H5N1 Bird Flu: A Slowly Looming Public Health Threat
Public health and infectious disease experts offered their perspectives on the 2024 US bird flu outbreak and its potential of becoming a greater public health crisis.
Throughout 2024, the US has experienced an unprecedented outbreak of avian influenza A(H5)—otherwise known as the bird flu—impacting dairy herds and poultry farms across the country. The current risk of humans contracting the virus remains low and no human-to-human transmission has been reported. Despite this, the bird flu has made its way from birds and cows to a total of 53 humans in 7 states,1 raising alarms to public health officials and infectious disease experts in fear of a future pandemic situation. [Editor’s note: These numbers are accurate as of the time of publication.]
“We've seen sporadic human cases, somewhere in between 5 and 10 cases in California. There were a couple of cases in Missouri earlier this year. There have been cases in Michigan and in Texas in humans, but to this point, no evidence of human-to-human transmission, which is a plus,” said Robert H. Hopkins, Jr., MD, medical director of the National Foundation for Infectious Diseases (NFID).
The first human case of the bird flu in the US was reported in April 2022 in a poultry worker from Colorado.2 After being examined and treated with the influenza antiviral oseltamivir (Tamiflu), the patient recovered; no human cases were reported in the US for nearly 2 years. Then, on March 27, 2024, a dairy farm worker in Texas reported conjunctivitis in their eyes and later tested positive for highly pathogenic avian influenza (HPAI) A(H5N1), making them just the second human living in the US to contract the bird flu virus.3
The bird flu has since infected poultry in 49 states and wild birds in 50 jurisdictions,1 with human cases gradually increasing amid reports in various states. One of the key factors for the bird flu possibly developing into a larger public health threat is human-to-human transmission, which has not yet occurred. However, with one of the more recent cases in Missouri being the first to report no contact with infected animals,4 the bird flu virus is noticeably progressing, despite public health sentiments regarding little to no risk.
Today, the escalation of the H5 bird flu to pandemic status is still considered unlikely, but vaccination options are being prepared. | image credit: Chanagun / stock.adobe.com
“These highly pathogenic avian influenza viruses were first detected in wild waterfowl in China in 1996, and in 1997, they were first found in a person causing infection” in Hong Kong, said Sonja Olsen, PhD, associate director for preparedness and response at the CDC’s Influenza Division, in a presentation sponsored by the NFID.5 The earliest reports of bird flu in 1997 resulted in 18 human infections and 6 deaths in Hong Kong. The virus would go on to infect over 850 individuals with nearly a 50% mortality rate in various parts of the world.
According to the NFID’s presentation, the virus continued to spread elsewhere in the world in the 21st century and beyond. Further outbreaks appeared in several Asian countries by 2003. With those outbreaks mainly occurring in wild birds, the virus eventually spread to Africa, the Middle East, and Europe by 2005. Once the virus began spreading to other continents, it also began to genetically diversify.
From 2014 to 2016, gene-swapping of wild birds and poultry led to the appearance of H5N6 and H5N8 virus subtypes, eventually reaching birds in the US. Between 2018 and 2020, these virus subtypes became prevalent across the world. Since then, further human cases have been reported while the virus persists in birds all around the world, with 2024 seeing the largest outbreak of its kind in the US.
“Flu of all types—bird flu, human flu—is transmitted by bodily fluids,” Hopkins explained. “If you cough or sneeze, you aerosolize the virus. It can then be breathed in by somebody else, and that can make them catch the flu. You can also get it from touching infected and dead animal tissue. You can also get viruses secreted in other bodily fluids…and so if you're in contact with an animal's diet of fluid, you can potentially catch flu from that animal.”
READ MORE: First Human Bird Flu Case Without Exposure to Infected Animal Discovered in Missouri
There are currently 16 known hemagglutinin (HA) and 9 known neuraminidase (NA) subtypes of influenza A viruses that have been discovered in various bird species. Outside of the H17N10 and H18N11 subtypes, only found in bats, bird species are susceptible to all subtypes of influenza A viruses.
