Highly pathogenic avian influenza (HPAI) viruses have occasionally infected humans, often with severe outcomes. Reports of HPAI in dairy cattle and the detection of high titres of the virus in cattle milk in the United States are therefore a cause for concern. Documented human infections linked to cattle infections, though mild, highlight the urgent need for enhanced biosecurity and vigilant monitoring.

Highly pathogenic avian influenza (HPAI) viruses of the A/goose/Guangdong/1/1996(H5N1) lineage (HPAI H5 viruses) have taught us many lessons since their emergence in 1997:

1.
Historically, outbreaks of HPAI viruses were limited to terrestrial poultry and controlled by culling of infected flocks in many countries. However, HPAI H5 viruses became established in wild water birds of the order Anseriformes (that is, ducks, geese and swans, the main reservoir of avian influenza A viruses). These viruses have caused multiple outbreaks in species of the order Charadriformes (that is, gulls, terns and shorebirds), and, in recent years, have infected an increasing number of raptor and peridomestic species. This expansion in host range has been paralleled by HPAI H5 virus spread across several continents.

2.
Avian influenza A viruses are typically benign in their natural reservoir; however, several HPAI H5 virus outbreaks in species of the orders Anseriformes and Charadriformes have caused mass mortality of infected birds.

3.
HPAI viruses had rarely infected mammals. Recently, however, HPAI H5 viruses have infected an increasing number of mammalian species, mostly scavengers and animals of prey1. While mammalian infections with HPAI viruses were limited to individual animals, recently, there have been reports of HPAI H5 infections of large groups of mammals including seals along the northeastern coast of the USA, mink in a mink farm in Spain and sea mammals in South America.

4.
HPAI H5 viruses were not known to transmit among mammals; however, HPAI H5 virus transmission among mammals may have occurred during outbreaks in mink in Spain in 2022 and sea lions and seals in South America. In laboratory settings, a mink HPAI H5 virus was transmitted via respiratory droplet to exposed ferrets in one study, whereas in another study conducted with a different mink virus isolate, respiratory droplet transmission among ferrets was not detected.