beaches simply because there are more white sharks in the water.