H5N1

H5N1 (A/H5N1) is a subtype of influenza A virus characterized by a type 5 hemagglutinin (H) surface protein and type-1 neuraminidase (N) protein. As an enveloped negative-sense RNA virus with a segmented genome, it causes avian influenza, commonly referred to as 'bird flu.' The virus is classified as either Low Pathogenic Avian Influenza (LPAI) or High Pathogenic Avian Influenza (HPAI) based on severity in domestic chickens, though this classification does not predict pathogenicity in other species.

In humans, H5N1 infection ranges from asymptomatic or mild illness to severe disease and death, with symptoms frequently being severe or fatal when cross-species transmission occurs. In birds, clinical presentation varies by species and viral strain, with signs including swollen head, watery eyes, unresponsiveness, lack of coordination, and respiratory distress such as sneezing or gurgling. In domestic chickens, HPAI H5N1 causes serious breathing difficulties, significant drops in egg production, and sudden death, while LPAI strains produce mild symptoms or no apparent illness.

H5N1 is enzootic in many bird populations and has demonstrated panzootic potential, affecting animals across a wide geographic area and multiple species. Wild aquatic birds act as natural asymptomatic carriers capable of spreading the virus over long distances during migration. An estimated half billion farmed birds have been slaughtered in containment efforts. As of February 2024, human-to-human transmission remains exceedingly rare, with each outbreak limited to a few individuals. H5N1 and H7N9 are considered the most significant avian influenza threats for cross-species transmission.

The virus is shed in the saliva, mucus, and feces of infected birds, with other infected animals potentially shedding in respiratory secretions and body fluids such as milk. Transmission occurs rapidly through poultry flocks and among wild bird populations. Human infections require close, unprotected contact with infected poultry, making occupational exposure a primary risk factor. Cross-species transmission to mammals, including humans, occurs through direct exposure to infected birds or contaminated environments.

Individuals with direct, unprotected contact with infected poultry or contaminated environments face the highest occupational risk. This includes poultry workers, farmers, and those involved in bird culling operations. Wild bird handlers and individuals in regions with active avian outbreaks are also at elevated risk. The general population remains at low risk absent direct exposure to infected birds.

Several candidate vaccines against H5N1, including Aflunov, Celldemic, and Seqirus/Audenz, have been developed as preparedness measures, though none were fully proven as of July 2024. Some governments have established strategic stockpiles of H5N1-targeted vaccines. However, due to the virus's high mutation rate characteristic of RNA viruses, any vaccine response would require targeting the specific circulating strain. Control measures in poultry populations have historically relied on mass culling of affected flocks.

H5N1 requires vigilant surveillance due to its potential for rapid evolution and adaptation. The virus's segmented genome and high mutation rate create ongoing risk for antigenic drift and potential host receptor switching, as evidenced by December 2024 research demonstrating a single mutation could enable specificity for human receptors. Monitoring should focus on poultry outbreaks, wild bird movements, and human cases with poultry exposure history. The rarity of sustained human-to-human transmission to date provides some reassurance but does not diminish the need for preparedness given the severity of human disease when it occurs.

1. 1 Wikidata. influenza A virus subtype H5N1.
2. 2 Wikipedia. Influenza A virus subtype H5N1 - Wikipedia.

ICD-10

J09

ICD-11

1E30.0

Total cases

19

Total deaths

13

Peak month

2015-03

Coverage

2 reporting countries · 2010-01-01 → 2026-05-02