H5N1

Highly pathogenic avian influenza A(H5N1) is a subtype of avian influenza A virus recognized as a novel influenza A virus of animal origin that is antigenically and genetically distinct from seasonal strains and has infected humans ^[1][4]. The currently circulating clade 2.3.4.4b emerged as a reassortant from H5N8 and shows enhanced ability to breach species barriers and infect multiple mammals ^[2][3].

Avian flu viruses can infect humans who are in close, unprotected contact with infected poultry, with cross-species transmission incidents being rare and symptoms ranging in severity from no symptoms or mild illness to severe disease that resulted in death ^[1]. Infected cats and poultry exposed via cattle showed severe respiratory disease, neurologic signs, and eventually died, whereas infection in humans and cows appears to be mild in most cases ^[2]. Sporadic human infections with highly pathogenic avian influenza A(H5N1) viruses present a wide spectrum of illness severity .

Avian influenza subtype H5N1 outbreaks have been occurring since 2020, with cases reported from every continent ^[1]. The global outbreak strain is H5N1 clade 2.3.4.4b, which has adapted to cause significant outbreaks in a broader range of species including mammals ^[1]. As of February 2024 there had been very few instances of human-to-human transmission, each limited to a few people ^[1]. The H5N1 clade 2.3.4.4b reassortant has led to massive worldwide outbreaks in wild and domestic birds and expanded its host range to ruminants such as dairy cattle in the United States, with cattle-to-cattle and cattle-to-other-animal transmission, and several human infections in dairy farm workers attributed to infected cattle representing the first mammalian-to-human transmission ^[2]. In October 2022 an H5N1 outbreak on a Spanish mink farm showed mammal-to-mammal transmission with 4% of mink dying from haemorrhagic pneumonia, and a December 2022 Caspian seal die-off involved 700 infected seals ^[1].

Humans are infected through close, unprotected contact with infected poultry or contaminated material, including feces or consumption of infected birds ^[1]. Cross-species transmission is rare, and human-to-human spread has been limited to very few instances ^[1]. The virus transmits among mammals, including cattle-to-cattle and from cattle to other animals, with dairy cattle serving as a source of human infection via exposure to infected animals ^[2][3]. Wild aquatic birds are natural asymptomatic carriers that can infect poultry, other birds, and mammals ^[1].

Avoiding close, unprotected contact with infected poultry reduces the risk of human infection ^[1]. Neuraminidase inhibitors such as zanamivir and oseltamivir may be used as postexposure prophylaxis to decrease infection risk among exposed persons ^[1][4]. Public health emphasis remains on limiting occupational and environmental exposure to infected birds and mammals rather than on population-wide interventions ^[1].

1. 1 Wikipedia contributors. Influenza A virus subtype H5N1 - Wikipedia [Internet]. Wikipedia. cited 23 Oct 2026. Available from: https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H5N1
2. 2 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/
3. 3 Peacock 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. 4 Uyeki TM et al. Antivirals for Novel Influenza A Virus Infections. J Infect Dis. 2025 Oct 17. PMID: 41102616. doi: 10.1093/infdis/jiaf296. PubMed: https://pubmed.ncbi.nlm.nih.gov/41102616/