“HA and NA are the 2 major surface glycoproteins, essential for virus entry and release and are the major targets for neutralizing antibodies. Mutations that occur in the HA and NA genes can lead to changes in the protein and facilitate evasion of host humoral immune responses. A subset of avian influenza A viruses are commonly found in domestic poultry, and numerous incidental hosts, including humans, have been described,” wrote authors of a study published in the British Medical Bulletin.6
The virus has been detected in over 10,600 wild birds and over 109 million poultry, and to a much lesser extent compared with cows and humans.1 While these infections begin in birds, they carry the slight—but very real—possibility of evolving and subsequently affecting mammals.
“Infected birds shed avian influenza viruses through their saliva, mucous, and feces. Other animals infected with avian influenza viruses may have virus present in respiratory secretions, different organs, blood, or in other body fluids, including animal milk. Human infections with avian influenza viruses can happen when [the] virus gets into a person's eyes, nose, or mouth, or is inhaled,” according to the CDC.7
While it is uncertain how the virus ended up in the US, experts learned about the 2024 outbreak when the bird flu began appearing in dairy cattle, which subsequently infected the first human patient since 2022. Since then, officials in 15 states have reported a bird flu outbreak amongst cows, with a total of 612 herds infected.8
In fact, the outbreak across America’s dairy herds was so concerning that a veterinary epidemiologist experimented with grocery store milk in the Midwest and found viral bird flu RNA in 58 samples. Despite samples of the virus appearing in pasteurized milk, there was no harm of humans contracting the bird flu because the virus would have been killed before the milk was placed on the grocery store shelf.9
However, this finding opened the eyes of public health experts to the more widespread nature of the potential bird flu threat than had been previously understood. For the past few months, government organizations have been putting forth a concerted effort to maintain the spread of the bird flu in the US, hoping to use lessons learned from the COVID-19 pandemic.
“When people get it, it's almost always been confirmed to be directly from animals to humans; the virus, in its current state, isn't evolved to readily spread from human to humans,” said David G. Hill, MD, director of clinical research at the American Lung Association. “That dual infection, where it changes to become more biologically designed to spread human to human, is where [an] epidemic or pandemic could occur. The reason for that is we don't have a lot of natural immunity to this virus, as opposed to the typical H1N1 flu that circulates where people have been exposed.”
Indeed, natural immunity to any infectious disease is what public health experts aim to target when population-wide health concerns appear. As was procedure during the COVID-19 pandemic, the immediate response of the federal government is to tame the spread of the bird flu and develop processes for how to manage the virus if it were to reach human-to-human transmission; the latter portion being the development of a bird flu vaccine.
“We rapidly created vaccines to a completely novel virus for COVID-19,” said Hill. “H5N1 bird flu is [an] influenza virus where we've had vaccine technology for decades. There are vaccines in development. There have been vaccines made in the past, so there are some bird flu vaccines in the national defense stockpile, but I think they could ramp up production of a vaccine relatively quickly.”
Like the COVID-19 pandemic, vaccines would be a saving grace for public health in the US if the bird flu escalates. While officials are prepared to address the bird flu should it become a larger threat to public health, experts are concerned about vaccine uptake, something that created yet another hurdle during the COVID-19 pandemic.
“The struggle we face in clinical care and in public health right now is there is so much misinformation about vaccination,” said Hill.
According to NFID survey data from September 2024, US adults’ concerns about getting vaccinated against respiratory viruses have decreased in recent history. Just 38% of survey respondents said they definitely plan to get a flu vaccine, 26% for the updated COVID-19 vaccine, 21% for the respiratory syncytial virus (RSV) vaccine, and 24% for the pneumococcal disease vaccine.10
While experts have seen a noticeable shift in vaccination rates, they are seemingly pointing at the COVID-19 pandemic for its part in exacerbating medical misinformation and skewing the trust patients have for their providers. As US vaccination rates decrease and the bird flu virus continues to infect humans, experts are hoping to stunt the spread of the virus before there is even a need for vaccination.
