#19,242
In the fall of 2024 - while we were watching the bovine B3.13 genotype spread in U.S. cattle - a new genotype (D1.1) of H5N1 appeared in Canada and the Pacific Northwest which very quickly became dominant in wild birds and swept eastward across the United States and Canada.
Since then, we've seen roughly 2 dozen confirmed human infections from this D1.1 genotype - several of them severe or fatal (see CDC Statement: First H5 Bird Flu Death Reported in United States) - along with studies suggesting it may better adapted to humans than B3.13 (see J.I.D.: Avian influenza virus A(H5N1) genotype D1.1 is better adapted to human nasal and airway organoids than genotype B3.13).
In early 2025 D1.1 was detected in dairy cattle in two states (Nevada & Arizona), and later in Wisconsin, further raising concerns over its ability to infect mammals.
Which is why the surprise announcement in February of 2025 that Oseltamivir Resistant H5N1 (Genotype D1.1) had been found On 8 Canadian Poultry Farms in the fall of 2024, raised so many concerns.
In a report from Canada's CFIA and PHAC - the authors described the detection of a (previously) rare H275Y substitution in the neuraminidase (NA) protein of a number of genotype D1.1 isolates, one which is associated with strong resistance to the NA inhibitors oseltamivir and peramivir.
Normally, we only see this mutation appear in a small percentage (1%) of patients receiving antiviral treatment. Because it is thought to exact a `fitness penalty' on influenza A viruses - limiting forward transmission - H275Y in the `wild' is fairly rare.
But over the past few years we've seen evidence of creeping oseltamivir resistance in seasonal H1N1 (FluView Week #20: EOY Review of Increased Detection of Oseltamivir Resistant H1N1 Viruses), along with growing concerns that oseltamivir monotherapy may no longer be the most effective treatment of H5N1.
Emerg Microbes & Inf: Antiviral Activities of Multiple Antivirals Against HPAI H5N1 in Vitro and in Mice
Nature Comms: Baloxavir outperforms oseltamivir, favipiravir, and amantadine in treating lethal influenza A(H5N1) HA clade 2.3.4.4b infection in mice
Nature Comms: Oseltamivir and Baloxavir Monotherapy and Combination Therapy Efficacy Against Clade 2.3.4.4b A(H5N1) Influenza Virus Infection in Ferrets
Characterization of oseltamivir-resistant A(H5N1) clade 2.3.4.4b, genotype D1.1 variants identified in poultry farms of British Columbia, Canada
Maxime Cochin , Yacine Abed , Robert Vendramelli , Katrina Dionne , Catherine Bourassa , Geneviève Laroche , show all
Article: 2686474 | Received 16 Feb 2026, Accepted 04 Jun 2026, Published online: 08 Jul 2026
https://doi.org/10.1080/22221751.2026.2686474
ABSTRACT
Highly pathogenic avian influenza A(H5N1) viruses of clade 2.3.4.4b, genotype D1.1, are responsible for widespread outbreaks in poultry and continue to cause sporadic, sometimes severe, human infections. Herein, we characterized a wild-type (WT) influenza A(H5N1) D1.1 isolate (BC-H5N1-WT) and its H275Y neuraminidase (NA) variant (BC-H5N1-H275Y), both of which emerged on farms in British Columbia, Canada, during the fall 2024 outbreak.
In vitro analysis assessed replication kinetics in MDCK cells, with supernatants collected at different days post-infection (p.i.) and titrated by TCID50 and qRT-PCR. Neuraminidase inhibitor (NAI) susceptibility was determined by NA inhibition assays, whereas susceptibility to baloxavir acid (BXA) was evaluated by plaque reduction assay. In vivo virulence was evaluated in BALB/c mice infected with serial 10-fold dilutions of each virus to monitor weight loss and mortality. Viral titers in lungs, brain, nose, kidney, spleen, and heart were quantified at day 4 p.i. The BC-H5N1-WT virus was susceptible to the four antivirals tested, whereas BC-H5N1-H275Y displayed resistance to oseltamivir and peramivir but remained susceptible to zanamivir and BXA.
