H5N1 Cases Decline, But Spring Migration Poses Fresh Pandemic Risk

The numbers tell a story of reprieve, at least for now. In March, American farmers destroyed two million birds infected with avian influenza—a sharp drop from the twelve point seven million culled in February and the twenty-three point two million lost in January. On dairy farms, the pattern mirrors the poultry side: a dozen new infections confirmed this month, down from seventeen in February and forty-three in December. The wholesale price of eggs, which had climbed steeply through the winter, is finally falling. By most measures, the outbreak that has ravaged U.S. farms appears to be loosening its grip.

But experts are not celebrating. The waterbirds that naturally carry the H5N1 virus have only just begun their northward migration, and April and May—peak migration season—will bring those birds back over farm country in massive numbers. When they arrive, they will likely bring the virus with them. Meanwhile, new cases have surfaced in unexpected places: sick cats in New York City, infections in live poultry markets in Philadelphia, and warnings from the United Nations that the global spread of the virus is destabilizing food supplies worldwide. The lull, in other words, may be temporary.

The reason for the current decline is straightforward. From November through March, migratory waterfowl winter in warmer southern regions, away from the commercial chicken farms and dairy herds of the north. Fewer birds overhead means fewer opportunities for the virus to jump from wildlife into livestock. The dairy farm situation is different—most of those infections trace back to a single transmission event in late 2023 or early 2024, when a wild bird passed the virus to a cow in Texas, which then spread through California's herds. By fall, the virus had infected 228 new dairy farms in December and 254 in November. The explosive phase has passed, but the damage persists: of the 755 dairies infected in California, only 422 have fully recovered. Three hundred thirty-three still have active infections.

The human toll has been surprisingly modest, which itself is a puzzle. Since H5N1 was first identified roughly twenty years ago, about half of all known infections globally have been fatal. That grim statistic is why the virus has commanded such close scientific attention. Yet in the United States, only one person has died among seventy confirmed cases. The Centers for Disease Control and Prevention recently offered a possible explanation: people who had previously been infected with the seasonal flu strain A(H1N1)—the virus behind the 2009 pandemic, now common in winter—appeared to get less severely ill when exposed to H5N1. Pre-existing antibodies, the agency suggested, might be blunting the virus's worst effects. More research is needed, but the theory offers a glimmer of understanding in an otherwise bewildering pattern.

Food safety officials have been consistent in their reassurances. The Food and Drug Administration confirms that pasteurization kills the virus, making pasteurized milk and dairy products safe. Eggs in retail markets are safe. The U.S. Department of Agriculture says the beef supply is secure and that properly cooked poultry poses no risk. The one concern flagged by health authorities in New York involves raw pet food, which has prompted recalls and warnings to pet owners. Raw milk cheese, while theoretically vulnerable, has not been linked to any known infections.

The federal government has begun mobilizing resources. The Department of Agriculture announced five hundred million dollars in new biosecurity funding and four hundred million in direct financial support for farmers. An additional hundred million will go toward vaccine research and development. On March 20, the agency opened applications for grants to develop new poultry vaccines, along with new treatments and diagnostic tools. But some experts argue the government should go further. Dr. Scott Gottlieb, a former FDA commissioner, has called for seriously considering vaccination campaigns for egg-laying hens—a strategy already employed in other countries. Logistical and technical challenges exist, but biosecurity measures alone, he and others suggest, may not be sufficient to prevent future outbreaks when the birds return.

What happens next depends largely on the calendar. The spring migration will test whether the current decline holds or whether the virus resurges. The farms are watching the sky.