Veterinary and public health officials share the important roles of surveillance and prevention strategies, insights on the virus's transmission pathways, historical context, the One Health approach, and highlights effective precautionary measures to mitigate H5N1 risks.
Avian influenza (HPAI) A(H5N1), commonly known as bird flu, has had an increase in human cases in the United States recently, particularly among poultry and dairy workers. H5N1 has primarily circulated in the Midwest, with these cases raising concerns about potential human transmission and broader public health implications. Understanding the current landscape of H5N1, including recent data and prevention strategies, is essential as health officials monitor the situation.
The CDC is monitoring influenza data at national, state, and local levels. As of October 9, 2024, 18 human cases of H5N1 have been reported in the US since 2022. Since February 2022, more than 15,500 individuals have been monitored after exposure to infected animals, with 590 tested for novel influenza A. Recently, approximately 6100 individuals have been monitored, including 3350 exposed to dairy cows and 2800 to birds and other animals.1
Currently, CDC surveillance systems indicate no unusual flu activity related to H5N1. In the week ending October 19, 2024, public health laboratories reported no positive tests for novel influenza A, and wastewater monitoring detected H5 at only 1 out of 320 sites.1
CDC continues to collaborate with the USDA, FDA, and state health officials using the One Health approach to monitor the situation. The agency has deployed a multilingual epidemiological field team to assist local health departments with outreach and surveillance efforts. To date, more than 5100 individuals exposed to potentially infected animals have been monitored, and more than 54,000 specimens have been tested for H5N1viruses.2
A notable case in Missouri marked the 14th reported case this year and was the first without documented occupational exposure. Identified through seasonal flu surveillance, the patient was hospitalized but has since recovered. Experts suggest that transmission routes may include incidental contact with infected animals, indicating a need for further research into risk factors.3
Map Of Human Cases By Year: 2024
Blue: 1-5 cases
Green: 6-10 cases
Red: Over 10 cases
Image credit: CDC
Map Of Human Cases: 2023
Image credit: CDC
Meghan Davis, DVM, PhD, MPH, an associate professor at Johns Hopkins Bloomberg School of Public Health, discussed potential alternative transmission routes for avian influenza, highlighting the Missouri case as an example of variable exposure routes beyond direct animal contact. “What the case in Missouri really highlights is that the transmission routes are quite variable. It's not just occupational contact with live animals; it could also be from incidental contact with animals, and we don't yet fully recognize what those risk factors might be for people,” she said.
The serological testing methods used in the CDC's investigation of the Missouri H5N1 case included several assays. The primary method was the Microneutralization (MN) assay, which identifies influenza virus-specific antibodies in blood sera by measuring the ability of these antibodies to neutralize the virus. The Hemagglutinin Inhibition (HI) assay was employed to detect antibodies to the hemagglutinin of specific influenza viruses; this test assesses the ability of antibodies to agglutinate red blood cells, using multiple types of red blood cells for detection.4
Another method used was the Multiplex Antibody Detection Assay (MIADA), which looks for antibody binding to multiple influenza antigen targets (28-plex) and measures total immunoglobulin responses, including IgG, IgM, and IgA. A Seasonal Influenza Serum Absorption step was included to remove antibodies against seasonal influenza viruses to reduce cross-reactivity, enhancing the specificity of the H5N1 testing.4
The CDC also developed 2 Reverse Genetics (RG) viruses that mirrored the antigenic changes found in the H5N1 virus from the Missouri case. This allowed for precise serological testing. Together, these methods provided a comprehensive assessment of immune responses and potential previous exposure or infection with H5N1 in the individuals tested.4
More recently, in California, the Department of Public Health confirmed 7 probable cases of H5N1 linked to dairy farm outbreaks. Most infections have occurred among farm workers, with mild symptoms reported.4
To continue, the CDC confirmed H5 infections in two of four poultry cullers from Washington state, marking the state's first cases. Despite the confirmed cases, public health officials maintain that the risk to the public remains low, although those with occupational exposure to sick animals face a higher risk.5
Despite the rise in cases, Davis said, “The good news, however, is that despite the human cases linked to poultry outbreaks and dairy cow infections primarily affecting workers, no human-to-human transmission has been identified yet. This is an important aspect to monitor closely.”
