Commonground
Senior Moderator
[bolding is mine]
Abstract
Highly pathogenic avian influenza viruses of the H5N1 subtype have recently undergone a major geographic expansion. At the same time, these viruses have substantially broadened their host range. Typically, H5N1 circulates in poultry, waterfowl, and shorebirds, but the breadth of avian species affected has increased and spillover to mammals has become common. The recent changes in H5N1 ecology have brought die-offs in wild birds and marine mammals, a sustained outbreak in US dairy cattle, and recurring spillover into domestic cats. Maintenance of H5N1 in wild birds in North America has furthermore produced a persistent risk to US poultry. Circulation of H5N1 in domestic animals has resulted in human infections, especially among farm workers and veterinarians, events that heighten the potential for a pandemic. Through their central position in One Health, veterinarians have a critical role to play in mitigating the impact of H5N1 on animal health, agriculture, and public health.
-snip-
An Evolving Threat
Globally, H5N1 has been around a long time—the progenitor emerged[SUP]2[/SUP] in 1996—but the virus is undergoing a burst of evolution that has increased its diversity and changed how it behaves. The last 5 years have seen a huge expansion across the world, driven by migrating birds.[SUP]3,4[/SUP] At the same time, H5N1 has begun to infect many different mammals.[SUP]5[/SUP] The US has seen H5N1 before, with poultry outbreaks in 2015 that were stopped,[SUP]6[/SUP] but this time is different. Outbreaks in different species are continuing to wax and wane, but it looks like H5N1 is now here to stay. Impact on Dairies
On dairy farms affected with H5N1, milk production drops, threatening producers’ financial stability and their employees’ livelihoods.[SUP]7[/SUP] The virus spreads easily to nearby farms, sowing concern among community members. As of May 2025, infection on 1,065 dairy premises in the US has been confirmed.[SUP]8[/SUP] Risks to Poultry
Poultry farms are being hit harder than dairy farms, with the virus coming from wild birds and dairy cows. When poultry are infected, highly pathogenic H5N1 is quickly lethal and, if not tightly contained, it spreads among poultry operations. For this reason, detections of H5N1 in poultry are addressed through depopulation. Between 2022 and May 2025, H5N1 has infected 1,700 poultry flocks across the country, leading to the culling of 169 million birds.[SUP]9[/SUP] It can take a farm months or years to recover from an outbreak. The magnitude of the problem has become apparent to American consumers through soaring egg prices. Spillover to Cats
Cases in cats were first detected in association with early dairy outbreaks, with farm cats suffering lethal infection.[SUP]10[/SUP]Detections in farm cats have continued,[SUP]11,12[/SUP] but cats without links to dairy farms are also being impacted. Raw pet food is also a confirmed source of some infections.[SUP]13[/SUP] Feeding on infected wild birds is a possible source of feline infection; however, it is difficult to gauge local risk levels associated with wild birds due to the sparseness of H5N1 surveillance data available to the public in real time. Threats to Wildlife
A large number of wildlife species that this virus has not previously threatened are continuing to be affected, including both birds and mammals. The most visible and at risk are keystone predators,[SUP]14,15[/SUP] such as the bald eagle, snowy owl, and peregrine falcon, and marine mammals, including several species of seals common in coastal areas.[SUP]16–18[/SUP] The virus has also swept through colonial nesting birds and infiltrated common species in human environments such as raccoons, skunks, and captive zoo animals.[SUP]19,20[/SUP] Spillover to
Humans
The effect of this virus on domestic animals is extremely damaging, and its impact on wildlife populations is worrying. But there is another layer to this problem—the virus can infect people. Although just 70 human infections with H5N1 have been confirmed[SUP]21[/SUP] in the US since 2024, a study[SUP]22[/SUP] of the immune responses of dairy and poultry workers reveals that many more have been infected. A serological survey[SUP]23[/SUP] of bovine veterinarians also reveals evidence of infection in 3 of 150 screened practitioners, 2 of whom had not knowingly attended to H5N1-afflicted animals. Fortunately, the disease experienced has been mild in most cases (there has been 1 death).[SUP]24[/SUP] However, even mild infections are a concern for public health because such spillover events increase the likelihood of a pandemic.[SUP]25[/SUP]
Pandemic Risk
The viruses that are spreading in cows and poultry have not spread between humans. But influenza viruses are constantly changing.[SUP]4[/SUP] When an H5N1 virus infects a human, it has an opportunity to adapt, either through mutation or by exchanging genes with those of human influenza viruses.[SUP]26[/SUP] Therein lies the pandemic threat. If the virus changes in ways that support transmission from person to person, it could trigger an outbreak that spreads across the globe in a matter of weeks. Pandemics can kill millions of people, infect billions, and cost the global economy trillions of dollars.[SUP]27[/SUP]
What Can Be Done?
