#17,728
The spillover of avian H5N1 into terrestrial and marine mammals raises concerns that - given enough time and opportunities - the virus could better adapt to mammalian hosts, increasing its pandemic potential.
Complicating matters, while researchers have identified a number of `mammalian adaptations' (e.g. PB2-E627K, PB2-D701N, PB2-T271A, etc.), there are certainly others we are unaware of. Nor do we know how many of these amino acid changes work in concert with one another.
Today the ECDC's Eurosurveillance Journal has published a Rapid Communications on the recent (July, 2023) discovery of an H5N1 spillover into foxes in Northern Ireland, and of amino acid substitutions thought to enhance its adaptation to mammalian hosts.
- One of these substitutions was PB2-T271A, was famously found in H5N1-infected mink in Spain, and is believed to `enhance the polymerase activity of influenza A viruses in mammalian host cells and mice'.
- Another mutation - PB2-M535I - had not previously been identified as a mammalian adaptation in H5N1, but a similar change (M535L) in the H7N9 virus has been shown to restore viral polymerase activity in mammalian cells in the absence of PB2-E627K.
- Over the past 6 months, 4 other instances of PB2-M535I have been reported in Europe (1 in Italy, 3 in Finland).
- A third mutation - PB1-F2-T7I - was also detected in one of these foxes, but there is insufficient evidence to classify it as authentic mammalian adaptation.
Due to its length, I've only posted the Abstract and some excerpts. Follow the link to read it in its entirety.
Rapid communication Open Access
Highly pathogenic avian influenza A(H5N1) virus infection in foxes with PB2-M535I identified as a novel mammalian adaptation, Northern Ireland, July 2023
Paula Lagan1,* , Robyn McKenna1,* , Salam Baleed1 , Bob Hanna1 , Jason Barley1 , Shirley McConnell1 , Anastasia Georgaki2 , Tarja Sironen3 , Ari Kauppinen4 , Tuija Gadd4 , Erika Lindh5 , Niina Ikonen5 , Michael J McMenamy1 , Ken Lemon1
The global spread of highly pathogenic avian influenza (HPAI) A(H5Nx) clade 2.3.4.4b has resulted in an unprecedented number of spillover events to wild and domestic mammals, leading to public health concerns over the increased zoonotic risks posed [1,2]. Here, we report the first confirmed cases of mammalian infection with HPAI A(H5N1) clade 2.3.4.4b in Northern Ireland, affecting two red foxes. We present analysis of the viral sequences obtained from the foxes, and two epidemiologically linked avian sequences, and identify a novel mammalian adaptation.
Event detection
On 25 July 2023, the Department of Agriculture, Environment and Rural Affairs (DAERA) was notified of two gulls and two red foxes found dead in sand dunes located on the north coast of Northern Ireland (Figure 1A and 1B). Given the unprecedented spread of HPAI in wild seabirds and findings of spillover to mammalian species, DAERA decided to investigate this event as part of passive surveillance. Veterinary officers attended the scene and confirmed that there was no human exposure to the infected animals. All four carcasses were collected by trained staff wearing appropriate personal protective equipment, packaged and transported following the competent authority's protocols, and delivered to the Agri-Food and Bioscience Institute (AFBI) for post-mortem examination and diagnostic testing. No further action was taken at the site of collection.
(SNIP)
Identification of candidate mammalian adaptations
We compared the amino acid sequences from the fox/avian coding regions to identify potential mammalian adaptations. Mutations at three locations were unique to the fox sequences, with the Fox 1 sequence bearing PB2-T271A and the Fox 2 sequence bearing PB2-M535I and PB1-F2-T7I (Table 2).
Since October 2022, PB2-T271A has been frequently identified in mammalian isolates of H5N1 clade 2.3.4.4b viruses and, along with PB2-E627K and PB2-D701N, is associated with increased virulence and replication in mammals [1,8].Discussion
In Northern Ireland, detection of the first case of HPAI A(H5N1) during the 2022–23 epidemiological year occurred in October 2022 in wild and captive Anatidae species (ducks, geese and swans) on a large wetland reserve. The WGS analysis identified the AB genotype (H5N1-A/duck/Saratov/29–02/2021-like). Over the winter months and into spring 2023, a small number of detections were observed in various wild bird species including geese, swans and raptors. From early June to late July 2023, a significant increase in detections occurred, with 14 cases involving 36 seabirds of the Laridae family (black-headed gulls, kittiwakes and terns) found dead around the northern and eastern coastal regions. While WGS was not performed for these cases, it seems likely that the epidemic in Northern Ireland has proceeded as elsewhere in Europe, with the AB genotype dominating at the start of the epidemiological year, followed by the rise of genotype BB in the latter half, with subsequent spillover to mammalian species.