The One Health approach is a collaborative framework that recognizes the interconnectedness of human, animal, and environmental health. It advocates for a multidisciplinary strategy to address health challenges, particularly zoonotic diseases, by integrating expertise from fields like medicine, veterinary science, and environmental science to promote holistic solutions and sustainable ecosystems.
Davis is a proponent of the One Health approach. She said, “One Health is foundational to many infectious diseases that are zoonotic. What is recognized with highly pathogenic avian influenza is that it has been present for a considerable period. Like seasonal influenza in humans, there are influenza strains that predominantly affect birds. These include reservoirs—types of birds that carry the virus but don't get very sick themselves—such as waterfowl like ducks. On the other hand, there are birds that can become severely ill and die, including domestic poultry like broiler chickens, layer chickens (for meat and eggs), turkeys, and other species kept by humans.”
According to the CDC spokesperson, “CDC’s One Health approach recognizes the interconnection between people, animals, plants, and their shared environment, which is highly relevant in our surveillance and response strategies for H5N1.”
According to the CDC’s October 11, 2024, update, the CDC has allocated more than 100,000 doses of seasonal flu vaccine for farm workers in 12 states with positive H5 cases to mitigate the risk of spread. Although the vaccine does not protect against H5N1, the CDC noted that it can reduce severe illness from seasonal flu as well as aid in the identification of potential H5N1 cases.6
Additionally, the CDC is testing existing candidate vaccines for effectiveness against current H5N1 strains. Recent studies suggest that licensed adjuvanted H5N1 vaccines may generate cross-neutralizing antibodies against circulating variants, aiding in pandemic preparedness.1
They are also genetic sequencing of the H5N1 viruses from infected individuals has shown no significant mutations associated with increased infectivity or reduced susceptibility to available antiviral treatments. This is important for understanding the current vaccine landscape, as it suggests that the current vaccines may still be relevant.1
A study published in Nature in July 2024 found the stockpiled adjuvanted H5N1 vaccines demonstrate potential effectiveness against H5N1 clade 2.3.4.4b, highlighting their importance in pandemic preparedness efforts. The study assessed the binding, hemagglutination inhibition, and neutralizing antibody responses in adults vaccinated with three licensed H5N1 vaccines derived from earlier strains (A/Vietnam, clade 1; A/Indonesia, clade 2.1) with and without adjuvants.7
Vaccination with the 2 adjuvanted H5N1 vaccines resulted in cross-reactive binding and cross-neutralizing antibodies against the HPAI clade 2.3.4.4b A/Astrakhan/3212/2020 virus. Seroconversion rates ranged from 60% to 95% after 2 doses of the AS03-adjuvanted A/Indonesia vaccine or 3 doses of the MF59-adjuvanted A/Vietnam vaccine.7
Oseltamivir is the primary antiviral, prescribed at 75 mg twice daily for 5 days for outpatients, and should be given promptly to hospitalized patients, regardless of illness onset. For immunocompromised patients, combination therapy may help prevent antiviral resistance. Treatment should not be delayed while awaiting lab results, and duration may be extended for severely ill patients based on clinical judgment.8
Intravenous peramivir is an alternative for those who cannot take oral medications, recommended for at least 5 days. Higher doses of oseltamivir have not shown additional benefits, so healthcare providers should consult CDC resources for dosing guidance. Treatment with antivirals like oseltamivir is recommended for suspected H5N1 cases. Healthcare providers are advised to start treatment without delay, even before laboratory confirmation. Monitoring for antiviral resistance, especially in hospitalized patients, is important.8
1. Avoid direct contact with wild birds; observe from a distance.
2. Do not touch sick or dead birds; report unusual deaths to authorities.
3. Avoid close exposure to cattle or materials possibly infected with H5N1.
4. Stay away from surfaces contaminated with bird saliva, mucus, or feces.
5. Do not consume raw milk or products made with it.
6. If contact with infected animals is necessary, wear appropriate personal protective equipment (PPE).
As avian influenza continues to rise as a threat to animals and the safety of veterinary professionals, it is becoming crucial for teams to understand the disease and how to protect patients, clients, and themselves—especially since avian influenza has now been transmitted to different species, including humans.9 In Spring 2024, Texas reported the detection of the disease in dairy farms and in other animals such as dolphins.
