Blue-winged teal in Texas. Inset shows avian influenza virus
The USGS National Wildlife Health Center (NWHC) conducts surveillance in wild birds to facilitate early detection and situational awareness for high consequence pathogens, including highly pathogenic avian influenza (HPAI) viruses.
Investigating Avian Influenza in U.S. Wildlife
Media
Sources/Usage: Public Domain. View Media Details
In addition to investigating wildlife mortality events since our founding in the 1970s, the NWHC conducts research to better understand processes and environmental factors that influence spread, distribution, and transmission of HPAI in wild birds and poultry. Surveillance activities conducted by NWHC, in support of national HPAI surveillance objectives, provide critical information regarding avian influenza strains currently circulating in the U.S. and enhance our understanding of disease impacts on wild birds. Wild bird surveillance also provides early warning for the agricultural sector and helps inform the need for increased biosecurity at poultry facilities.
The NWHC routinely tests samples from wild birds for avian influenza viruses. In December 2014, the NWHC detected HPAI viruses of Asian origin in wild waterfowl in the state of Washington (Ip et al. 2015). By the end of 2015, losses associated with this HPAI outbreak exceeded 50 million poultry, resulting in over $3 billion dollars in economic impacts.
Between 2016 and 2021, the NWHC tested over 3,400 wild-bird carcasses and over 12,400 swab samples from healthy wild birds for the presence of avian influenza viruses. While HPAI was not detected in any of these samples, over 2,600 low pathogenicity avian influenza viruses were identified and characterized, providing critical information to wildlife and agricultural officials regarding the spatial distribution and strains of avian influenza viruses circulating in our nation’s wildlife.
Media
Sources/Usage: Public Domain. View Media Details
HPAI Surveillance Update 2021-2025
Highly pathogenic avian influenza was detected in North American wild birds in December 2021 for the first time since 2015. Initial detections occurred in Canada (Newfoundland and Labrador) and the United States (South Carolina). Multiple introductions of HPAI H5N1 into North America have now been documented to have occurred in 2021 and 2022. Since that time, HPAI H5/H5N1 has been detected in over 10,000 wild birds representing over 160 unique North American species, and over 400 wild mammals representing over 20 unique North American species. Confirmed detections in wild birds, mammals, poultry, and livestock in the United States are reported online by the U.S. Department of Agriculture and a map of confirmed detections in wild birds in the United States is available from the NWHC.
Interagency Partnerships
The NWHC serves on the U.S. Interagency Steering Committee for Surveillance for Highly Pathogenic Avian Influenza in Wild Birds and conducts morbidity and mortality event investigations in support of the Interagency Strategic Plan for Early Detection and Monitoring for Avian Influenzas of Significance in Wild Birds. The NWHC also serves as an affiliate member of the National Animal Health Laboratory Network (NAHLN) and collaborates with the U.S. Department of Agriculture's National Veterinary Services Laboratory on diagnostic testing of samples collected from wild birds for avian influenza surveillance.
Additional Resources
- Distribution of Highly Pathogenic Avian Influenza in wild birds in North America, 2021-2025 (Map)
- Avian influenza (USDA)
- Information on Avian Influenza (CDC)
- Implementation Plan for Avian Influenza Surveillance in Waterfowl in the United States
- NWHC Wildlife Health Bulletins
- Winter 2025 Update on Highly Pathogenic Avian Influenza H5 - March 2025
- Status and Response to Detections of Highly Pathogenic Avian Influenza H5N1 in North America – February 2022
- Detection of Highly Pathogenic Avian Influenza H5N1 in an Exhibition Farm in Newfoundland and Labrador, Canada - December 2021
- Winter 2021 Update on Highly Pathogenic Avian Influenza Viruses Circulating Globally in Wild Birds - December 2021
More Information on Avian Influenza
Avian influenza is a viral disease caused by various strains of avian influenza viruses that can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), which refers to a specific virus’ capability to cause mortality in domestic poultry. It remains a high consequence global disease with the potential to threaten wildlife, agriculture, and human health. Check out the main Avian Influenza page to learn more.