Interspecies transmission of HPAI to mammals is rare. However, the current panzootic caused by HPAI A(H5N1) virus has witnessed unprecedented numbers of mammalian spillover events in both the Americas and Europe [1]. This observation has raised concerns that the virus may eventually adapt to mammalian hosts and become capable of sustained mammal-to-mammal spread. Widespread surveillance, including genetic characterisation of mammalian detections, is therefore critical to monitor HPAI A(H5N1) virus evolution.
A major barrier to interspecies transmission is the inability of avian-origin viral polymerases to carry out efficient replication in mammalian cells. Approximately half of all mammalian viruses from the current panzootic characterised to date contain at least one of three mutations that enable the replicative ability of the viral polymerase in mammalian cells [1]. The most commonly observed is PB2-E627K. In the absence of PB2-E627K, the mutations PB2-T271A and PB2-D701N are also thought confer similar functionality. A recent outbreak of HPAI A(H5N1) in domestic cats in Poland identified PB2-K526R in combination with PB2-E627K [9]. PB2-K526R is a known mammalian adaptation thought to enhance the effect of PB2-E627K [10].
In our current study, we identify PB2-M535I as a newly emerging mammalian adaptation of clade 2.3.4.4b viruses. The close epidemiological link between the infected gulls and foxes and the genetic similarity of the viral sequences allows the observed differences between the avian and mammalian sequences to be assessed with high confidence. Further in vivo/in vitro studies are needed to clearly define the effect of this mutation on the biological characteristics of the virus.
The mutation PB2-M535I has not been previously described as a mammalian adaptation during the current global outbreak of HPAI A(H5Nx) clade 2.3.4.4b viruses. Analysis of multiple human cases of influenza A(H7N9) virus infection, which first emerged in eastern China in 2013, identified PB2-M535L as one of three mutations (along with PB2-Q591K and PB2-D701N) that could restore viral polymerase activity in mammalian cells in the absence of PB2-E627K [11]. Methionine at position 535 of PB2 is completely conserved in European avian H5N1 clade 2.3.3.4b strains. Examination of all available mammalian sequences shows that PB2-M535I has emerged on four other occasions in addition to the case described here.
It was first detected in a fox, collected on 7 April 2023 in the Rovigo province of Italy (A/fox/Italy/23VIR3885–1/2023 EPI_ISL_17679728). Since 14 July 2023, a major outbreak of H5N1 has been ongoing in farmed foxes, minks and racoon dogs on a large number of fur farms in the South and Central Ostrobothnia region of Finland [12]. A subset of WGS from this outbreak showed canonical PB2 mammalian adaptations (PB2-E627K and PB2-T271A). However, we noted that PB2-M535I occurred in at least three animals: A/arctic-fox/Finland/621/2023 EPI_ISL_18122439 (collected 24 July 2023), A/blue fox/Finland/2023AI06820 015/2023 EPI_ISL_18131276 (collected 3 August 2023) and A/silver fox/Finland/2023AI06834 029/2023 EPI_ISL_18131279 (collected 4 August 2023). Taken together, we propose PB2-M535I as a newly emerged mammalian adaptation.
The PB1-F2-T7I substitution has not been previously described as a mammalian adaptation. PB1-F2-T7 is highly conserved in both avian and mammalian derived H5Nx clade 2.3.4.4b European sequences, with PB1-F2-I7 occurring at a frequency of 1.2% and 2.1%, respectively (data not shown). Therefore, although the available data indicate that PB1-F2-T7I emerged during replication within the fox host, there is insufficient evidence to suggest that this is an authentic mammalian adaptation.
The two fox-derived sequences differ at only three nucleotide positions, strongly suggesting a common source of infection, with Gull 2 or another closely related avian source most likely. The distinct patterns of mammalian adaptation displayed (PB2-T271A vs PB2-M535I) also suggest that these mutations arose independently during replication within each fox host rather than via mammal-to-mammal spread.
Conclusion
Analysis of an epidemiologically linked cluster of HPAI A(H5N1) virus infected foxes and gulls has identified PB2-M535I as a recently emerged mammalian adaptation of clade 2.3.4.4b viruses. Retrospective analysis of submitted genetic sequences shows that this mutation has arisen on at least five occasions in three European countries since April 2023. Surveillance efforts should monitor future mammalian derived sequences for this mutation in addition to the other well characterised adaptations.
