Bird flu ‘not going away,’ experts warn
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.
With enough time, the virus can mutate
Experts have commented on the possible dangers of H5N1 mutating to become more transmissible between humans and capable of causing more severe illness.
“As the virus has opportunities to jump from species to species and within species, it has opportunities to develop mutations that can have important implications for transmissibility and virulence,” Roess said. “The longer a virus hangs around in a host the more likely one of the many mutations that arise will lead to a strain that is problematic for human and animal health.”
The patient in Louisiana who died after being hospitalized in December was infected with an H5N1 virus of the D1.1 genotype, which had been detected among wild birds and poultry in the U.S. and Canada before that, according to the CDC. The virus is different from the B3.13 genotype circulating among dairy cattle, people and poultry in the U.S.
The U.S. Department of Agriculture Animal and Plant Health Inspection Service (APHIS) confirmed on Jan. 31 that D1.1 was detected in Nevada dairy cattle through the USDA’s National Milk Testing Strategy, which requires national milk testing for H5N1.
On Feb. 13, APHIS announced that D1.1 was also identified in dairy cattle in Arizona. At the time, the agency said that the detection was “not unexpected.” However, “it may indicate an increased risk of highly pathogenic avian influenza introduction into dairies through wild bird exposure.”
“The virus is still not particularly great at infecting humans,” Nuzzo said. “It's still largely a bird virus, although now it's becoming more of a mammal virus by spending more time in cows.”
Nuzzo issued another warning related to what has been a historically bad influenza season: If H5N1 and seasonal influenza end up in the same person, the viruses could “swap genes” and spit out a brand new, more transmissible virus, a process known as reassortment.
In an update published in August, the CDC warned of this possibility but called such dual infections “very rare” and noted that seasonal influenza vaccination can reduce the risk even further.
“We keep seeing the virus change, and we very much worry about the possibility that it could gain the ability to more easily infect and spread between people,” Nuzzo said.
Political decisions complicate response
The outbreak has been complicated by recent changes at the nation’s public health agencies and an order issued by HHS to temporarily cease public communication, which included the delayed publication of multiple scientific reports about the outbreak.
During the communications freeze, the CDC’s primary medical journal, MMWR, went unpublished for the first time in its history, going dark for 2 weeks. The CDC resumed publishing the journal on Feb. 6, but new reports about the bird flu outbreak that were expected to be included in the Jan. 23 issue were not among the articles that appeared.
Eventually, a new H5N1 report appeared in the Feb. 13 issue, describing a multistate serosurvey that showed evidence that bovine veterinary practitioners had undetected bird flu infections. Other reports about the outbreak have followed, including one this week on wastewater surveillance for H5 viruses.
Amesh A. Adalja, MD, FACP, FACEP, FIDSA, senior scholar at the Johns Hopkins Center for Health Security, said the pause in communications “makes it very hard to have full situational awareness of what is going on and what new developments have occurred.”
“This is a very fluid situation, and constant updates are required,” Adalja told Healio.
The day after his inauguration, President Trump signed an executive order withdrawing the U.S. from WHO, citing what he characterized as unfair financial contributions made by the U.S. in comparison to other countries, especially China.
Experts agreed that a withdrawal from WHO, which would take a year to complete, could cause major disruptions in global public health, resulting in “less coordination, less situational awareness and less sharing of scientific data,” Adalja said.
In a media briefing on Feb. 12, WHO Director-General Tedros Adhanom Ghebreyesus, PhD, MSc, said the agency was receiving “limited information” about the spread of H5N1 among animals and people in the U.S.
“What has already happened is that CDC is not allowed to call WHO to communicate with them on emerging health crises. They have not been able to share or even reference WHO-developed materials,” Nuzzo said. “Basically, it's a censoring of science and science communications, and it is really to our detriment.”
According to Roess, there is an expectation among the 196 countries that are a party to the International Health Regulations to share outbreak information in real time — an essential part of answering questions about emerging pathogens.