Recent Scientific Publications
The following publications include NWHC contributions to avian influenza research:
- Elsmo, E.J., Wünschmann, A., Beckmen, K.B., Broughton-Neiswanger, L.B., Buckles, E.L., Ellis, J.H., Fitzgerald, S.D., Gerlach, R., Hawkins, S., Ip, H.S., Lankton, J.S., Lemley, E.M., Lenoch, J.B., Killian, M.L., Lantz, K., Long, L., Maes, R., Mainenti, M., Melotti, J., Moriarty, M.E., Nakagun, S., Ruden, R.M., Shearn-Bochsler, V.I., Thompson, D.A., Torchetti, M.K., Van Wettere, A.J., Wise, A.G., Lim, A.L., 2023. Highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b infections in wild terrestrial mammals, United States, 2022. Emerging Infectious Diseases 29, 2451-2460. https://doi.org/10.3201/eid2912.230464
- Gass, J.D., Jr., Dusek, R.J., Hall, J.S., Hallgrimsson, G.T., Halldórsson, H.P., Vignisson, S.R., Ragnarsdottir, S.B., Jónsson, J.E., Krauss, S., Wong, S.-S., Wan, X.-F., Akter, S., Sreevatsan, S., Trovão, N.S., Nutter, F.B., Runstadler, J.A., Hill, N.J., 2023. Global dissemination of Influenza A virus is driven by wild bird migration through arctic and subarctic zones. Molecular Ecology 32, 198-213. https://doi.org/10.1111/mec.16738
- Hall, J.S., Grear, D.A., Krauss, S., Seiler, P., Dusek, R.J., Nashold, S., Webster, R.G., 2021. Highly pathogenic avian influenza virus H5N2 (Clade 2.3.4.4) challenge of mallards age appropriate to the 2015 midwestern poultry outbreak. Influenza and Other Respiratory Viruses 15, 767-777. https://doi.org/10.1111/irv.12886
- Hill, N.J., Bishop, M.A., Trovao, N.S., Ineson, K., Schaefer, A., Puryear, W.B., Zhou, K., Foss, A., Clark, D., McKenzie, K., Gass, J.D., Jr., Borkenhagen, L., Hall, J.S., Runstadler, J.A., 2022. Ecological divergence of wild birds drives avian influenza spillover and global spread. PLoS Pathogens18. https://doi.org/10.1371/journal.ppat.1010062
- Ip, H.S., Uhm, S., Killian, M.L., Torchetti, M.K., 2023. An evaluation of avian influenza virus whole-genome sequencing approaches using nanopore technology. Microorganisms 11. https://doi.org/10.3390/microorganisms11020529
- Kent, C.M., Ramey, A.M., Ackerman, J.T., Bahl, J., Bevins, S.N., Bowman, A.S., Boyce, W., Cardona, C., Casazza, M.L., Cline, T.D., De La Cruz, S.E.W., Hall, J.S., Hill, N.J., Ip, H.S., Krauss, S., Mullinax, J.M., Nolting, J.M., Plancarte, M., Poulson, R.L., Runstadler, J.A., Slemons, R.D., Stallknecht, D.E., Sullivan, J.D., Takekawa, J.Y., Webby, R.J., Webster, R.G., Prosser, D.J., 2022. Spatiotemporal changes in influenza A virus prevalence among wild waterfowl inhabiting the continental United States throughout the annual cycle. Scientific Reports 12. https://doi.org/10.1186/s13567-024-01397-5
- Prosser, D., Chen, J., Ahlstrom, C., Reeves, A.B., Poulson, R.L., Sullivan, J.D., McAuley, D., Callahan, C.R., McGowan, P.C., Bahl, J., Stallknecht, D.E., Ramey, A.M., 2022. Maintenance and dissemination of avian-origin influenza A virus within the northern Atlantic Flyway of North America. PLoS Pathogens 18. https://doi.org/10.1371/journal.ppat.1010605
- Ramey, A.M., Hill, N.J., DeLiberto, T.J., Gibbs, S.E.J., Hopkins, M.C., Lang, A.S., Poulson, R.L., Prosser, D., Sleeman, J.M., Stallknecht, D.E., Wan, X.-F., 2022. Highly pathogenic avian influenza is an emerging disease threat to wild birds in North America. Journal of Wildlife Management 86. https://doi.org/10.1002/jwmg.22171
- Tawidian, P., Torchetti, M.K., Killian, M.L., Lantz, K., Dilione, K.E., Ringenberg, J.M., Bevins, S.N., Lenoch, J., Ip, H.S., 2024. Genotypic clustering of H5N1 avian Influenza viruses in North America evaluated by ordination analysis. Viruses 16. https://doi.org/10.3390/v16121818
- Youk, S., Torchetti, M.K., Lantz, K., Lenoch, J.B., Killian, M.L., Leyson, C., Bevins, S.N., Dilione, K., Ip, H.S., Stallknecht, D.E., Poulson, R.L., Suarez, D.L., Swayne, D.E., Pantin-Jackwood, M.J., 2023. H5N1 highly pathogenic avian influenza clade 2.3.4.4b in wild and domestic birds: Introductions into the United States and reassortments, December 2021–April 2022. Virology 587. https://doi.org/10.1016/j.virol.2023.109860
Learn about other NWHC work on avian influenza.