The CDC advises that it is currently unlikely humans can become infected through an infected stray, wild, feral, or domestic cat, but human infection can be possible if there is prolonged unprotected exposure to infected animals in individuals not wearing personal protective equipment.10
“If you have an animal that has a fever, is lethargic, maybe [they are] having some discharge from the eyes or nose, coughing, trouble, breathing, wheezing, and especially an elevated body temperature, any of those flu-like symptoms will be things that you'll want to be thinking about as potential symptoms of avian influenza,” Kemba Marshall, MPH, DVM, DABVP (Avian), SHRM-CP, founder of Marshall Recruiting Consortium, explained.
“Now, specifically, let’s turn our attention towards backyard poultry. Chickens can do the same things, they can still cough, wheeze, and have discharged from the eyes and nose. Their comb, the skin that you see on the top of their head, you may see that become discolored. Affected laying hens may have an abnormally shaped eggs, which may be kind of lumpy, bumpy, or what we just call like a misshapen egg,” she continued.
Veterinary professionals who suspect that they have a case of avian influenza based on clinical signs, will need to run a polymerase chain reaction (PCR) test. Veterinary teams should wear personal protection equipment (PPE) for all suspect avian influenza cases and while collecting diagnostic samples. For testing, veterinary teams are encouraged to reach out to their state veterinarian prior to sample collection because state labs, which are part of the National Veterinary Diagnostic Laboratory, are often the only labs permitted to perform avian influenza testing. State labs should be contacted before any post-mortem samples are collected to ensure proper sampling techniques. As a reminder, carcass can be refrigerated or chilled but should never be frozen prior to diagnostic sample collection.
Nasal and ocular swabs, urine, and whole blood samples can be submitted to the appropriate diagnostic laboratory. If the suspected animal is deceased, veterinary teams should also submit a 1” x 2” section of brain.10 All samples should be fresh, refrigerated, and shipped in an insulated box with ice packs.
Marshall explained that biosecurity is the key to managing the virus on farms and amongst animals. She encourages anyone coming into your house or farm to wear clean clothing, hats, and shoes to avoid cross contamination and disease spread via fomites. If animals must be transported, it is important to thoroughly disinfect all shipping, handling, and transport areas and vehicles, especially for larger animals on the road for shows or competitions.
“Biosecurity also means we should be able to have a record and a log of every person that came to the fair, animal exhibition, or farm. We want to keep a record of who was there, how long they were there, and if that person had been to other farms that day or that week. Farms should have designated entry points, wheel or vehicle wash stations, and separate holding areas or pens for incoming and outgoing animals. All state ordinances and laws should be followed regarding intra and interstate health certificates” Marshall continued.
“We cannot understate the importance of PPE…. which includes gloves, goggles, gowns, and face masks. In addition, hand hygiene is critical and includes very frequent and thorough hand washing with soap and water. Individuals should be mindful of not handling animals or animal matter and then immediately handling their cell phones. Individuals should not be on their cell phones texting or making notes or anything like that, and then turn around and scroll on that same cell phone when you're eating your ham sandwich or drinking your coffee,” Marshall noted.
As avian influenza has now been detected in felines, it is recommended to use caution when letting cats go outside, especially if they live within an agricultural setting. The CDC also recommends feline owners who have interacted with animals or birds with unknown health status change their clothes and shoes before interacting with personal pets.11 For owners of potentially infected pets, veterinarians should refer them to their state's Department of Public Health and Environment in order to monitor human exposure and potential human disease transmission.
