H5N1 Bird Flu Cases Have Slowed in Animals and People

Infectious Disease
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Bird Flu

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However, CDC researchers recommend continued surveillance

by
Kristina Fiore, Director of Enterprise & Investigative Reporting, MedPage Today
May 6, 2025 • 3 min read

Detections of H5N1 avian influenza have slowed in both animals and humans, but continued surveillance is warranted, CDC researchers said.

In dairy cattle, cases surged over the fall and early winter but eased in January, while cases in poultry flocks fell after February, and came down last month in backyard flocks, according to data on CDC’s website that was shared during a clinician outreach and communication activity (COCA) call on Tuesday.

“Most of our human cases are known to be associated with animal exposures, so fewer infections in the animals leads to fewer infections in people,” Alicia Budd, MPH, team lead of the national surveillance and outbreak response team at the National Center for Immunization and Respiratory Diseases (NCIRD), said during the call.

“It’s certainly great to see these declines in both animal and human cases, but it’s also critical that we maintain targeted monitoring and our general surveillance, so that if this situation changes, we’d be able to identify that quickly,” Budd added.

During the question and answer session of the call, someone questioned whether there was seasonality to H5N1, and whether that may be tied to the lull in cases.

Todd Davis, PhD, chief of the virology, surveillance, and diagnosis branch at NCIRD, said any seasonality noted in other countries “primarily corresponds to migratory bird patterns.”

“It’s quite different from seasonality for human influenza viruses, in that the distribution of the virus among wild birds tends to follow migratory patterns, so we expect to see more circulation of H5N1 in wild birds in the fall and early winter as birds are migrating south,” Davis said.

That’s when poultry flocks and dairy cattle are most at risk of exposure, “and then subsequently the chances of human exposure correspond with when those viruses are circulating either in poultry flocks or dairy cattle,” he said.

However, that picture may be different in the U.S., noted James Lawler, MD, MPH, of the University of Nebraska Medical Center’s Global Center for Health Security in Omaha, who was not involved in the CDC call.

Transmission among dairy cows was “primarily due to cow-to-cow transmission and not reintroduction from wild birds,” so migration patterns are unlikely to be the main reason for the recent decline in cases.

On the other hand, “migration and wild bird mixing likely does drive emergence of new subtypes,” he cautioned.

Overall, there have been 70 cases of H5N1 detected in humans since the outbreak began in the U.S. in 2022.

Of those, 64 were detected through targeted H5 surveillance, and six were detected through national influenza surveillance, Budd said.

Those 70 cases have occurred in 13 states, with the majority (41) being associated with dairy cattle; 24 with commercial poultry; two with backyard poultry; and three with unknown exposures.

A total of four patients were hospitalized, and there was one death — a patient in Louisiana who had been exposed to sick birds in a backyard flock.

Among animals, more than 169 million commercial poultry or backyard birds have been infected in the current H5N1 outbreak, along with 1,049 herds from 17 states, Tim Uyeki, MD, MPH, chief medical officer of NCIRD, said during the call.

Budd emphasized that the overall risk to the public remains low, and those at the greatest risk are those with close, prolonged exposure to infected animals.

Lawler said that while some reporting challenges remain, and that the U.S. still “has limited visibility into the true scope,” there does appear to be a lull in H5N1 transmission.

“The real question,” he said, “is what happens next? I doubt we will be so lucky that H5 has run its course in the U.S. and we are done with it.”

“I assume we will see a resurgence, perhaps of a new subtype,” Lawler said. “2.3.4.4b has been unique among H5 clades, and I think it likely has more surprises left for us.”

Kristina Fiore leads MedPage’s enterprise & investigative reporting team. She’s been a medical journalist for more than a decade and her work has been recognized by Barlett & Steele, AHCJ, SABEW, and others. Send story tips to [email protected]. Follow