Avian Influenza
Avian influenza is a viral disease caused by various strains of avian influenza viruses that can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI). It remains a global disease with potential high consequence with the potential to threaten wildlife, agriculture, and human health.
Below are photos related to avian influenza surveillance.
Blue-winged teal in Texas. Inset shows avian influenza virus
Blue-winged teal in Texas. Inset shows avian influenza virus
Testing for Avian Influenza
A USGS scientist takes a sample from a northern pintail duck (Anas acuta) to be tested for avian influenza.
A USGS scientist takes a sample from a northern pintail duck (Anas acuta) to be tested for avian influenza.
USGS biologists swab a hen mallard duck for avian influenza
USGS biologists swab a hen mallard duck for avian influenza in Rio Grande County, Colorado.
USGS biologists swab a hen mallard duck for avian influenza in Rio Grande County, Colorado.
Below are FAQs related to avian influenza.
What is Avian Influenza? What is Avian Influenza?
Avian influenza (AI) is caused by influenza type A viruses that historically have spread between wild birds (waterfowl and shorebirds) with occasional outbreaks in poultry (chickens, turkeys). These viruses can change over time and there has been a fundamental change with AI viruses now causing illness and death in wild birds, wild mammals, poultry, dairy cattle, cats, and people. Learn more...
How do scientists know if individual wild birds are infected with avian influenza? How do scientists know if individual wild birds are infected with avian influenza?
The U.S. Department of Agriculture implements a plan every year to monitor wild birds for highly pathogenic avian influenza. This national plan could not be implemented without the engagement of State partners, the National Flyway Council, the Department of the Interior, and other agencies. This coordinated effort is revised annually based on new science and data shared through the Interagency...
How is avian influenza spread? How is avian influenza spread?
Avian influenza is spread when wild and domestic birds shed the virus into the environment through their oral and nasal secretions as well as excrement. Following the 2015 avian influenza outbreak, USGS, USDA, and other partners confirmed through data on wild bird migratory patterns—including intercontinental movements and genetic analyses of viral strains—that the global spread of HPAI has been...
Can people get avian influenza? Can people get avian influenza?
While rare, human infections with avian influenza viruses have occurred. The Centers for Disease Control (CDC) consider the risk to the general public from infections to be low. There have been human cases associated with the dairy cattle and wild bird strains of HPAI. To date, most human cases have been associated with farm workers (poultry and dairy cattle operations) as well as people who have...
Can wild birds spread avian influenza to domestic poultry? Can wild birds spread avian influenza to domestic poultry?
Wild birds can directly or indirectly spread avian influenza to domestic poultry. The indirect route is likely most common due to viral contamination of the farm environment (feed, equipment, boots, clothes). The U.S. Department of Agriculture has found that farm-to-farm movement is also a major route for avian influenza spread, noting that farm biosecurity is critical to reduce the risk of avian...
What are the different types of avian influenza? What do the numbers next to H and N mean? What are the different types of avian influenza? What do the numbers next to H and N mean?
Avian influenza (AI) type A viruses are divided into subtypes based on two proteins on the surface of the virus: Hemagglutinin (HA), of which there are 16 subtypes (H1-H16). Neuraminidase (NA), of which there are 9 subtypes (N1-N9). Many combinations of HA and NA proteins are possible, for example H5N1, H5N2, H7N2, and H7N8. AI viruses are also classified into two groups based on their ability to...
