Avian influenza is caused by strains of influenza virus A that usually infect only wild birds and domestic poultry. Infections due to some of these strains have been detected in humans. Human-to-human transmission is limited; most cases are acquired from animals, typically poultry. Typical symptoms include fever, coryza, cough, headache, and malaise. Diagnosis is usually clinical and includes reverse transcriptase–polymerase chain reaction (RT-PCR) testing. Treatment is with antiviral medications.
Most cases of avian influenza in humans have been caused by Asian strains H5N1 and H7N9, but other strains have also caused some human infections. The most frequently identified subtypes of avian influenza that have caused human infection are H5, H7, and H9. H6 and H10 cause human infection infrequently (1).
Avian influenza infections are often asymptomatic in wild birds but may cause highly lethal illness in domestic poultry. Marine mammals can also become infected with avian influenza strains (eg, H10N7 in harbor seals), with subsequent zoonotic disease potential (ie, possibility of seal-to-human transmission) (2).
See Viral Diseases of Marine Mammals and Avian Influenza in Poultry and Wild Birds for more information on veterinary disease. See also Understanding Highly Pathogenic Avian Influenza (HPAI) and Minimizing Transmission Risk.
Humans can become infected with avian influenza viruses through inhalation of or direct contact with secretions (saliva, mucus, or feces) from infected birds. It is likely that avian influenza viruses of any antigenic specificity can cause influenza in humans whenever the virus acquires mutations, enabling it to attach to human-specific receptor sites in the respiratory tract. Because all influenza viruses are capable of rapid genetic change, they have the potential to acquire enhanced person-to-person transmissibility through spontaneous mutation or genetic reassortment during replication within a human, animal, or avian host. If these strains acquire such human transmissibility, an influenza pandemic could result.
All cases of human infection with an influenza A subtype other than H1 or H3 are nationally notifiable in the United States (3).
The first human cases of H5N1 were discovered in Hong Kong in 1997 (4). Many of the 18 people affected had severe respiratory symptoms, and the mortality rate was 33% (5). Spread to humans was contained by culling domestic bird populations. However, in 2003 and 2004, H5N1 infections in humans reappeared, and occasional cases continue to be reported, primarily in Asia and the Middle East. H5N1 was reported among dairy cattle in several states in the United States in 2024. The resulting outbreak caused 71 infections and 2 deaths in humans between 2024 and 2025 (6). There has been no known person-to-person spread. Although the current public health risk is low, national surveillance is ongoing.
Since 2014, 93 people have been infected with H5N6, and 58 people have died of it; all of these documented cases occurred in the Western Pacific Region, mostly in mainland China (7).
In 2021, the first human infections with H5N8 were reported in 7 poultry workers in Russia; all cases were reported to be asymptomatic. No human-to-human transmission was observed, and the risk to the general public was considered to be very low (8).
In early 2013, an extensive outbreak of H7N9 avian influenza in humans occurred in several provinces of southeastern China. About one-third of cases were fatal (9), but significant illness typically occurred only in older adults. Sustained human-to-human transmission did not occur, although there was some evidence of limited human-to-human transmission. Human infection appeared to result from direct exposure to infected birds in live (wet) poultry markets, where birds are purchased for subsequent consumption at home. Seasonal outbreaks typically correspond with increased poultry traffic and handling for Chinese New Year celebrations.
China's sixth wave of H7N9 avian influenza in humans peaked in 2016–2017 with nearly 800 cases; only sporadic cases have been reported since that time. Worldwide, 1568 human cases and 616 deaths have been reported to the World Health Organization since 2013 (7). Some cases of Asian H7N9 avian influenza have been reported outside of mainland China, but most have occurred in people who had traveled to mainland China before becoming ill.
Human infection with other avian influenza strains have occurred sporadically, including H7N3 in Canada, H7N7 in the Netherlands, and H7N4 and H9N2 mainly in China and most recently in Vietnam (10).
Surveillance data indicate that many avian influenza infections may cause mild respiratory symptoms or even be subclinical. However, severe pneumonia with high case fatality rates has been reported in H5N1, H5N6, and H7N9 clusters.
General references
1. Centers for Disease Control and Prevention (CDC). Avian Influenza Type A. December 20, 2024. Accessed June 22, 2026.
2. Philippon DAM, Wu P, Cowling BJ, Lau EHY. Avian Influenza Human Infections at the Human-Animal Interface. J Infect Dis. 2020;222(4):528-537. doi:10.1093/infdis/jiaa105
3. CDC. U.S. Influenza Surveillance: Purpose and Methods. December 2, 2025. Accessed April 22, 2026.
4. Bender C, Hall H, Huang J, et al. Characterization of the surface proteins of influenza A (H5N1) viruses isolated from humans in 1997-1998. Virology. 1999;254(1):115-123. doi:10.1006/viro.1998.9529
5. Chan PK. Outbreak of avian influenza A(H5N1) virus infection in Hong Kong in 1997. Clin Infect Dis. 2002;34 Suppl 2:S58-S64. doi:10.1086/338820
6. CDC. A(H5) Bird Flu: Current Situation. March 6, 2026. Accessed December 3, 2025.
7. World Health Organization (WHO). Avian Influenza Weekly Update Number 1045. May 18, 2026. Accessed December 3, 2025.
8. WHO. Avian influenza A(H5N8) infects humans in Russian Federation. March 2, 2021. Accessed May 18, 2026.
9. Li Q, Zhou L, Zhou M, et al. Epidemiology of human infections with avian influenza A(H7N9) virus in China. N Engl J Med. 2014;370(6):520-532. doi:10.1056/NEJMoa1304617
10. CDC. Vietnam Reports First Human Infection with Avian Influenza H9N2 Virus. April 12, 2024. Accessed December 3, 2025.
