Where Did Bird Flu Go?

Scientific American

15-07-2025

For months, bird flu was seemingly everywhere in the U.S.: news headlines reported the highly pathogenic H5N1 avian influenza virus was rapidly sweeping through hundreds of herds of dairy cattle and leading to massive culls of poultry flocks, concerning infections in humans and grocery store aisles where nary an egg could be found.
But nearly as quickly as bird flu took hold in daily conversations, it disappeared from them and most people's thoughts—making it easy for the public to think avian influenza's threat had waned. Far from it, experts say. 'The flu is still there, and we just don't know enough about it,' says Angela Rasmussen, a virologist at the University of Saskatchewan.
What made the virus apparently fade away—and what does that mean for the future of bird flu?
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One scenario experts have definitively ruled out is that the currently circulating bird flu virus—a member of a subtype of influenza called H5N1 for the proteins on its surface—is simply vanishing on its own, says Jennifer Nuzzo, an epidemiologist at Brown University. 'There has been this wishful thinking that it's just going to wipe through and be gone, and we've just not seen that, and that's just not how flu viruses work,' Nuzzo says. 'This isn't going away.'
Experts are still monitoring for H5N1 avian influenza in a variety of animals: wild birds, commercial poultry animals, wild mammals, dairy cattle and humans—and finding it, albeit at lower rates. But the virus is tricky, behaving somewhat differently in each host. Here's what we know about the current state of the virus.
The most reliable data on bird flu prevalence come from poultry operations. That's because the virus is so devastating in chickens and turkeys that farmers must cull flocks as soon as they detect an infection to reduce spread. They are also able to report outbreaks to the federal government to receive partial compensation. There's no way to ignore a sick flock or any incentive to hide one.
And right now poultry tolls to avian influenza are relatively low. Farmers reported just three million poultry birds killed by the virus or culled to stop it in March and April combined compared with 23 million and 12 million in January and February, respectively. May saw more than five million birds dead after the virus infiltrated several massive egg-laying facilities in Maricopa County, Arizona. But June rates fell far below one million birds, and July cases to date remain very low, with just one commercial facility affected so far.
These lower rates of bird flu aren't particularly surprising, given the virus's past behavior in poultry to date, says Mike Persia, a poultry specialist at Virginia Tech. 'We generally see a reduction in infections over the summer,' he says. Since the current outbreak began in early 2022, U.S. Department of Agriculture data show that, each year, the monthly count of affected poultry birds has tended to dip to under five million in June, July and August.
Two factors seem to contribute to the apparent seasonal trend, Persia says. The virus appears to falter in higher ambient temperatures, and the migratory wild birds that typically introduce the virus into poultry flocks aren't traveling as widely now that breeding season is in full swing.
But the outbreak's history tells a cautionary tale: each autumn, the number of affected poultry birds rises again—so it would be premature to assume H5N1 is done with us. 'I'm optimistic that maybe this was the last of it, and it goes away forever. I wouldn't take the lull as proof of that, though,' says Jada Thompson, an agricultural economist at the University of Arkansas. 'We need to maintain vigilance.'
Evaluating the outbreak in U.S. dairy cattle has been more difficult. Cows that are sick with bird flu eat less and produce thick and discolored milk. But the infection isn't nearly as fatal in cattle as it is in poultry, making the virus harder to see in the former. And there's no recompense for lost milk to encourage farmers to report being hit.
In addition, the virus's jump into dairy cattle in late 2023 was wildly unexpected and not publicly confirmed until March 2024, giving dairy farmers and virologists little time to understand bird flu's tendencies in the species. Last year cases continued throughout the summer, particularly in the hard-hit state of Colorado. Spread proved to be difficult to contain, in part because of the movement of animals required by the dairy industry. And although the virus can be monitored through milk, officials only began mandating such testing last December, after a full year of viral circulation.
This year reported infections have trailed off, with only two herds confirmed to have the virus in all of June. But it's unclear how to interpret the trend—dairy farmers, too, are left poised between caution and optimism.
Throughout the outbreak, bird flu risk to humans has been low, although dairy and poultry workers with exposure to infected animals have been more vulnerable. The first detected human infection in 2024 came shortly after confirmation that dairy cattle had become sick with H5N1. Additional human cases came in flurries throughout the intervening months, totaling 70 confirmed infections, including one death, by mid-February. Since then, infection tallies at the Centers for Disease Control and Prevention have stalled.
Experts doubt that's a good thing. 'I can't rule out that part of why we're not finding infections is: we're just simply not looking for them,' Nuzzo says.
Throughout the outbreak, the CDC has kept a running tally of the testing it is conducting, and those numbers paint a clear picture. As of July 1, the CDC noted that more than 880 people had been subject to targeted testing after exposure to infected animals. On March 1 that number had been more than 840; in contrast, the February 1 number was more than 660. The CDC tested more than four times as many people in February as in March, April, May and June combined. Another way experts have kept tabs on bird flu has been through existing national flu surveillance—but because normal flu infections are in a seasonal lull, so are tests through that network.
The result is a lot of question marks. 'We are in sort of a perfect storm of no testing,' Rasmussen says.
Even wastewater monitoring, which has proven helpful in understanding levels of the virus that causes COVID as testing rates have fallen, is of limited help. The approach looks for the presence of viruses in community water processing plants, but H5N1 is spread so broadly across species that it is nearly impossible to use these detections to definitively trace sources.
'You don't know how it got there,' Nuzzo says of the virus in wastewater. 'You don't know if people are infected; you don't know if [the virus is present] because birds were hanging out in the wastewater.' In some cases, spikes in wastewater levels of H5N1 have even been linked to farmers dumping milk from their infected cows.
Nuzzo suspects that there have certainly been more human cases of avian influenza than the 70 confirmed to date but that the virus is not spreading widely. 'I don't think there's some huge iceberg of infections that we're missing,' Nuzzo says.
Nuzzo and Rasmussen find that cold comfort, however. Instead they emphasize how vital it is to have as much intel as possible about what H5N1 is doing. Choosing not to seek out evidence of the virus's behavior means passing up on the opportunity to catch any early signs of a pandemic in the making.
'No news in my world is not good news,' Rasmussen says. 'We're just not collecting any data, and those are two very, very different things.'
The U.S.'s current approach is simply further shrouding a situation that is already difficult to parse—given the complexity of a multispecies outbreak and the unpredictable nature of rapidly changing influenza viruses.
'This is the kind of thing that could become a pandemic tomorrow, [or] it could never become a pandemic. And I don't know which one is going to happen,' Rasmussen says.
'This is a huge risk, but it's also a risk that may never come to pass,' she says. 'But we won't know if we just stop looking for it.'