The USGS National Wildlife Health Center (NWHC) conducts surveillance in wild birds to facilitate early detection and situational awareness for high consequence pathogens, including highly pathogenic avian influenza (HPAI) viruses.
Investigating Avian Influenza in U.S. Wildlife
Media
Sources/Usage: Public Domain. View Media Details
In addition to investigating wildlife mortality events since our founding in the 1970s, the NWHC conducts research to better understand processes and environmental factors that influence spread, distribution, and transmission of HPAI in wild birds and poultry. Surveillance activities conducted by NWHC, in support of national HPAI surveillance objectives, provide critical information regarding avian influenza strains currently circulating in the U.S. and enhance our understanding of disease impacts on wild birds. Wild bird surveillance also provides early warning for the agricultural sector and helps inform the need for increased biosecurity at poultry facilities.
The NWHC routinely tests samples from wild birds for avian influenza viruses. In December 2014, the NWHC detected HPAI viruses of Asian origin in wild waterfowl in the state of Washington (Ip et al. 2015). By the end of 2015, losses associated with this HPAI outbreak exceeded 50 million poultry, resulting in over $3 billion dollars in economic impacts.
Between 2016 and 2021, the NWHC tested over 3,400 wild-bird carcasses and over 12,400 swab samples from healthy wild birds for the presence of avian influenza viruses. While HPAI was not detected in any of these samples, over 2,600 low pathogenicity avian influenza viruses were identified and characterized, providing critical information to wildlife and agricultural officials regarding the spatial distribution and strains of avian influenza viruses circulating in our nation’s wildlife.
Media
Sources/Usage: Public Domain. View Media Details
HPAI Surveillance Update 2021-2025
Highly pathogenic avian influenza was detected in North American wild birds in December 2021 for the first time since 2015. Initial detections occurred in Canada (Newfoundland and Labrador) and the United States (South Carolina). Multiple introductions of HPAI H5N1 into North America have now been documented to have occurred in 2021 and 2022. Since that time, HPAI H5/H5N1 has been detected in over 10,000 wild birds representing over 160 unique North American species, and over 400 wild mammals representing over 20 unique North American species. Confirmed detections in wild birds, mammals, poultry, and livestock in the United States are reported online by the U.S. Department of Agriculture and a map of confirmed detections in wild birds in the United States is available from the NWHC.
Interagency Partnerships
The NWHC serves on the U.S. Interagency Steering Committee for Surveillance for Highly Pathogenic Avian Influenza in Wild Birds and conducts morbidity and mortality event investigations in support of the Interagency Strategic Plan for Early Detection and Monitoring for Avian Influenzas of Significance in Wild Birds. The NWHC also serves as an affiliate member of the National Animal Health Laboratory Network (NAHLN) and collaborates with the U.S. Department of Agriculture's National Veterinary Services Laboratory on diagnostic testing of samples collected from wild birds for avian influenza surveillance.
Additional Resources
- Distribution of Highly Pathogenic Avian Influenza in wild birds in North America, 2021-2025 (Map)
- Avian influenza (USDA)
- Information on Avian Influenza (CDC)
- Implementation Plan for Avian Influenza Surveillance in Waterfowl in the United States
- NWHC Wildlife Health Bulletins
- Winter 2025 Update on Highly Pathogenic Avian Influenza H5 - March 2025
- Status and Response to Detections of Highly Pathogenic Avian Influenza H5N1 in North America – February 2022
- Detection of Highly Pathogenic Avian Influenza H5N1 in an Exhibition Farm in Newfoundland and Labrador, Canada - December 2021
- Winter 2021 Update on Highly Pathogenic Avian Influenza Viruses Circulating Globally in Wild Birds - December 2021
More Information on Avian Influenza
Avian influenza is a viral disease caused by various strains of avian influenza viruses that can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), which refers to a specific virus’ capability to cause mortality in domestic poultry. It remains a high consequence global disease with the potential to threaten wildlife, agriculture, and human health. Check out the main Avian Influenza page to learn more.
