Over the last three years, a highly lethal form of avian influenza has whipped around the world, felling birds in Europe, Africa and Asia before jumping across the ocean and setting off the worst bird flu outbreak in United States history.

Last fall, the virus, known as H5N1, finally arrived in South America. It raced quickly down the Pacific coast and killed wild birds and marine mammals in staggering numbers. Peru and Chile alone have reported more than 500,000 dead seabirds and 25,000 dead sea lions, according to a new report, which was published last week by OFFLU, a global network of flu experts.

Now, scientists are worried that the virus will make its way to Antarctica, one of only two continents — along with Australia — that have not yet been hit by the pathogen. “The negative impact of this virus on Antarctic wildlife could be immense — likely worse than that on South American wildlife,” the report warns.

More than 100 million birds breed in Antarctica and on the islands nearby, and many marine mammals swim in the surrounding waters. Some of those species, including the distinctive emperor penguin and Antarctic fur seal, crowd together in large colonies. “And that could be a recipe for disaster,” said Dr. Ralph Vanstreels, a researcher at a Latin American wildlife health program at the University of California, Davis, and an author of the new report. “We could be looking at a very high death toll.”

This bird flu variant, which emerged in 2020, has caused enormous outbreaks on poultry farms, resulting in the deaths of nearly 60 million farmed birds in the United States alone. But unlike earlier versions of the virus, it has also spread widely in wild birds and routinely spilled over into wild mammals.

The virus first appeared in South America in October 2022, spreading from Colombia down to Chile in just three months. “As soon as it started moving south, it did so very, very rapidly,” said Dr. Marcela Uhart, who directs the U.C. Davis Latin American wildlife health program and is an author of the OFFLU report.

The casualties are difficult to tally because many infected animals were probably never detected, scientists said, and not all of the dead animals that did turn up were tested for the virus. But hundreds of thousands of dead seabirds, including boobies, cormorants and gulls, were reported in South America. The losses accounted for 36 percent of Peru’s population of Peruvian pelicans and 13 percent of Chile’s Humboldt penguins, according to the report.

South American sea lions also died by the thousands, representing 9 percent of the population in Peru and Chile. (Scientists are still not sure exactly how marine mammals are contracting the virus or whether it is spreading readily among them.)

The virus has continued to move south. In June, it turned up in a South American sea lion in the far south of Chile, just 670 miles from the Antarctic Peninsula. Some birds routinely wander between South America and Antarctica, feeding in both locations. Others will make their way to their Antarctic breeding sites as spring arrives in the Southern Hemisphere, potentially bringing the virus with them.

Antarctica has never had an outbreak of highly pathogenic bird flu before and its residents are likely to have few immune defenses against the virus. “The populations are completely naïve,” said Dr. Thijs Kuiken, a veterinary pathologist at Erasmus University Medical Center in the Netherlands and an author of the new report. “The worry is that the first time that it goes through, it will really have a high impact in terms of rate of mortality.”

Many of the region’s birds, including emperor penguins and sooty shearwaters, are already facing other threats, from sources including climate change, the fishing industry or other human activities. Some species, like the southern pintail and the Macquarie shag, are restricted to just a few islands. “So if you were to get an outbreak in those islands, basically the whole species collapses,” Dr. Vanstreels said.

Local marine mammals could be at risk, too. Although the Antarctic fur seal can range widely, 95 percent of the population lives around just one island, making it vulnerable to an outbreak.

At this point, the virus is so widespread that it may not be possible to stop it from reaching Antarctica. “At the moment, there’s nothing we can do to prevent it,” Dr. Kuiken said. “So it’s important in the coming months to be as alert as possible.”

It will be critical to monitor wild populations to learn more about how the virus is spreading, what species might be most at risk and what conservation actions might be needed to help them recover, scientists said. “What we’re trying to do is document this really well, trying to understand how the virus is moving to see how we can better protect the species going forward,” Dr. Uhart said.