Public Health

What is Highly Pathogenic H5N1 Avian Influenza?

H5N1 was first detected at a goose farm in China in 1996. A big poultry outbreak in Hong Kong in 1997 led to the first documented human deaths and sparked the first pandemic fears. Around 2005, the virus spilled over into migratory birds, which have since spread it across the world in several big waves.

The H5N1 strain is a fast-mutating, highly pathogenic avian influenza virus (HPAI) found in multiple bird species. It is both epizootic (an epidemic in non-humans) and panzootic (a disease affecting animals of many species especially over a wide area).

HPAI A(H5N1) 2.3.4.4b Variant

A new variant named 2.3.4.4b that emerged in the Netherlands in October 2020 has spread faster and farther than any predecessor, dealing huge blows to the poultry industry in Europe and North America before arriving in Central and South America in the fall of 2022. “It seems this virus is just more adapted to all birds than any others have been,” says Richard Webby, an influenza researcher at St. Jude Children’s Research Hospital.

Because the receptors the virus binds to in the upper airways of birds are less common in mammalian upper airways, H5N1 largely spares mammals. But this time around many mammalian species have become infected, including foxes, cats, ferrets, seals, and dolphins, presumably through contact with infected birds.

In January 2021, an A(H5N1) related to the 2020–2021 European viruses was reported in West Africa and subsequently in South African countries. Since then, it has been persistently circulating in this geographic area, and in West Africa it has reassorted with the A(H9N2) subtype of the zoonotic G1 lineage.

From late 2021, different clade 2.3.4.4b A(H5N1) genotypes, one of them previously identified in Europe, have been detected in South and East Asia, including China. In December 2021, the A(H5N1) clade 2.3.4.4b strongly related to the A(H5N1) identified in Northern Europe during the 2020-2021 epidemic season was introduced in North America.

At the beginning of 2022, a new introduction in North America from the Pacific flyway of an A(H5N1) related to the viruses circulating in Japan was identified (Alkie et al., 2022). Since then the virus has spread all over North America undergoing reassortment events with LPAI viruses of the American lineages (Alkie et al., 2022).

In October 2022, A(H5N1) viruses spread for the first time to Mexico and soon after to South America, including Peru, Colombia, Ecuador and Venezuela, with mass mortality events reported in wild birds (BBC, online) and detections in poultry farms. The characterized viruses belong to clade 2.3.4.4b first detected in January 2022 in the Atlantic coast of North America (ProMed, online-a) and have been described as reassortant viruses between the Eurasian and American lineages (WOAH, 2022).