For veterinarians, it is recommended to use isolation and appropriate patient handling protocols for suspected infectious and/or zoonotic disease cases, including negative airflow if possible. Staff should contain and thoroughly disinfect any equipment used to treat the potentially infected patient. Staff should wear proper PPE and limit staff contact with animals in isolation facilities to the best of their abilities.
The CDC H5N1 Science Agenda outlines key research issues regarding the H5N1 virus outbreak in dairy cows and other animals in the U.S. It includes:
1. Updated recommendations for agricultural fair organizers to reduce the risk of influenza A viruses spreading between animals and people.
2. Strategies to protect farm workers, utilizing outreach in affected counties via social media and digital displays in English and Spanish.
3. Collaboration with public health labs to monitor influenza testing and surveillance data, focusing on unusual trends in areas where H5N1 has been detected.
Wastewater contaminated with the droppings of infected animals may harbor the virus. Individuals who encounter contaminated water or who work in wastewater management could be at risk, especially if proper precautions are not taken.
Genetic testing in wastewater is increasingly used to track the spread of H5N1 in the US, particularly among dairy herds, raising public health concerns. While PCR testing is standard for virus detection, its specificity issues may miss some variants. Recent research found H5N1 in wastewater across nine Texas cities, suggesting animal rather than human sources. Individuals in close contact with infected animals or consuming unpasteurized milk face greater risk. Overall, wastewater testing is becoming essential for disease surveillance.12
If poultry infected with avian influenza contaminates milk products, consuming unpasteurized or raw milk could pose a risk of transmission. Pasteurization typically eliminates such viruses, but raw products carry a higher risk.
Health officials warn against consuming unpasteurized milk due to the H5N1 bird flu virus found in more than 140 US dairy herds and high levels in raw milk. The virus can remain infectious on milking equipment for several hours and in unpasteurized milk for at least 1 hour. The milking process, which involves automated equipment and manual cleaning, increases exposure risks for workers.13
Avian flu can be traced back to northern Italy in 1878, when a contagious disease of poultry with a high mortality rate was described, and then referred to as “fowl plague.” It was eventually determined that the “fowl plague” described was caused by a virus, which in 1955 was determined to be a type A influenza virus. In 1971, influenza viruses were originally classified based on the antigenic properties of their structural and surface proteins nucleoprotein hemagglutinin and neuraminidase proteins, as well as the species of origin. In 1981, the name “fowl plague” was officially replaced with avian influenza at the First International Symposium of Avian Influenza.
According to Justin Brown, DVM, PhD, wildlife veterinarian and assistant teaching professor at Penn State University, the avian influenza virus has historically been most commonly detected within wild birds—mostly waterfowl, shorebirds, and gulls. However, this unique strain of the virus, H5N1 highly pathogenic avian influenza virus, has very unique properties that are very different from most avian influenza viruses, including the ability to infect and cause disease in a wide diversity of avian and mammal hosts.
“A wide diversity of avian influenza viruses circulate in wild birds without causing any outward signs of disease.Historically, when those have been low pathogenic and caused mild or no disease if they transmitted to chickens.However, if those low pathogenic avian influenza viruses were of the H5 or H7 subtype and spilled over into chickens, they could adapt to chickens and mutate to become highly pathogenic viruses that were associated with acute and high mortality.This historically has been a rare event.Once the virus adapts to chickens or related poultry species, these highly pathogenic avian influenza viruses have rarely transmitted back to wild birds.”
“This one-way spillover of avian influenza virus from a wild bird reservoir to poultry no longer explains what we are seeing with the H5N1 strains that are impacting the health of wild birds, poultry, and an increasing number of mammal species in multiple continents.This group of highly pathogenic avian influenza viruses was first isolated in China in 1997. Since then, these viruses have evolved into a group of viruses that differ genetically and biologically.Specifically, these H5N1 viruses have exhibited a unique ability to infect and cause disease in a wide range of bird and mammal hosts. As opposed to the historic “one-way” spillover events, some strains of this virus have re-adapted back to wild birds.What this means is that certain wild bird species can serve as reservoirs for the virus; this is very rare for a highly pathogenic avian influenza virus and makes control or eradication of the virus extremely difficult.
