By Paul Wigley
The UK government recently announced that as of April 18, poultry and captive birds can be kept outside again as the threat from bird flu eases. These mandatory housing measures were introduced across England and Wales in the autumn of 2022 following the unprecedented spread of bird flu in the UK and Europe.
Indeed, the past two winters in the UK have seen our largest and most prolonged outbreak of bird flu in modern times, leading to substantial controls in poultry production, the mass death of seabirds, cases in wild mammals and a single human case.
So how did we get here, and has the threat in the UK really receded? Let's take a look.
Avian influenza, commonly known as bird flu, is caused by the influenza A virus which can lead to disease in many hosts including humans. However, its ability to infect and transmit to different hosts depends on which subtype or serotype of influenza A is involved.
Influenza A serotypes are designated based on two proteins found on the virus's surface called neuraminidase (N) and haemagglutinin (H), each of which come in several different types. The combination of H and N types (for example, H5N1, H1N1 or H3N2) defines the serotype. Within the serotype there can also be a number of variants called clades.
The current outbreak in the UK and elsewhere is caused by a 2.3.4.4b clade of the H5N1 serotype. Typically, horse strains of influenza A are H3N8 and H7N7, pigs and humans are susceptible to H1N1 and H3N2, and avian influenza encompasses H5 and H7 serotypes.
In general, human influenza viruses have little capacity to infect animal species and vice versa, but sporadic infections can sometimes occur between species.
How did the current outbreak spread?
The origins of all influenza A serotypes are thought to be in aquatic birds such as ducks, geese, gulls and wading birds. The 2.3.4.4b clade of the H5N1 serotype, for example, is descended from the goose/Guangdong lineage, first isolated in a goose in Guangdong, China.
This clade emerged around 2021 and has caused several outbreaks globally, including in Europe, Asia, and the Americas, most likely spread by migratory birds. As many geese and duck species can carry the virus and shed it in their droppings without becoming ill they can easily transmit virus to other bird species.
Normally in the UK we see a number of localized bird flu outbreaks each winter, but the levels of the current virus—identified in more than 300 UK poultry farms in the past two years—is unprecedented in modern times.
And while avian flu has traditionally been a problem primarily for chickens and other domestic birds, this outbreak has been unusual in its capacity to cause disease and death in wild birds too. This strain has been found to cause disease in over 60 wild bird species in the UK, including many not previously known to be affected.
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