Highly pathogenic avian influenza H5N1 is not new, but its behavior is changing. H5N1 has historically been a disease of poultry and wild birds, but recent spillover events in mammals—ranging from domestic cats to dairy cattle—raise new questions about how this virus is evolving and the implications for veterinary medicine. This article answers key questions about the impact of H5N1 in 2025.
What is the history of H5N1?
H5N1 was first identified in wild waterfowl in China in 1996 and has caused regular outbreaks of disease in poultry and sporadic poultry-associated infection in humans, mainly in Asia.1 H5N1 is one of the most closely watched influenza viruses because of its presence in wild birds, its significant impact on poultry, and the potential for spillover and fatal infection in mammals, including humans. Historical mortality rates in humans with H5N1 are generally assumed to be 30% to 50%.
H5N1 emerged in North America in late 2021, and at least 6 different strains have been introduced since that time. These strains have continued to evolve and have recombined with circulating low-pathogenicity strains regularly found in North America. The molecular epidemiology is thus complex, but the net result is that different subtypes of H5N1 influenza are circulating in birds throughout the world.
How is H5N1 spreading in the United States? Which species are most affected?
H5N1 is widespread in wild birds. The susceptibility of different bird species is variable, with high mortality rates in some species and mild or rare disease in others. Commercial and backyard poultry have been severely impacted. More than 160 million birds have been infected in the United States since 2022.2 A wide range of wild mammals has been affected, most often species that catch or scavenge birds.
Spillovers from exposure to infected wild birds have been rare in cats and rarer in dogs as H5N1 emerged in wild birds, but the epidemiology of the virus has become more complicated in the past 1 to 2 years. A major development was emergence of a specific strain (B3.13 variant of H5N1) in US dairy cattle that spread widely.3 Recently, another strain (D1.1) has been found in US dairy cattle and resulted in infection and death in cats on dairy farms and cats fed raw poultry or raw milk.4
Can dogs, cats, or other companion animals be infected with H5N1?
Cats are susceptible to H5N1 and often have fatal infections.5-8 Dogs are less commonly affected but are susceptible.9 Ferrets are highly susceptible.10 The susceptibility of other domestic mammals is less understood, but H5N1 can presumably infect any mammal. Pet birds are also at risk, with the degree of illness varying between species.
What are the clinical signs of H5N1 in cats & dogs?
The range of disease in cats is not well understood. Most reported infections have been severe, with neurologic disease predominating and concurrent respiratory disease present in some cases.7,11-14 Severe respiratory disease without apparent neurologic disease has also been identified. Whether severe disease is the typical presentation or testing has been limited to cats with severe disease and whether cats can also develop mild influenza-like disease similar to that which occurs with other common causes of upper respiratory tract disease is unknown. This knowledge gap hampers risk assessment and determination of when to use enhanced infection-control measures.
Infection in dogs is rare. Whether dogs are less likely to be exposed or are inherently less susceptible is unclear; both could be true, but dogs are likely more resistant to infection (or at least clinically apparent infection). Nonetheless, severe disease has been reported, including fatal neurologic and respiratory disease.9,15,16
What H5N1 testing is available at diagnostic laboratories? When is testing indicated?
PCR testing of respiratory specimens (eg, oropharyngeal swab, nasal swab) is the main diagnostic approach. H5N1 testing varies between laboratories and is changing as testing methods are adapted. Testing is indicated if H5N1 is suspected in a patient. Laboratories that do not specifically indicate H5N1 testing should be contacted to determine what type of testing they perform. Some laboratories perform specific H5N1 testing, while others perform a test that detects all influenza A strains (ie, pan influenza). Laboratories that test for pan influenza should submit positive samples to a reference laboratory for specific H5N1 testing. A positive pan influenza result in a cat is most likely due to H5N1 but can be due to other avian influenza strains that normally circulate in birds or rare spillovers of human influenza. A positive pan influenza test in a dog could be due to H3N2 canine influenza, avian influenza, or human influenza; specific strain testing is required.
Are H5N1 vaccines available for animals besides poultry?
Vaccination is not an option for domestic mammals, as no licensed H5N1 vaccines are currently available. Canine H3N2 and H3N8 influenza vaccines are unlikely to offer protection against H5N1.
Can H5N1 be transmitted from pets to humans and between pets? What precautions are needed in veterinary medicine?
The transmission risk posed by infected dogs and cats is unclear. Concern is highest with cats because they are more commonly affected, and PCR results from some investigations have suggested cats can have a reasonably high viral burden in respiratory secretions. Without additional data, infected cats (and likely dogs) can be assumed to pose some degree of transmission risk through respiratory secretions and, potentially, aerosols. Influenza virus can also be shed in feces. Viral levels tend to be low, and the risk from feces is likely low but cannot be ruled out.