Recent Scientific Publications
The following publications include NWHC contributions to avian influenza research:
- Elsmo, E.J., Wünschmann, A., Beckmen, K.B., Broughton-Neiswanger, L.B., Buckles, E.L., Ellis, J.H., Fitzgerald, S.D., Gerlach, R., Hawkins, S., Ip, H.S., Lankton, J.S., Lemley, E.M., Lenoch, J.B., Killian, M.L., Lantz, K., Long, L., Maes, R., Mainenti, M., Melotti, J., Moriarty, M.E., Nakagun, S., Ruden, R.M., Shearn-Bochsler, V.I., Thompson, D.A., Torchetti, M.K., Van Wettere, A.J., Wise, A.G., Lim, A.L., 2023. Highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b infections in wild terrestrial mammals, United States, 2022. Emerging Infectious Diseases 29, 2451-2460. https://doi.org/10.3201/eid2912.230464
- Gass, J.D., Jr., Dusek, R.J., Hall, J.S., Hallgrimsson, G.T., Halldórsson, H.P., Vignisson, S.R., Ragnarsdottir, S.B., Jónsson, J.E., Krauss, S., Wong, S.-S., Wan, X.-F., Akter, S., Sreevatsan, S., Trovão, N.S., Nutter, F.B., Runstadler, J.A., Hill, N.J., 2023. Global dissemination of Influenza A virus is driven by wild bird migration through arctic and subarctic zones. Molecular Ecology 32, 198-213. https://doi.org/10.1111/mec.16738
- Hall, J.S., Grear, D.A., Krauss, S., Seiler, P., Dusek, R.J., Nashold, S., Webster, R.G., 2021. Highly pathogenic avian influenza virus H5N2 (Clade 2.3.4.4) challenge of mallards age appropriate to the 2015 midwestern poultry outbreak. Influenza and Other Respiratory Viruses 15, 767-777. https://doi.org/10.1111/irv.12886
- Hill, N.J., Bishop, M.A., Trovao, N.S., Ineson, K., Schaefer, A., Puryear, W.B., Zhou, K., Foss, A., Clark, D., McKenzie, K., Gass, J.D., Jr., Borkenhagen, L., Hall, J.S., Runstadler, J.A., 2022. Ecological divergence of wild birds drives avian influenza spillover and global spread. PLoS Pathogens18. https://doi.org/10.1371/journal.ppat.1010062
- Ip, H.S., Uhm, S., Killian, M.L., Torchetti, M.K., 2023. An evaluation of avian influenza virus whole-genome sequencing approaches using nanopore technology. Microorganisms 11. https://doi.org/10.3390/microorganisms11020529
- Kent, C.M., Ramey, A.M., Ackerman, J.T., Bahl, J., Bevins, S.N., Bowman, A.S., Boyce, W., Cardona, C., Casazza, M.L., Cline, T.D., De La Cruz, S.E.W., Hall, J.S., Hill, N.J., Ip, H.S., Krauss, S., Mullinax, J.M., Nolting, J.M., Plancarte, M., Poulson, R.L., Runstadler, J.A., Slemons, R.D., Stallknecht, D.E., Sullivan, J.D., Takekawa, J.Y., Webby, R.J., Webster, R.G., Prosser, D.J., 2022. Spatiotemporal changes in influenza A virus prevalence among wild waterfowl inhabiting the continental United States throughout the annual cycle. Scientific Reports 12. https://doi.org/10.1186/s13567-024-01397-5
- Prosser, D., Chen, J., Ahlstrom, C., Reeves, A.B., Poulson, R.L., Sullivan, J.D., McAuley, D., Callahan, C.R., McGowan, P.C., Bahl, J., Stallknecht, D.E., Ramey, A.M., 2022. Maintenance and dissemination of avian-origin influenza A virus within the northern Atlantic Flyway of North America. PLoS Pathogens 18. https://doi.org/10.1371/journal.ppat.1010605
- Ramey, A.M., Hill, N.J., DeLiberto, T.J., Gibbs, S.E.J., Hopkins, M.C., Lang, A.S., Poulson, R.L., Prosser, D., Sleeman, J.M., Stallknecht, D.E., Wan, X.-F., 2022. Highly pathogenic avian influenza is an emerging disease threat to wild birds in North America. Journal of Wildlife Management 86. https://doi.org/10.1002/jwmg.22171
- Tawidian, P., Torchetti, M.K., Killian, M.L., Lantz, K., Dilione, K.E., Ringenberg, J.M., Bevins, S.N., Lenoch, J., Ip, H.S., 2024. Genotypic clustering of H5N1 avian Influenza viruses in North America evaluated by ordination analysis. Viruses 16. https://doi.org/10.3390/v16121818
- Youk, S., Torchetti, M.K., Lantz, K., Lenoch, J.B., Killian, M.L., Leyson, C., Bevins, S.N., Dilione, K., Ip, H.S., Stallknecht, D.E., Poulson, R.L., Suarez, D.L., Swayne, D.E., Pantin-Jackwood, M.J., 2023. H5N1 highly pathogenic avian influenza clade 2.3.4.4b in wild and domestic birds: Introductions into the United States and reassortments, December 2021–April 2022. Virology 587. https://doi.org/10.1016/j.virol.2023.109860
Learn about other NWHC work on avian influenza.
