Bird flu decimates seal colonies worldwide; scientists struggle to contain spread

In the waters off Maine and along the Pacific coast, seals are dying by the hundreds. In South America, the toll is far grimmer—more than 20,000 sea lions in Chile and Peru, thousands of elephant seals in Argentina, their bodies washing ashore in numbers that have forced scientists to confront a crisis they do not yet know how to stop.

The culprit is avian influenza, the same virus that has killed millions of domesticated birds since 2020 and spread across the globe in ways no one fully anticipated. What began as a poultry problem has become something far more complex: a disease that moves through wildlife with devastating efficiency, jumping from infected seabirds to marine mammals that have never encountered it before and possess no natural immunity. The virus is not considered a major threat to humans, but its ecological consequences are already severe and growing.

In New England, researchers at Tufts University documented an outbreak in 2022 that killed more than 330 harbor and gray seals along the North Atlantic coast. The seals likely contracted the virus through contact with infected gulls—either by scavenging dead birds or coming into contact with contaminated feces. The U.S. government classified it as an unusual mortality event. But what seemed contained at the time has proven to be part of a much larger pattern. The virus arrived in South America in late 2022 and has since killed tens of thousands of marine mammals across the region. In February, it reached mainland Antarctica for the first time.

Marcela Uhart, who directs the Latin America program at UC Davis's wildlife health center, explained the fundamental problem: once the virus establishes itself in wildlife, it spreads without restraint as long as susceptible animals remain. In domesticated flocks, humans can implement control measures. In the ocean, there is no such option. The seals and sea lions of South America had never been exposed to this virus before, and their populations paid the price. Hundreds of thousands of birds in Peru and Chile have also died from the infection, creating a reservoir of virus that continues to circulate.

The ecological damage extends beyond the raw numbers. Seals and sea lions occupy a critical position in marine food webs, controlling fish populations and maintaining balance throughout coastal ecosystems. Their sudden removal creates cascading effects that scientists are only beginning to understand. Some species affected, like South American sea lions and Southern elephant seals, maintain relatively stable populations overall, but the concern runs deeper: the virus has already been linked to deaths of endangered Caspian seals in Russia. If it reaches other vulnerable species, the consequences could be irreversible.

Researchers have identified mutations in the virus that suggest it is adapting to spread between birds and mammals simultaneously. A study published in Nature Communications last fall warned that these changes warrant urgent surveillance. Another study in Emerging Infectious Diseases found nearly identical virus samples in dead sea lions, seals, and seabirds—evidence of a multispecies outbreak that confirms the virus is evolving in ways that cross traditional species boundaries. The World Organisation for Animal Health has warned that wildlife losses at this scale present an unprecedented risk of population collapse.

Some scientists point to climate change as a contributing factor. Warming ocean temperatures off northern Chile reduce populations of forage fish, weakening sea lions and making them more susceptible to disease. Liesbeth van der Meer, director of Oceana in Chile, noted that authorities have launched public campaigns urging people to avoid contact with sick or dead seabirds and marine mammals. Vaccinating poultry may help reduce the virus's circulation, though no vaccine exists for wild animals. Even the New England Aquarium has implemented strict protocols—staff cannot bring backyard poultry to work, and an awning shields the seal exhibits from wild birds overhead.

What remains unclear is whether this crisis can be managed at all. The virus continues to spread, and the seals continue to die. Scientists are watching, documenting, and warning, but the machinery of prevention and control that works on land has no equivalent in the ocean. The question now is not whether the ecosystem will change, but how much it will change, and whether the animals that depend on these waters can survive the transformation.