The BC-H5N1-WT exhibited significantly higher viral replication titers than BC-H5N1-H275Y at all tested time points and showed larger plaque sizes. In mice, BC-H5N1-WT was more virulent with LD50 values of 1.78 × 103 PFUs compared to 8.71 × 104 PFUs for BC-H5N1-H275Y, and produced higher viral titers in lungs and other organs. Despite the reduced fitness of the resistant H5N1 D1.1 variant, its emergence in the absence of viral selection pressure underscores the need for continued surveillance.
(SNIP)
In October 2024, several chicken farms in the province of BC, Canada, faced an outbreak of HPAIV. Whole genome sequencing confirmed that the virus was an H5N1 virus of the clade 2.3.4.4b [Citation7]. Phylogenetic analysis of the hemagglutinin (HA) and neuraminidase (NA) genes confirmed that the viruses involved were reassortant H5 viruses of clade 2.3.4.4b with an avian NA of N1 subtype from a North American wild bird lineage [Citation7].
Notably, this NA has a longer stalk compared to the truncated NA stalk of traditional HPAI H5N1 strains [Citation8]. Fully Eurasian 2.3.4.4b H5N1 viruses also possess this untruncated (long-stalk) NA, differing from the short-stalk NA common in most pre-2020 highly pathogenic H5N1 strains.
Interestingly, the NA gene from 8 of the 45 sampled poultry farms harboured the well-described H275Y (N1 numbering) substitution that confers high levels of resistance to oseltamivir (OSV) and peramivir (PER), which are influenza NA inhibitors (NAI).
Of note, there was no evidence of OSV or PER use in the related farms that could explain the presence of such H275Y mutants in these animals. Although old H1N1 viruses with the H275Y substitution were associated with a fitness loss [Citation9–11], more recent seasonal and pandemic H1N1 mutant viruses were more fit than the wild-type (WT) virus due to the presence of permissive NA mutations [Citation12–14].
(SNIP)
It has been well described that the H275Y NA substitution can be selected under OSV pressure either clinically or in experimental in vitro/in vivo procedures. However, to the best of our knowledge, there was no use of OSV in poultry that could explain the emergence of the H275Y NA substitution in BC farms.
- One possible explanation could be poultry exposure to antivirals in the environment (e.g. via contaminated water near human treatment centers) [Citation40].
- Another possibility is cross-species contamination where the mutation would emerge in humans and spills back into poultry but the only human Canadian case with H5N1 infection so far was not infected by this mutant [Citation6].
- A more probable scenario seems to be that the H275Y substitution has emerged spontaneously as influenza viral replication involves an error-prone RNA-dependent RNA polymerase, with random mutations occurring frequently. Such mutation could potentially favour a more optimal balance between HA and NA in some viruses.
OSV is considered the preferred antiviral for the control of a potential influenza pandemic involving avian influenza viruses such as H5N1. Consequently, stockpiles of this compound have been established in several countries worldwide. This study and other observations suggest that it would be unwise not to consider alternatives in the event of the emergence of the H275Y substitution.
This should include the consideration of other antivirals such as BXM, as well as combinations of antivirals with different mechanisms of action, i.e. ZAN (NAI) and BXM (polymerase inhibitor). Also, our findings confirm the need for extensive surveillance studies for influenza drug resistance not restricted to humans.
While the origin of this cluster of H275Y bearing H5N1 viruses remains unclear, the good news is these viruses (at least, back in 2024) showed diminished virulence (in BALB/c mice).
But as we saw with seasonal H1N1 in 2008, `fitness penalties' can be offset by permissive mutations elsewhere in the NA, which is why we follow reports like this one with interest.
D1.1 isn't the only genotype of concern, and new genotypes will undoubtedly emerge in the future. Most will be evolutionary failures, but the success of B3.13 and D1.1 remind us there are exceptions.
Even if widespread oseltamivir resistance doesn't emerge, our antiviral stockpile is limited, and we've previously seen problems getting them to patients during the first critical 48 hours of infection (see Sporadic Tamiflu (Oseltamivir) Shortages Reported In U.S. & Canada).
Once again, our first line of defense will likely rely heavily on NPIs (non-pharmaceutical interventions), like face masks, hand washing, ventilation, staying home while sick, and avoiding crowds.
Which is why I'm recommending that people seriously consider now (see #Natlprep 2025: Personal Pandemic Preparedness) what they will do if another pandemic virus should embark on a new world tour.