Brown also explained there are multiple routes for the virus to spread from wild birds, including direct contact with an infected bird (dead or alive) or contact with an infected birds feces (a common route that the virus is shed).Exposure to the virus through infected environmental samples has occurred and is possible.This often may be in habitats in which large numbers of infected birds are present and can contaminate the environment with virus, such as waterfowl habitat.
As the threat of infectious diseases continues to grow, veterinarians and animal health experts are facing an increasing challenge in managing species that may serve as reservoirs for transmission. Although the traditional focus for control of avian influenza has been on species like waterfowl and gulls, the list of potential infected bird or mammal species is expanding. This list includes wild and domestic felines. Brown stated that ensuring separation between naive host species and known reservoirs remains a key strategy.
“The real challenge is that we're seeing more and more species involved in the transmission and maintenance of H5N1, which includes highly susceptible species that are infected and die, as well as the species that are infected in the absence of outward disease.. Before we focused on the “high risk species”, such as ducks, gulls and shorebirds, for controlling the spread of avian influenza. Now our focus has to expand to new species that we’ve identified as reservoirs, and we have to continue to conduct research to identify new hosts involved in the epidemiology of this virus. This is critically important for understanding the risk of exposure to the virus and for preventing spillover into unique or atypical hosts like cats,” Brown explained.
“Over the last few years, we have seen increasing cases of H5N1 in wild and domestic carnivores and scavengers. Certainly species appear to be highly susceptible to infection and are likely to experience severe and fatal disease, including red foxes, domestic cats, and large felids. Presumably, the increasing cases we see in these animals is related to multiple factors, including a high susceptibility of the currently circulating strain and increased numbers of sick or dead infected animals on the landscape that are available to be scavenged.Understanding the wild and domestic animals that are most likely to be infected is an important step for preventing exposure in our highly susceptible companion animals, such as cats. In some systems, such outdoor cats, this may be more challenging,” he continued.
With the rise in cases infecting novel species, Brown discussed that avian influenza has the potential to change the way veterinary teams look at sick patients in their clinics. In cats and other mammals, H5N1 virus infection can present with neurologic and/or respiratory signs. Unfortunately, this presentation is not specific to highly pathogenic avian influenza, and looks similar to many other more common diseases in domestic cats. However, with the cases in domestic cats, highly pathogenic avian influenza virus should be on the rule-out list for any cat exhibiting consistent clinical signs, particularly if there is a history of potential exposure to infected animals or their habitats.
“Avian influenza virus is widespread in wild birds and the virus is continuing to evolve. Influenza viruses have a unique ability to adapt and change, and this is particularly true for H5N1. Over the last 20 years, I can’t count the number of times we’ve said ‘well this is something new we haven't seen before’ .This massive ability to adapt and change makes avian influenza virus challenging to control or manage," he continued.
A lot of times, we learn about a pathogen in veterinary school or early in our careers, and then we retain the old textbook version in our mind. This won’t work with highly pathogenic avian influenza virus, because of how much the virus is changing.Rather, as practitioners or anyone in the animal health field, it is important that you keep a current understanding of the disease. Fortunately or unfortunately, it wouldn't surprise me if in 1, 2, [or] 3, years, the epidemiology of the highly pathogenic avian influenza virus outbreak in North American is quite different” Brown concluded.
As H5N1 poses a multifaceted threat to both animal and human health, ongoing research, surveillance, and public health preparedness strategies are essential. The adaptability of the virus calls for a proactive approach to disease management, ensuring a coordinated response to safeguard public health.
As H5N1 poses a multifaceted threat to both animal and human health, ongoing research, surveillance, and public health preparedness strategies are essential. The adaptability of the virus calls for a proactive approach to disease management, ensuring a coordinated response to safeguard public health.