Unlike COVID-19, however, the bird flu is something medical experts within the US are much more familiar with: it has been observed in birds for the past 30 years, and the virus has similar strands as one of the most prominent infectious diseases, making the federal government’s current clinical infrastructure much more well-suited to address the bird flu than it was able to address the novel SARS-CoV-2 virus. With public health concerns remaining low at the time, right now, experts are aiming to address the bird flu at its source and amongst the humans most susceptible to interacting with the virus.
“There's a low risk to the average American out there. The people who should be concerned are those who work in an occupation that puts them at risk. People working on poultry farms or with close contact to livestock should be listening to their public health authorities and wearing appropriate protective equipment and reporting symptoms, should they get [the virus],” said Hill. For the general public, he urged to stay away from dead or injured birds and wild birds in general.
The CDC and US Department of Agriculture are working around the clock to assist workers at risk of contracting bird flu while also dedicating millions of dollars to support businesses impacted by the outbreak. Today, the escalation of the H5 bird flu to pandemic status is still considered unlikely and vaccination options are being prepared before the virus has even reached 100 people. But human-to-human transmission is key; it’s something experts and the general public should keep an eye out for to ensure they are playing their parts in keeping the population safe from this virus.
“If we're seeing H5 influenza viruses, local health departments can investigate that and make sure that this is coming from potentially animal sources, as opposed to something that’d make us worry about more human contact,” said Hopkins. “[There’s] lots of activity going on, but if we see human-to-human transmission, that's when we have to step things up to another level.”
READ MORE: Everything You Need to Know About the US Bird Flu Outbreak
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References
1. H5 bird flu: current situation. CDC. November 1, 2024. Accessed November 1, 2024. https://www.cdc.gov/bird-flu/situation-summary/index.html
2. U.S. case of human avian influenza A(H5) cirus reported. News Release. CDC. Published April 28, 2022. Accessed November 1, 2024. https://www.cdc.gov/media/releases/2022/s0428-avian-flu.html
3. Health Alert Network (HAN) - 00506 | Highly pathogenic avian influenza A(H5N1) virus: identification of human infection and recommendations for investigations and response. News Release. CDC. April 5, 2024. Accessed November 1, 2024. https://emergency.cdc.gov/han/2024/han00506.asp
4. CDC confirms human H5 bird flu case in Missouri. News Release. CDC. Published September 6, 2024. Accessed November 1, 2024. https://www.cdc.gov/media/releases/2024/s0906-birdflu-case-missouri.html
5. Updates on avian influenza A(H5N1). National Foundation for Infectious Diseases. Webinar. June 18, 2024. Accessed June 20, 2024. https://www.youtube.com/watch?v=BLz6RJLP13E
6. Li YT, Linster M, Mendenhall IH, et al. Avian influenza viruses in humans: Lessons from past outbreaks. Br Med Bull. 2019 Dec 11;132(1):81-95. doi: 10.1093/bmb/ldz036. https://pmc.ncbi.nlm.nih.gov/articles/PMC6992886/
7. What causes bird flu virus infections in humans. CDC. May 3, 2024. Accessed November 7, 2024. https://www.cdc.gov/bird-flu/virus-transmission/avian-in-humans.html
8. Current H5N1 bird flu situation in dairy cows. CDC. June 21, 2024. Accessed November 14, 2024. https://www.cdc.gov/bird-flu/situation-summary/mammals.html
9. Molteni M. Early tests of H5N1 prevalence in milk suggest U.S. bird flu outbreak in cows is widespread. STAT News. April 25, 2024. Accessed April 26, 2024. https://www.statnews.com/2024/04/25/h5n1-bird-flu-cows-outbreak-likely-widespread/
10. 2024 national survey: attitudes and behaviors about influenza, COVID-19, respiratory syncytial virus, and pneumococcal disease. National Foundation for Infectious Diseases. September 25, 2024. Accessed September 27, 2024. https://www.nfid.org/resource/2024-national-survey-attitudes-and-behaviors-about-influenza-covid-19-respiratory-syncytial-virus-and-pneumococcal-disease/
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