From H5N1 to SARS-CoV — the virus that causes COVID-19 disease — there is an unnerving sense of déjà vu about how viruses can jump to humans from animals in the food supply chain.

Chickens and ducks have provided cheap and abundant sources of protein for humans throughout the ages, accompanied by small-scale virus outbreaks dating back to the 1800s. Bird flu evolved to become virulent in domestic geese in 1996, seeding a lineage capable of killing poultry, wild birds, and even humans. This lineage of H5N1 was first detected in southern China, coinciding with a major increase in the industrialization of poultry, and its initial spread to nearby countries in East and South Asia appeared to track closely with poultry trade. Between 2005 and 2021, over 300 million poultry died or were slaughtered in an effort to stamp out bird flu, but H5N1 has now become endemic in parts of Asia, the Middle East, Africa, and recently Europe.

The introduction of bird flu in North America in late 2021 marked a turning point in the evolution and spread of the virus. First, the introduction of the virus to New Brunswick, Canada, in November 2021 is consistent with the movement of a tag-team of gull, shorebird, and geese species from Northern Europe. While bird flu was in the United States once before, in 2014-2015, the spread of the virus was halted due to poultry culling in the Midwest.

This time around, the culling of 60 million chickens and turkeys from California to Maine may have slowed transmission but failed to contain the spread of the virus. Of the 1,000 species of wild birds native to the United States, over 140 species have now been infected with H5N1, including iconic species such as bald eagles, red-tailed hawks, and nesting seabirds, such as common and roseate terns. If the virus is adapting to wild birds across the globe — a scenario that would make controlling outbreaks infinitely harder — the spread of bird flu between countries may become more frequent.

H5N1 now has a host range that has expanded well beyond its avian reservoir. Infections have been reported in wild mammals ranging from grizzly bears, raccoons, bobcats, and skunks, with red foxes accounting for half of the 110 infections occurring across 17 US states. The common thread with mammalian cases is carnivores that are typically solitary or live in small groups and eat wild birds. Symptoms such as blindness and loss of coordination leading to death have been common in carnivores, and while harrowing, the severity of disease may reduce transmission. However, outbreaks in gregarious mammals that live in large groups such as seals, porpoises, sea lions, and on a single mink farm in Spain have sparked concerns about the virus finally gaining capacity for human transmission. Luckily, we have seen only individual mutations and not the required two to three mutations that would allow H5N1 to replicate in the upper airways rather than deeper in the lungs — a transition that would open the door for human-to-human spread.

With a window of time to rewrite the ending to this potential pandemic playbook, how should the world respond? Immediate solutions include stepping up monitoring the health of wildlife that play an increasing role in the evolution and spread of H5N1. Understanding what ecological conditions create a tipping point for outbreaks in wildlife will be important for predicting spread. Surveillance of people who work with poultry, pigs, and domestic animals in high-density environments should also be a priority.

Human infections are rare but deadly with a 56 percent fatality rate compared to about 1.1 percent for Americans infected with COVID-19. Bird flu vaccine stockpiles in the United States are small, offering protection for 20 million individuals at the highest risk of occupational exposure. Due to COVID-19, mRNA technology now exists to help scale-up production of vaccines for the public against the latest H5N1 strain. Yes, pandemic fatigue is real, but ignoring this threat could also be costly.

Nichola Hill is an assistant professor of biology who studies disease ecology and evolution at the University of Massachusetts Boston.

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