Circulation of H5N1 in wild animals is largely out of our ability to control, but spillover to domestic animals and humans can be mitigated and this can help prevent new variants from arising. Knowledge is our first defense. Recent variants of H5N1 are changing everything we thought we knew about influenza. Infection of mammary tissues, rapid spread to the brain in many species, and mortality in species that we didn’t think were influenza hosts all signal a need to add H5N1 to the differential diagnosis for many presentations. For example, cats with neurological signs and a high index of suspicion should be tested for H5N1 and, while results are pending, these animals should be handled with personal protective equipment appropriate for influenza.[SUP]28[/SUP] If tests are positive, appropriate communication of data and samples is critical.[SUP]29[/SUP]
Recent changes in how H5N1 behaves also call for changes in biosecurity practices on farms.[SUP]30[/SUP] Just as the potential for influenza virus transmission to poultry and pigs has long shaped biosecurity measures for these species, the sustained circulation of H5N1 in dairy cattle and potential for introductions into other new species call for a recalibration of risk across the agricultural sector. For example, practices should be updated to limit the movement of equipment and personnel between dairy and poultry premises and dairy and swine premises (Figure 1).
-snip-
The Unique Role of Veterinarians
Veterinary practitioners have a critical role to play in educating clients about the risks H5N1 presents and making recommendations to lower those risks. Those caring for companion animals can communicate the risk to felines and other animals from wild birds, raw pet food, and raw dairy products. Keeping cats indoors would mitigate the risk of their infection. Large animal veterinarians can communicate the threat to agriculture and recommend limiting exposure of livestock to wild birds, controlling the movement of equipment and personnel between farms, and testing herds so that spread can be interrupted where H5N1 is present. Veterinarians focused on wildlife and public health can inform clients and colleagues of the risks that bird flu presents to wildlife, themselves, and the general population. Measures to reduce risks include the proper disposal of infected carcasses, showering and changing clothes after working with infected animals and before returning home, the pasteurization of dairy products, and thorough cooking of meat products. Veterinarians are on the front lines of this outbreak and should be aware of their role in identifying potential infection and communicating with public health and research professionals who respond to and track this panzootic virus.
Closing
H5N1 is rewriting the rules and, in so doing, presenting major challenges to the veterinary profession. Interested veterinarians may get involved more by working with their clients and researchers to understand transmission to veterinary hosts and exploring avenues to mitigate the impacts of continued H5N1 circulation. To reduce the One Health risk H5N1 presents, affected communities need to understand the problem and come together to enact the best solutions.
https://avmajournals.avma.org/view/...5.0294/javma.25.05.0294.xml?tab_body=fulltext
Abstract
Highly pathogenic avian influenza viruses of the H5N1 subtype have recently undergone a major geographic expansion. At the same time, these viruses have substantially broadened their host range. Typically, H5N1 circulates in poultry, waterfowl, and shorebirds, but the breadth of avian species affected has increased and spillover to mammals has become common. The recent changes in H5N1 ecology have brought die-offs in wild birds and marine mammals, a sustained outbreak in US dairy cattle, and recurring spillover into domestic cats. Maintenance of H5N1 in wild birds in North America has furthermore produced a persistent risk to US poultry. Circulation of H5N1 in domestic animals has resulted in human infections, especially among farm workers and veterinarians, events that heighten the potential for a pandemic. Through their central position in One Health, veterinarians have a critical role to play in mitigating the impact of H5N1 on animal health, agriculture, and public health.
-snip-
An Evolving Threat
Globally, H5N1 has been around a long time—the progenitor emerged[SUP]2[/SUP] in 1996—but the virus is undergoing a burst of evolution that has increased its diversity and changed how it behaves. The last 5 years have seen a huge expansion across the world, driven by migrating birds.[SUP]3,4[/SUP] At the same time, H5N1 has begun to infect many different mammals.[SUP]5[/SUP] The US has seen H5N1 before, with poultry outbreaks in 2015 that were stopped,[SUP]6[/SUP] but this time is different. Outbreaks in different species are continuing to wax and wane, but it looks like H5N1 is now here to stay. Impact on Dairies
On dairy farms affected with H5N1, milk production drops, threatening producers’ financial stability and their employees’ livelihoods.[SUP]7[/SUP] The virus spreads easily to nearby farms, sowing concern among community members. As of May 2025, infection on 1,065 dairy premises in the US has been confirmed.[SUP]8[/SUP] Risks to Poultry
Poultry farms are being hit harder than dairy farms, with the virus coming from wild birds and dairy cows. When poultry are infected, highly pathogenic H5N1 is quickly lethal and, if not tightly contained, it spreads among poultry operations. For this reason, detections of H5N1 in poultry are addressed through depopulation. Between 2022 and May 2025, H5N1 has infected 1,700 poultry flocks across the country, leading to the culling of 169 million birds.[SUP]9[/SUP] It can take a farm months or years to recover from an outbreak. The magnitude of the problem has become apparent to American consumers through soaring egg prices. Spillover to Cats
