2020–2025 H5N1 outbreak

Since 2020, outbreaks of avian influenza subtype H5N1 have been occurring, with cases reported from every continent except Australia as of February 2025.^[1][2][3][4] Some species of wild aquatic birds act as natural asymptomatic carriers of a large variety of influenza A viruses, which can infect poultry, other bird species, mammals (including humans) if they come into close contact with infected feces or contaminated material, or by eating infected birds.^[5] In late 2023, H5N1 was discovered in the Antarctic for the first time, raising fears of imminent spread throughout the region, potentially leading to a "catastrophic breeding failure" among animals that had not previously been exposed to avian influenza viruses.^[6] The main virus involved in the global outbreak is as H5N1 clade 2.3.4.4b, genetic diversification of which with other clades (such as 2.3.2.1c) has seen an evolution in the ability to cause significant outbreaks in a broader range of species, mammals included.^[7][8][9]

H5N6 and H5N8 viruses with the H5-2.3.4.4b hemagglutinin (HA) gene became prominent globally in 2018–2020.^[8][10] In 2020, reassortment (genetic "swapping") between these H5-2.3.4.4b viruses and other strains of avian influenza led to the emergence of a H5N1 strain with a H5-2.3.4.4b gene.^[8] The virus then spread across Europe, first detected there in the autumn of 2020, before spreading to Africa and Asia.^[1] It continues to swap genes with local flu viruses as it crosses the globe.^{[11]: (fig.1)}