CHICAGO, ILLINOIS—When wildlife pathologists arrived at Hay Island off the coast of Nova Scotia in March 2012, they met an eerie sight. Of the thousands of silver and black-speckled gray seals that lay on the rocky outcrop, roughly a fifth were dead, despite showing no outward signs of disease. Necropsies revealed that 406 dead seals were infested with a crescent moon-shaped parasite that had destroyed their livers, but it wasn't clear what the organism was or how the seals had contracted the parasite. Researchers revealed the parasite's identity here today at the annual meeting of AAAS, which publishes Science. They also explained how melting ice in the Arctic Circle is helping such pathogens disperse throughout the world's oceans.

The parasite in question belongs to the genus Sarcocystis. It doesn't always kill its host—ring seals, for example, have long lived with the microbe with little ill effect. In gray seals, however, the parasite appears to follow a different pattern of infestation. Rather than low-level infection, "it completely destroys the architecture" of liver cells, says Michael Grigg of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. After examining the dead gray seals' tissue under an electron microscope and tracing the parasite's genetic lineage, Grigg and colleagues at other institutions identified it as a close relative of Sarcocystis canis, found in dogs. They named the new strain Sarcocystis pinnipedi. (Pinniped is the scientific term for seals.)

Because ring seals depend on ice to build caves for their young, they have historically remained isolated from gray seals, which inhabit warmer waters. As roughly a third of the ice covering the Arctic has disappeared over the past 30 years, however, gray seals have begun to follow their favorite prey into the ring seals' territory, says Sue Moore, a biological oceanographer at the U.S. National Oceanic and Atmospheric Administration Fisheries Office of Science and Technology in Seattle, Washington. Because their habitats are starting to overlap, these species are beginning to mix for the first time, exposing them to new diseases and parasites, she says.

Loss of Arctic ice isn't just allowing pathogens once trapped in higher latitudes to escape—it may also be enabling parasites once found only in lower latitudes to move north, says Stephen Raverty, a veterinary pathologist at the British Columbia Ministry of Agriculture in Vancouver, Canada. For the first time, Raverty and colleagues have discovered the cat parasite Toxoplasma gondii—which can cause infectious blindness and abortions in people—in western Arctic beluga whale meat, a staple food for the Inuit people. It's not yet clear how the pathogen first reached the whales, he notes. One possibility is that the Inuit are keeping more cats as pets, because kitty litter flushed down the toilet is a major source of the parasite in the environment. But loss of Arctic ice is also likely eroding the ecological barrier that has previously protected beluga whales, allowing the parasite to move northward, he says.

"Time will tell" if these exposures are flukes or "the tip of the iceberg" as ice in the Arctic melts, Grigg says. As some scientists estimate that all ice in the Arctic will be gone within 20 years, more research linking climate change to the health of marine mammals is critical, Moore notes.