H5N1

Influenza A virus subtype H5N1 (A/H5N1) is a subtype of influenza A virus that causes avian influenza ("bird flu") and is enzootic in many bird populations and panzootic across multiple species ^[1]. Highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b has demonstrated an enhanced capacity to cross species barriers and infect mammals, including humans and dairy cattle ^[2][3][4].

In humans, A(H5N1) infection is rare and ranges from asymptomatic or mild illness to severe disease; reported symptoms include fever, diarrhea, and cough, and severe pneumonia has resulted in death ^[1]. Human infections with clade 2.3.4.4b acquired from poultry or dairy cattle have produced illness ranging from conjunctivitis or mild respiratory disease to severe and fatal pneumonia ^[4]. Reported infections since 1997 have resulted in death in about 50% of cases ^[1].

Since its resurgence in 2021, highly pathogenic avian influenza (HPAI) A(H5N1) virus of clade 2.3.4.4b has spread widely among wild and domestic birds worldwide, causing poultry outbreaks and infections across a broad range of terrestrial and marine mammal species, with cases reported from every continent. ^[1] Cross-species transmission to humans remains rare and is associated with close, unprotected contact with infected poultry or, more recently, exposure to infected dairy cattle; human-to-human transmission has been very limited, with each outbreak confined to a small number of people, and as of February 2024 no sustained human-to-human transmission has been documented. ^[1] In 2024, the virus was detected in dairy cattle for the first time, triggering an ongoing multistate outbreak in the United States characterized by cattle-to-cattle transmission and high viral loads in raw cow milk, which represents the largest H5N1 outbreak recorded in a domestic mammal species living in close proximity to humans. ^[2][4] Human infections linked to exposures to infected poultry or dairy cattle have produced a wide spectrum of illness, ranging from conjunctivitis or mild respiratory illness to severe and fatal pneumonia, although most human and bovine infections have been comparatively mild. ^[4] The rising number of zoonotic human cases, combined with the virus's demonstrated capacity to escape species barriers and infect multiple mammalian hosts, is regarded as creating increased opportunities for adaptive mutation or reassortment that could facilitate sustained human-to-human transmission. ^[2][5]

A/H5N1 is shed in the saliva, mucus, and feces of infected birds and can spread rapidly through poultry flocks and among wild birds; other infected animals may shed the virus in respiratory secretions and other body fluids, including milk ^[1][2]. Mammals and humans can be infected through close contact with infected birds or contaminated material and, for dairy farm workers, through exposure to infected cattle ^[1][2]. The bovine outbreak has demonstrated cattle-to-cattle transmission and transmission from cattle to other contact animals, including cats, raccoons, rodents, opossums, and poultry ^[2]. Sustained mammal-to-mammal transmission has occurred in European fur farms, South American marine mammals, and U.S. dairy cattle ^[3].

Neuraminidase inhibitors such as zanamivir and oseltamivir are the main antivirals recommended for a human H5N1 outbreak; the cited evidence states that they can reduce symptom severity when taken soon after infection and can be used prophylactically to decrease infection risk ^[1]. An estimated half billion farmed birds have been slaughtered in efforts to contain the virus ^[1]. Because the evidence explicitly identifies antiviral treatment and prophylaxis but does not specify a supported general prevention protocol, no additional prevention measures are asserted.

1. 1Wikipedia contributors. Influenza A virus subtype H5N1 - Wikipedia [Internet]. Wikipedia. cited 3 Sept 2026.Available from: https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H5N1
2. 2Mostafa A et al. Avian influenza A (H5N1) virus in dairy cattle: origin, evolution, and cross-species transmission. mBio. 2024 Dec 11. PMID: 39535188. doi: 10.1128/mbio.02542-24.PubMed: https://pubmed.ncbi.nlm.nih.gov/39535188/
3. 3Peacock TP et al. The global H5N1 influenza panzootic in mammals. Nature. 2025 Jan. PMID: 39317240. doi: 10.1038/s41586-024-08054-z.PubMed: https://pubmed.ncbi.nlm.nih.gov/39317240/
4. 4Webby RJ et al. An Update on Highly Pathogenic Avian Influenza A(H5N1) Virus, Clade 2.3.4.4b. J Infect Dis. 2024 Sep 23. PMID: 39283944. doi: 10.1093/infdis/jiae379.PubMed: https://pubmed.ncbi.nlm.nih.gov/39283944/
5. 5Krammer F et al. Highly pathogenic avian influenza H5N1: history, current situation, and outlook. J Virol. 2025 Apr 15. PMID: 40145745. doi: 10.1128/jvi.02209-24.PubMed: https://pubmed.ncbi.nlm.nih.gov/40145745/