H5N1

H5N1 is a highly pathogenic avian influenza A virus, described in the source material as clade 2.3.4.4b and as a novel reassortant virus related to subtype H5N8 ^[1][2]. The available sources characterize it as a zoonotic and cross-species pathogen with an expanding host range that now includes birds, several mammalian species, and dairy cattle ^[1][2][3]. Source-backed detail on its full virologic classification beyond these descriptions is not yet available in the provided material ^[1][2].

The reported clinical spectrum in humans ranges from conjunctivitis and mild respiratory illness to severe and fatal pneumonia ^[2]. The sources also note human infections in dairy farm workers attributed to exposure to infected dairy cattle, but do not provide further symptom inventories or timing of illness ^[1][2]. In animals, infected cats and poultry were reported to develop severe respiratory disease and neurologic signs and to die, while infection in cows appears to be mainly confined to mammary tissue with high viral loads in milk ^[1]. Source-backed detail on incubation period, complication frequency, or recovery course is not yet available ^[1][2].

The sources describe a large and ongoing H5N1 panzootic with massive outbreaks worldwide in wild and domestic birds, including poultry outbreaks ^[1][2][3]. Since 2021, clade 2.3.4.4b viruses have spread widely among birds and have also been detected in a range of terrestrial and marine mammals, with a first detection in dairy cattle in 2024 and an ongoing multistate outbreak in the United States ^[2][3]. The virus has been reported to infect multiple mammalian species, including cats, raccoons, rodents, opossums, and humans, and the sources state that the rise in zoonotic cases creates concern for further adaptive change ^[1][3]. Source-backed burden estimates, case counts, and geographic details beyond these outbreak descriptions are not yet available ^[1][2].

The evidence provided supports transmission through exposure to infected animals, including poultry and dairy cattle, with human infections attributed to such exposures ^[1][2]. The sources also document cattle-to-cattle transmission and spread from cattle to contact animals such as cats, raccoons, rodents, opossums, and poultry ^[1]. One report notes that viral loads were high in raw cow milk, but the exact exposure pathways for human infection are not further specified in the supplied material ^[2]. Human-to-human transmission has not been observed in the sources provided ^[3].

The provided sources most directly identify people with exposures to infected poultry or dairy cattle, including dairy farm workers, as affected groups ^[1][2]. They also indicate risk in animals with contact to infected cattle, including cats, raccoons, rodents, opossums, and poultry, and in broader wildlife and mammalian populations involved in outbreaks ^[1][2][3]. Source-backed detail on age-specific, occupational, or medical high-risk groups beyond these exposure-defined populations is not yet available ^[1][2].

The source material emphasizes vigilance, stronger global virologic surveillance, virus characterization, and rapid data sharing across birds, poultry, terrestrial and marine mammals, and humans ^[2]. It also frames pandemic risk reduction as urgently required in the context of ongoing viral evolution and rising zoonotic cases ^[3]. Source-backed detail on specific farm, laboratory, household, or vaccination measures is not yet available in the provided snippets ^[1][2][3].

In surveillance terms, H5N1 should be read as an evolving zoonotic and panzootic event rather than only a poultry disease, because the supplied sources document spread in wildlife, poultry, mammals, and dairy cattle ^[2][3]. Human cases currently appear to occur after animal exposure, and the sources note a broad severity spectrum, so case finding should be interpreted together with animal outbreak data and virologic characterization ^[2][3]. Because the virus continues to evolve and has demonstrated cross-species transmission, integrated monitoring of animals and humans is explicitly supported by the source material ^[2][3].

1. 1 Mostafa 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/
2. 2 Webby 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/
3. 3 Krammer 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/
4. 4 H5N1 moratorium. Bioengineered. 2012. doi: 10.4161/bbug.20004. DOI: https://doi.org/10.4161/bbug.20004
5. 5 H5N1 Antibodies. JAMA. 2007. doi: 10.1001/jama.298.1.32-c. DOI: https://doi.org/10.1001/jama.298.1.32-c
6. 6 H5N1. Revolver. None. doi: 10.2307/j.ctt20ks0p9.15. DOI: https://doi.org/10.2307/j.ctt20ks0p9.15

ICD-10

J09

ICD-11

1E30.0

Total cases

19

Total deaths

13

Peak month

2015-03

Coverage

2 reporting countries · 2010-01-01 → 2026-06-20