The bird flu threat to humans in the United States

The United States recorded its first human death from bird flu on January 6, heightening concerns that the latest strains of influenza A/H5N1 virus could become a significant public health threat to humans in 2025. So far, much of the impact of bird flu in the U.S. has been limited to poultry and dairy farms, where more than 10 million egg-laying chickens have died and 928 herds of dairy cattle have been infected, according to the U.S. Department of Agriculture.

As of January 15, there were 67 confirmed human cases of bird flu in the U.S, with most of those cases involving poultry and dairy workers with direct exposure to infected animals. The Louisiana man who died earlier this month is believed to have contracted the virus from his exposure to a non-commercial backyard flock of birds and wild birds. While federal authorities say the risk to humans remains low, the Biden administration, in response to growing public health concerns, recently authorized $306 million dollars to bolster the nation’s bird flu preparedness.

Dr. Colin J. Carlson, PhD, a Yale School of Public Health assistant professor of epidemiology (microbial diseases), is an expert in virus outbreak reporting and monitoring. He is co-founder and executive director of Verena, a cross-university collaboration involving over a dozen scientists who are developing data-driven approaches to assessing when viruses pose a threat to human health. Dr. Carlson recently took a moment to share his thoughts on the current bird flu outbreak in the United States and the potential threat it poses for humans.

Colin Carlson: Scientists who work on influenza usually say the next pandemic is not a question of “if” there will be one, but “when.” Usually, it takes about 10 to 40 years – and it’s been 15 years now since the swine flu pandemic, so we’re definitely on the clock – but it really comes down to luck. We’ve been seeing cases of H5N1 influenza in humans for decades now. But for the most part, these viruses haven’t spread from person to person. The current situation is a bit different. We’ve seen this lineage spreading in other species of mammals, both on farms and in the wild, so we’re worried it could eventually happen in humans. This is probably the closest we’ve been to an H5N1 pandemic—but it’s hard to say exactly how close that is.

It’s also hard to say what an H5N1 pandemic would look like, if it does happen. The last few flu pandemics were a lot less deadly than COVID-19, but the 1918 “Spanish flu” was the deadliest pandemic in modern history. I don’t think we can quite conceptualize what it would look like to lose 2% to 3% of the world’s population in a couple of years. We should take that risk seriously.

CC: The longer that these viruses circulate, the more chances they’ll have to infect humans. It’s especially bad if we’re letting them adapt to mammalian immune systems, like swine flu did in 2009, or like we’re seeing now with H5N1 in cattle in North America. But the good news is, we know a lot about flu, so we know what an H5N1 virus that’s “good at infecting humans” would probably look like. In the last few months, the scientists who are tracking flu genomes have seen some mutations that are worrisome, but nothing that suggests human-to-human spread will start today. But like I said, virus evolution is all about luck: the right mutation could crop up tomorrow, or in 10 years. For now, scientists will keep monitoring the virus as it continues to circulate and evolve.

Right now, the big shift in risk is political, not biological. The worst thing we could do right now is stop tracking this virus just because it’s inconvenient for politicians and the agriculture industry. We saw what that leads to with COVID—cases don’t go away just because you stop counting.

CC: The greatest risk right now is on farms. Agricultural workers need protective equipment to stay safe, and they need to know why it’s important—management may not always tell employees what’s going on. If they do get sick, they need to be able to go to the doctor and isolate, so they don’t get other people sick. That means paid sick leave, and protection from retaliation.

If you’re keeping poultry in your backyard, you should try to minimize their interactions with wild birds—ideally by bringing them inside, or at least their food and water—and obviously wash your hands. Wear a high-quality respirator (like an N95) while caring for them, and especially while cleaning their litter.

The risk to everyone else is currently pretty low, but not zero. Don’t drink unpasteurized milk, and don’t give raw food to your pets. Just don’t do it! If you have cats, keep them inside; don’t let your pets (or kids) play with wild birds. If you have a birdfeeder or nest box, it’s never a bad idea to wash your hands or wear gloves, for any number of reasons—and if you’re also keeping backyard poultry, you should probably take that birdfeeder down.