#18,421
Over the past two decades HPAI H5 influenza has continued to evolve, producing dozens of clades and subclades, some with hundreds of genotypes, spanning 9 different subtypes (H5N1-H5N9). Many of these variants have been short-lived, being unable to compete with more biologically fit variants.
Until recently Clade 2.2.1.2 viruses circulated widely in Egypt, while clade 2.3.2.1a viruses are still occasionally reported in India and Bangladesh. But the two most active HPAI H5 clades today are clade 2.3.4.4b (global) and clade 2.3.2.1c (in Cambodia).
The H5N1 subtype dominates today, but between 2014 and 2019, H5N8 was the most reported H5 subtype. Still, H5N6 continues to circulate in China, and increasingly we're seeing signs that an upstart H5N5 reassortant is making inroads in both Canada and Europe.
We've seen reports of H5N5 in European birds going back to 2016, but it was unexpectedly detected in dead raccoons on Prince Edward Island about 18 months ago (see CIDRAP Report Canada reports first H5N5 avian flu in a mammal).Last July, in Cell Reports: Multiple Transatlantic Incursions of HPAI clade 2.3.4.4b A(H5N5) Virus into North America and Spillover to Mammals, researchers reported finding the mammalian adaptive E627K mutation in a number of samples. They wrote:
Thus, while A(H5N5) viruses are comparably uncommon, their high virulence and mortality potential demand global surveillance and further studies to untangle the molecular markers influencing virulence, transmission, adaptability, and host susceptibility.
We looked in on H5N5's continued incursion into Canada back in April and again in May, and in the Spring edition of ECDC/EFSA Quarterly Avian Influenza Overview Dec 2023 - Mar 2024, there were several mentions of HPAI H5N5, including:
- Subtype A(H5N5), genotype EA-2021-I, which has persistently been detected in Norway since the 2021–2022 epidemiological year, has since September 2023 spread to Iceland, the United Kingdom, Greenland and Germany, and – besides infecting wild birds – infected red foxes in Norway. A(H5N5) was also detected in Faroe Islands and Japan during the current epidemiological year.
- Unlike the mammalian infections reported in Europe during the summer months in 2023, mainly caused by the EA-2022-BB genotype, the recent A(H5) viruses identified in wild mammals belonged to different A(H5N1) and A(H5N5) genotypes. The detection of A(H5N5) virus in two red foxes from Norway represents the first detection of this subtype in mammals in Europe.
Just 10 days ago we noted the increased detection in UK wild birds and an outbreak in poultry in Yorkshire in UK: HPAI H5N5 Rising.
Over the past year Norway's Veterinary Institute has reported 13 detections of HPAI H5 in wild birds (see chart below), of which 8 were H5N8 (H5N5), while only 3 were H5N1.
Much like we saw earlier this month in the UK, this increase in detections in wild birds has led to an outbreak in domesticated birds; in this case on a hobby farm in Trøndelag. This from Norway's Veterinary Institute:
(translation)(Excerpt)Avian flu detected in hobby bird farms in Trøndelag
Published 16/11/2024
The birds were sampled after several hens died suddenly. Due to suspicion of illness, the Norwegian Food Safety Authority was notified and then took samples which were sent to the Veterinary Institute for analysis. At the time of sampling, 16 hens and two ducks were dead. The virus that has been detected in the samples from the birds is a highly pathogenic bird flu virus of the type H5N5. This is the first time this subtype of the virus has been detected in domesticated birds in Norway. Several discoveries of this type of virus have been made in wild birds in Norway during October and November this year.
The Norwegian Food Safety Authority has introduced a number of restrictions around bird keeping to prevent further spread of infection. A protection zone of three kilometers and a surveillance zone of ten kilometers have been created around the farm. The zones include parts of Frøya and Hitra municipalities. Read more about this at the Norwegian Food Safety Authority.
The Veterinary Institute has also detected bird flu virus of the same subtype in seagulls in Nord-Troms following the discovery of a large number of dead seagulls in a small area. Read more about it here.
While it is too soon to know if H5N5's recent surge is a sustainable trend, or a flash in the pan, it does remind of HPAI H5's ability to continually reinvent itself over time. Until just over 10 years ago, H5N1 was essentially the only HPAI H5 threat on our radar.
In 2014 HPAI H5N8 emerged in South Korea, followed a few months later by a particularly deadly H5N6 subtype in China. After years of being overshadowed by H5N8, H5N1 re-emerged in 2020-2021, a far more formidable foe.
This tug-of-war between H5 viruses continues, and while H5N1 seems firmly ensconced today, we always need to be prepared for surprises.
