The outbreak of bird flu that began in the United States in 2022 among wild birds and poultry has since spread to dairy cattle, at least one pig, a growing number of people — and even cats.

“We're dealing with, by all definitions, an endemic disease now,” Maurice Pitesky, DVM, MPVM, BMEA, a faculty member at the University of California, Davis School of Veterinary Medicine-Cooperative Extension, told Healio.

In interviews, Pitesky and other experts described the bird flu outbreak as smoldering, still mostly a threat to animals, but more likely to pose a larger danger to humans the longer it sticks around and the less engaged the U.S. becomes in global public health. (The CDC continues to rate the risk that bird flu poses to the general public to be low.)

“The procedures that we’ve used to eradicate the virus have been somewhat futile,” Pitesky said. “We’re dealing with a virus that is ubiquitous in the environment. We knew that several months ago. We’re now at a point where we just haven’t acknowledged that, and that really has kind of stifled our response.”

‘Fortunately,’ most cases have been mild
According to the CDC, around 12,000 wild birds, 166 million poultry and 976 dairy herds have been affected by the virus at the center of the outbreak, highly pathogenic avian influenza A(H5N1). Various other animals have been infected, including zoo animals in several states. Spread among commercial poultry and dairy cattle has had implications for egg and milk supplies.

The first human H5N1 case in the U.S. was reported in April 2022 — 2 months after the virus was detected among turkeys in a commercial poultry facility, according to the CDC.

Two years later, in April 2024, the second reported human case — and first linked to dairy cattle — was reported in a dairy worker in Texas in what was believed to be the first instance of mammal-to-human transmission.

There have now been 70 reported human cases, according to the CDC. Unlike some past outbreaks of the virus, most U.S. cases have been mild, but there has been one death: a 65-year-old patient in Louisiana with underlying medical conditions who had contact with sick and dead birds in a backyard flock.

“We are very fortunate that the current strains of bird flu circulating do not appear to cause severe illness in most individuals,” Amira A. Roess, PhD, MPH, professor of global health and epidemiology at George Mason University, told Healio. “However, as the first bird flu death in Louisiana illustrates, those who are immunocompromised are the most vulnerable.”

For now, Roess said, most healthy people do not have to be very concerned about bird flu. But she noted that immunocompromised people, older adults and infants are at a greater risk for severe illness.

The CDC lists poultry and dairy workers, owners of backyard flocks, and veterinary staff as being among the people most at risk for infection.

“There have been a handful of cases who have gotten sick and we don't know how they got it, but the majority of cases are in people who have worked with or been exposed to sick animals,” Jennifer B. Nuzzo, DrPH, director of the Pandemic Center at Brown University School of Public Health, told Healio. “That alone is enough to make me worry, because this is historically a deadly virus.”

It is unclear why there have not been more severe illnesses among the U.S. cases, Nuzzo said. Past outbreaks of H5N1 have had significant mortality rates. Six of 18 patients — all children — died in the first human H5N1 outbreak in Hong Kong in 1997. Outbreaks in Thailand and Vietnam had mortality rates of 67% or higher, according to a summary published in Emerging Infectious Diseases in 2004.

“Fortunately, the majority of recent cases have had mild symptoms, [but] I see nothing about this virus that makes me not worried that future cases won't be severe,” Nuzzo said.

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