“The idea is that by sharing information we work together to reduce infectious and other diseases and death among populations across the world,” she said. “Imagine how much worse it will be when the U.S. is not at the proverbial WHO table.”
Nuzzo said the U.S. could be “imperiling the rest of the world” by not being transparent about the outbreak.
“I think back to the early days of COVID and the criticisms that were lobbed at China for not being transparent and open in response to that the start of the global pandemic,” she said. “We've really lost any leg to stand on in terms of pointing fingers at countries and accusing them of not being open.”
‘We just have to be prepared’
A CDC spokesperson told Healio that the agency and its government partners continue to conduct preparedness activities in response to H5N1, including planning for a potential vaccination program, should it be necessary.
The spokesperson said the U.S. has doses of H5 vaccine in ready-to-use, prefilled syringes and multidose vials, and is currently supporting additional manufacturing efforts to increase the inventory.
“These vaccines are well matched against the A(H5) viruses circulating among wild birds and dairy cows, as well as those infecting poultry,” the spokesperson said, noting that the vaccines are not commercially available and are not authorized or approved by FDA.
Still, “Having a candidate vaccine virus that protects against H5 bird flu in humans is an important step in being prepared for an H5 vaccination program, as it can cut up to 8 weeks off of a vaccine development timeline,” the spokesperson said. “That means a vaccine can be ready up to 8 weeks earlier, and more lives can be saved in a pandemic.”
According to the CDC, candidate vaccine viruses have been shared with manufacturers and could be used to make vaccines if needed for a larger outbreak or pandemic.
For now, it is unclear if bird flu will reach pandemic-level status. Based on currently available data, Michael T. Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said the major question — Will it evolve to spread easily among people? — remains unanswered.
“Maybe it will. Maybe it won't. We don't know. We just have to be prepared,” he said.
“This is not going away,” Nuzzo said. “Our entire response to this virus has been based on the hope that it will somehow just disappear, and I see absolutely no evidence of that happening.”
References:
- APHIS confirms D1.1 genotype in dairy cattle in Nevada. https://www.aphis.usda.gov/news/program-update/aphis-confirms-d11-genotype-dairy-cattle-nevada-0. Published Jan. 31, 2025. Accessed Feb. 14, 2025.
- APHIS identifies third HPAI spillover in dairy cattle. https://www.aphis.usda.gov/news/program-update/aphis-identifies-third-hpai-spillover-dairy-cattle. Published Feb. 13, 2025. Accessed Feb. 14, 2025.
- Apisarnthanarak A, et al. Emerg Infect Dis. 2004;doi:10.3201/eid1007.040415.
- CDC. About bird flu. https://www.cdc.gov/bird-flu/about/index.html. Updated Dec. 30, 2024. Accessed Feb. 23, 2025.
- CDC A(H5N1) bird flu response update August 2, 2024. https://www.cdc.gov/bird-flu/spotlights/h5n1-response-08022024.html. Published Aug. 2, 2024. Accessed Feb. 24, 2025.
- CDC confirms first severe case of H5N1 bird flu in the United States. https://www.cdc.gov/media/releases/2024/m1218-h5n1-flu.html. Published Dec. 18, 2024. Accessed Feb. 23, 2025.
- CDC. H5 bird flu: Current situation. https://www.cdc.gov/bird-flu/situation-summary/index.html. Updated Feb. 14, 2025. Accessed Feb. 14, 2025.
- Uyeki TM, et al. N Engl J Med. 2024;doi:10.1056/NEJMc2405371.
For More Information:
Amesh A. Adalja, MD, FACP, FACEP, FIDSA, can be reached at aadalja1@jhu.edu.
Jennifer B. Nuzzo, DrPH, can be reached at pandemic_center@brown.edu.
Michael T. Osterholm, PhD, MPH, can be reached at cidrap@umn.edu.
Maurice Pitesky, DVM, MPVM, BMEA, can be reached mepitesky@ucdavis.edu.
Amira A. Roess, PhD, MPH, can be reached at aroess@gmu.edu.
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