Symptoms and Signs of Avian Influenza
Manifestations of avian influenza are similar to those of seasonal influenza but sometimes with more pronounced conjunctival irritation causing chemosis, erythema, and discharge; however, disease severity and case fatality rate may be higher, albeit with wide variation depending on the viral strain.
Complications such as pneumonia and acute respiratory distress syndrome may develop (1).
Symptoms and signs reference
1. CDC. Signs and Symptoms of Bird Flu in People. December 20, 2024. Accessed June 22, 2026.
Diagnosis of Avian Influenza
History and physical examination
Reverse transcriptase–polymerase chain reaction (RT-PCR)
The diagnosis of avian influenza should be suspected when there is an appropriate clinical syndrome in a patient exposed to a person known to be infected or who has been exposed to birds, particularly in an area where there is an ongoing avian influenza outbreak (1). History of exposure to birds, and also dairy cows, or to infected people should prompt testing for influenza A.
Testing is typically performed with RT-PCR using a nasal, conjunctival, or throat swab. Patients with lower respiratory tract illness can have samples taken from sputum, endotracheal aspirate, or bronchoalveolar lavage fluid. Culture of the organism should not be attempted because special precautions are required for these highly pathogenic viruses.
Suspected and confirmed cases should be reported to the appropriate government health authority (eg, Centers for Disease Control and Prevention [CDC] in the United States) (2).
Diagnosis references
Treatment of Avian Influenza
Oseltamivir or zanamivir (neuraminidase inhibitors)
Baloxavir marboxil (endonuclease inhibitor)
Treatment with oseltamivir or zanamivir at usual doses is indicated (1).
The H7N9 and H5N1 viruses are resistant to the earlier antiviral medications amantadine and rimantadine; resistance or reduced susceptibility to oseltamivir has also been reported.
The antiviral medication baloxavir marboxil is a polymerase acidic endonuclease inhibitor that is indicated for treatment of patients ≥ 5 years of age with acute uncomplicated influenza who have been symptomatic for ≤ 48 hours (2). It has been used as treatment for avian influenza in humans and shows effectiveness in animal models.
Treatment reference
1. CDC. Interim Guidance on the Use of Antiviral Medications for Treatment of Human Infections with Novel Influenza A Viruses Associated with Severe Human Disease. July 3, 2025. Accessed June 17, 2026.
2. Andreev K, Jones JC, Kandeil A, Vogel P, Webby RJ, Govorkova EA. Baloxavir outperforms oseltamivir, favipiravir, and amantadine in treating lethal influenza A(H5N1) HA clade 2.3.4.4b infection in mice. Nat Commun. 2026;17(1):2937. Published 2026 Feb 19. doi:10.1038/s41467-026-69721-5
Prevention of Avian Influenza
People should avoid contact with sick or dead animals, especially wild and domesticated birds, poultry, and cattle. Raw or undercooked food products, such as unpasteurized milk and dairy products, should also be avoided. In the United States, initial testing of commercial milk samples has shown no live, infectious virus, suggesting that pasteurization is effective in inactivating avian influenza virus (1). Dairies are required to send only milk from healthy animals into processing for human consumption, and pasteurization is required for milk to be sold in stores.
Spread is contained by identifying and culling infected flocks of domestic birds.
China has active poultry vaccination campaigns for H5 and H7 influenza viruses to help prevent the spread from wild to domestic birds, which are more likely to come in contact with and spread the virus to humans.
Several vaccines for H5N1 have been developed and stockpiled for pandemic preparedness. No vaccines are currently available for the other avian influenza viruses rarely associated with human disease (H7N7, H9N2, H7N3, and H7N9).
Prevention reference
1. U.S. Food and Drug Administration (FDA). Investigation of Avian Influenza A (H5N1) Virus in Dairy Cattle. March 14, 2025. Accessed June 18, 2026.
Key Points
Avian influenza affects mainly birds, but several strains of avian influenza virus have caused severe respiratory illness and death in humans.
Human infection is typically acquired from infected birds, but human-to-human transmission has occurred (not in the 2024–2025 H5N1 outbreak).
Diagnosis is typically with reverse transcriptase–polymerase chain reaction (RT-PCR) testing; these viruses should not be cultured because they are highly pathogenic, and special precautions are required.
Treat with oseltamivir or zanamivir at usual doses or with baloxavir marboxil.
Drug Information for the Topic