Because of the likely risk for transmission, patients with suspected H5N1 should be managed as infectious. Hospitalized patients should be housed in an adequately designed and operated isolation ward (see Suggested Reading). A high-efficiency particulate air (HEPA) filter can be added to the room to reduce aerosol risks. Personal protective equipment, including a proper-fitting N95 mask, eye protection (goggles or face shield), gown, and gloves, should be worn when handling patients. Patients should be handled as little as possible, and aerosol-generating procedures (eg, intubation) should be avoided unless absolutely necessary. Influenza virus is readily inactivated by routine clinic disinfectants.
In the home, companion animals with suspected H5N1 should be isolated (eg, housed alone in a room with a HEPA filter). Contact with humans and other animals should be minimized, and humans who have contact with the animal should wear personal protective equipment when in the same room. Public health should be contacted for management of humans with potential exposure; management may include prophylactic antivirals for high-risk individuals.
What should pet owners know about the risk for H5N1 associated with raw diets?
Cases of H5N1 influenza in cats, some fatal, have been linked to contaminated raw poultry in surveillance reports. High-pressure pasteurization should reduce the risk, but infections from high-pressure–pasteurized diets have also been reported in cats. High-pressure pasteurization is therefore a risk-reduction—not a risk-elimination—method. If a raw diet is fed, a high-pressure–pasteurized diet that does not contain poultry should be used. The risk from beef is much lower compared with chicken, but raw beef from a region where H5N1 is present in dairy cattle should be avoided. Currently, H5N1 only affects dairy cattle in the United States, but it could affect cattle in other countries.
Does raw milk pose a risk?
Yes; fatal H5N1 influenza infections in cats have been linked to raw milk ingestion, both on farms and from commercial raw milk, in surveillance reports. Raw milk should not be fed to cats. Pasteurization effectively eliminates H5N1.
How likely is H5N1 to become endemic in North American wildlife populations? What are the implications for domestic species?
H5N1 has been widespread in wild birds in North America for several years and is unlikely to disappear quickly. If H5N1 becomes endemic in wild birds, risk for exposure of domestic animals and humans will be ongoing, and H5N1 influenza will remain a clinical consideration in domestic animals. The impact on domestic mammals will depend on the frequency of spillovers, whether mammalian adaptations allow for transmission between domestic animals, and the severity of disease. Even if an adaptation for mammal-to-mammal transmission does not occur, further (often fatal) infections in cats and, to a lesser degree, dogs are expected.
What is the potential for H5N1 to mutate and increase mammalian transmissibility?
A primary concern is antigenic drift, gradual mutation of H5N1 into a virus that can more easily infect and spread between mammals. Mutations occur through viral replication and are an ever-present risk. Many mutations are required for H5N1 to become a true human, canine, or feline virus, but every mammalian infection increases that risk.
Another main concern is antigenic shift, whereby different influenza viruses recombine to create a new virus. Recombination of H5N1 with human influenza virus could result in a virus that is highly transmissible between humans and different from human influenza viruses, resulting in minimal antibody protection from previous infection or vaccination. This recombinant virus could cause a pandemic, particularly if it is able to cause severe disease. Similar concern would apply if H5N1 recombined with H3N2 canine influenza; however, because canine influenza is much rarer than human influenza, most dogs have no natural or vaccination protection against H3N2, and a recombinant strain would therefore have fewer implications unless it was more capable of causing serious disease. Regardless, this recombinant strain would be of concern because it would be an additional circulating influenza strain that could recombine with human influenza virus.
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How should pet owners expressing concern or asking about H5N1 be advised?
Basic discussions with owners about H5N1 are important to increase awareness, allay fears, and address potentially negative decisions (eg, surrendering cats because of disease concerns). Owners should be counseled to keep cats indoors when possible and not feed raw poultry or raw milk to keep the risk for H5N1 exposure low. Enclosed outdoor spaces for cats (ie, catios) can pose some risk in geographic areas where H5N1 is circulating. Songbirds and other small birds seem to be less susceptible to H5N1 than waterfowl but are potential sources of exposure for cats; therefore, birds should be prevented from entering catios, and cats should not be allowed in these enclosures when H5N1 is present in the area.
Conclusion
The overall risk of H5N1 in companion animals and livestock remains unclear. Staying informed, practicing good biosecurity, and monitoring emerging research are key. Clinicians are at the forefront of surveillance and education, fielding owner concerns, reporting suspected cases, and adjusting infection-control protocols. The role of clinicians in mitigating the impact of H5N1 is more important than ever.
Frequent updates from Dr. Weese about H5N1 in North American livestock and companion animals are available at Worms & Germs Blog.