Cases in cats were first detected in association with early dairy outbreaks, with farm cats suffering lethal infection.[SUP]10[/SUP]Detections in farm cats have continued,[SUP]11,12[/SUP] but cats without links to dairy farms are also being impacted. Raw pet food is also a confirmed source of some infections.[SUP]13[/SUP] Feeding on infected wild birds is a possible source of feline infection; however, it is difficult to gauge local risk levels associated with wild birds due to the sparseness of H5N1 surveillance data available to the public in real time. Threats to Wildlife
A large number of wildlife species that this virus has not previously threatened are continuing to be affected, including both birds and mammals. The most visible and at risk are keystone predators,[SUP]14,15[/SUP] such as the bald eagle, snowy owl, and peregrine falcon, and marine mammals, including several species of seals common in coastal areas.[SUP]16–18[/SUP] The virus has also swept through colonial nesting birds and infiltrated common species in human environments such as raccoons, skunks, and captive zoo animals.[SUP]19,20[/SUP] Spillover to
Humans
The effect of this virus on domestic animals is extremely damaging, and its impact on wildlife populations is worrying. But there is another layer to this problem—the virus can infect people. Although just 70 human infections with H5N1 have been confirmed[SUP]21[/SUP] in the US since 2024, a study[SUP]22[/SUP] of the immune responses of dairy and poultry workers reveals that many more have been infected. A serological survey[SUP]23[/SUP] of bovine veterinarians also reveals evidence of infection in 3 of 150 screened practitioners, 2 of whom had not knowingly attended to H5N1-afflicted animals. Fortunately, the disease experienced has been mild in most cases (there has been 1 death).[SUP]24[/SUP] However, even mild infections are a concern for public health because such spillover events increase the likelihood of a pandemic.[SUP]25[/SUP]
Pandemic Risk
The viruses that are spreading in cows and poultry have not spread between humans. But influenza viruses are constantly changing.[SUP]4[/SUP] When an H5N1 virus infects a human, it has an opportunity to adapt, either through mutation or by exchanging genes with those of human influenza viruses.[SUP]26[/SUP] Therein lies the pandemic threat. If the virus changes in ways that support transmission from person to person, it could trigger an outbreak that spreads across the globe in a matter of weeks. Pandemics can kill millions of people, infect billions, and cost the global economy trillions of dollars.[SUP]27[/SUP]
What Can Be Done?
Circulation of H5N1 in wild animals is largely out of our ability to control, but spillover to domestic animals and humans can be mitigated and this can help prevent new variants from arising. Knowledge is our first defense. Recent variants of H5N1 are changing everything we thought we knew about influenza. Infection of mammary tissues, rapid spread to the brain in many species, and mortality in species that we didn’t think were influenza hosts all signal a need to add H5N1 to the differential diagnosis for many presentations. For example, cats with neurological signs and a high index of suspicion should be tested for H5N1 and, while results are pending, these animals should be handled with personal protective equipment appropriate for influenza.[SUP]28[/SUP] If tests are positive, appropriate communication of data and samples is critical.[SUP]29[/SUP]
Recent changes in how H5N1 behaves also call for changes in biosecurity practices on farms.[SUP]30[/SUP] Just as the potential for influenza virus transmission to poultry and pigs has long shaped biosecurity measures for these species, the sustained circulation of H5N1 in dairy cattle and potential for introductions into other new species call for a recalibration of risk across the agricultural sector. For example, practices should be updated to limit the movement of equipment and personnel between dairy and poultry premises and dairy and swine premises (Figure 1).
-snip-
The Unique Role of Veterinarians
Veterinary practitioners have a critical role to play in educating clients about the risks H5N1 presents and making recommendations to lower those risks. Those caring for companion animals can communicate the risk to felines and other animals from wild birds, raw pet food, and raw dairy products. Keeping cats indoors would mitigate the risk of their infection. Large animal veterinarians can communicate the threat to agriculture and recommend limiting exposure of livestock to wild birds, controlling the movement of equipment and personnel between farms, and testing herds so that spread can be interrupted where H5N1 is present. Veterinarians focused on wildlife and public health can inform clients and colleagues of the risks that bird flu presents to wildlife, themselves, and the general population. Measures to reduce risks include the proper disposal of infected carcasses, showering and changing clothes after working with infected animals and before returning home, the pasteurization of dairy products, and thorough cooking of meat products. Veterinarians are on the front lines of this outbreak and should be aware of their role in identifying potential infection and communicating with public health and research professionals who respond to and track this panzootic virus.
Closing
H5N1 is rewriting the rules and, in so doing, presenting major challenges to the veterinary profession. Interested veterinarians may get involved more by working with their clients and researchers to understand transmission to veterinary hosts and exploring avenues to mitigate the impacts of continued H5N1 circulation. To reduce the One Health risk H5N1 presents, affected communities need to understand the problem and come together to enact the best solutions.
https://avmajournals.avma.org/view/...5.0294/javma.25.05.0294.xml?tab_body=fulltext