Avian Influenza
Avian influenza is a viral disease caused by various strains of avian influenza viruses that can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI). It remains a global disease with potential high consequence with the potential to threaten wildlife, agriculture, and human health.
Below are photos related to avian influenza surveillance.
Blue-winged teal in Texas. Inset shows avian influenza virus
Blue-winged teal in Texas. Inset shows avian influenza virus
Blue-winged teal in Texas. Inset shows avian influenza virus
Testing for Avian Influenza
A USGS scientist takes a sample from a northern pintail duck (Anas acuta) to be tested for avian influenza.
A USGS scientist takes a sample from a northern pintail duck (Anas acuta) to be tested for avian influenza.
USGS biologists swab a hen mallard duck for avian influenza
USGS biologists swab a hen mallard duck for avian influenza in Rio Grande County, Colorado.
USGS biologists swab a hen mallard duck for avian influenza in Rio Grande County, Colorado.
Below are FAQs related to avian influenza.
What is Avian Influenza? What is Avian Influenza?
Avian influenza (AI) is caused by influenza type A viruses that historically have spread between wild birds (waterfowl and shorebirds) with occasional outbreaks in poultry (chickens, turkeys). These viruses can change over time and there has been a fundamental change with AI viruses now causing illness and death in wild birds, wild mammals, poultry, dairy cattle, cats, and people. Learn more...
How do scientists know if individual wild birds are infected with avian influenza? How do scientists know if individual wild birds are infected with avian influenza?
The U.S. Department of Agriculture implements a plan every year to monitor wild birds for highly pathogenic avian influenza. This national plan could not be implemented without the engagement of State partners, the National Flyway Council, the Department of the Interior, and other agencies. This coordinated effort is revised annually based on new science and data shared through the Interagency...
How is avian influenza spread? How is avian influenza spread?
Avian influenza is spread when wild and domestic birds shed the virus into the environment through their oral and nasal secretions as well as excrement. Following the 2015 avian influenza outbreak, USGS, USDA, and other partners confirmed through data on wild bird migratory patterns—including intercontinental movements and genetic analyses of viral strains—that the global spread of HPAI has been...
Can people get avian influenza? Can people get avian influenza?
While rare, human infections with avian influenza viruses have occurred. The Centers for Disease Control (CDC) consider the risk to the general public from infections to be low. There have been human cases associated with the dairy cattle and wild bird strains of HPAI. To date, most human cases have been associated with farm workers (poultry and dairy cattle operations) as well as people who have...
Can wild birds spread avian influenza to domestic poultry? Can wild birds spread avian influenza to domestic poultry?
Wild birds can directly or indirectly spread avian influenza to domestic poultry. The indirect route is likely most common due to viral contamination of the farm environment (feed, equipment, boots, clothes). The U.S. Department of Agriculture has found that farm-to-farm movement is also a major route for avian influenza spread, noting that farm biosecurity is critical to reduce the risk of avian...
What are the different types of avian influenza? What do the numbers next to H and N mean? What are the different types of avian influenza? What do the numbers next to H and N mean?
Avian influenza (AI) type A viruses are divided into subtypes based on two proteins on the surface of the virus: Hemagglutinin (HA), of which there are 16 subtypes (H1-H16). Neuraminidase (NA), of which there are 9 subtypes (N1-N9). Many combinations of HA and NA proteins are possible, for example H5N1, H5N2, H7N2, and H7N8. AI viruses are also classified into two groups based on their ability to...