Canadian Researchers Discover New H5N1 Bird Flu Strains Resistant to Oseltamivir (Tamiflu)!

Medical News: Worrisome Discovery in Canada
Canadian researchers have identified a new strain of the highly pathogenic H5N1 avian flu that shows resistance to the antiviral drug oseltamivir (Tamiflu). The discovery has raised concerns about the effectiveness of current influenza treatments. The mutated strain was found on eight chicken farms in British Columbia during an investigation of a widespread outbreak affecting 45 poultry farms in October 2024.

Canadian Researchers Discover New H5N1 Bird Flu Strains Resistant to Oseltamivir (Tamiflu)!
 
The research team from the Canada Food Inspection Agency (CFIA) sequenced the virus and determined it to be a clade 2.3.4.4b A(H5N1) strain. This particular variant possesses a neuraminidase surface protein originating from a low-pathogenic flu virus from a North American lineage. Despite initial findings suggesting that this substitution might reduce viral fitness, the strain spread rapidly across eight farms within just 15 days. This Medical News report delves into the details of this alarming development and its potential implications.
 
Uncommon Mutation Raising Concerns
The mutation, designated as H275Y, was found in the neuraminidase (NA) protein of the virus. This particular genetic change is well-known for conferring resistance to oseltamivir, one of the most commonly used antiviral drugs for treating and preventing influenza infections. According to the researchers, the H275Y mutation is extremely rare among clade 2.3.4.4b viruses, appearing in just 0.045% of publicly available isolates.
 
This finding has serious implications for public health. As oseltamivir is widely used to combat influenza A viruses, including those with pandemic potential, the continued spread of a drug-resistant H5N1 strain could limit treatment options and necessitate a reassessment of influenza management strategies in Canada and beyond.
 
Tracing the Emergence of the Strain
The researchers believe that the resistant H5N1 strain first emerged in Canada around September 2024. Shortly after its detection, the U.S. Department of Agriculture (USDA) updated its North American A(H5N1) genotyping tool, GenoFlu, to classify the virus as genotype D1.1. While the exact source of the mutation remains uncertain, genomic sequencing suggests that it resulted from a reassortment event between the H5N1 virus and a North American low-pathogenic avian influenza (LPAI) virus.
 
When CFIA experts analyzed oropharyngeal and cloacal swabs collected from affected poultry farms, they confirmed the presence of the H5 clade 2.3.4.4b virus. However, they noted that the NA segment was of North American lineage, rather than the Eurasian lineage typically associated with recent H5N1 outbreaks. This unusual finding indicates a genetic exchange between different flu viruses, leading to the emergence of a reassortant strain with unique properties.
 
Genetic Analysis and Resistance Confirmation
To confirm the mutation’s impact on drug resistance, scientists at the Public Health Agency of Canada’s National Microbiology Laboratory conducted laboratory tests to measure the virus’s sensitivity to oseltamivir, as well as other neuraminidase inhibitors like peramivir and zanamivir. Their findings were concerning:
 
-The H5N1 strain with the H275Y mutation exhibited a 161.4-fold increase in oseltamivir resistance compared to sensitive control strains.
 
-A moderate increase in resistance to peramivir was also observed.
 
-Zanamivir, however, remained effective against the strain, suggesting that alternative treatments could still be viable.
 
These results highlight the urgent need to monitor the spread of oseltamivir-resistant H5N1 and to explore alternative antiviral strategies.
 
Rapid Spread Raises Alarm
Despite evidence suggesting that the H275Y mutation may reduce viral fitness, the resistant H5N1 strain spread rapidly between poultry farms in British Columbia. Within 27 days of its first detection, the virus had infected 44 farms, prompting concerns over its potential to spread further.
 
The ability of the virus to maintain transmissibility despite acquiring drug resistance is a significant concern. In previous cases, viruses with the H275Y mutation exhibited limited ability to spread efficiently. However, the Canadian strain appears to have overcome this limitation, making it a potential threat to both animal and human health.
 
Potential Human Health Risks
While the primary impact of H5N1 has been on poultry, the risk of spillover to humans remains a pressing issue. Infections with H5N1 in humans are rare but often severe, with a high fatality rate. The emergence of an oseltamivir-resistant strain raises concerns about the effectiveness of current antiviral stockpiles in the event of a human outbreak.
 
The World Health Organization (WHO) and other health agencies closely monitor H5N1 outbreaks for signs of human adaptation. Although the newly discovered Canadian strain has not yet been reported in humans, its potential to reassort with other flu viruses could increase the risk of a future pandemic.
 
Conclusion and Future Implications
The discovery of an oseltamivir-resistant H5N1 strain in Canada is a wake-up call for health authorities worldwide. While previous reassortments of clade 2.3.4.4b viruses have been relatively rare, this case highlights the virus’s ability to acquire genetic changes that could impact treatment options. The fact that this resistant strain spread so quickly among poultry suggests that it could pose a significant challenge for outbreak containment.
 
This development underscores the need for continuous surveillance, rapid genomic sequencing, and the exploration of alternative antiviral treatments. Health authorities may need to consider updating stockpiles with additional antiviral drugs, such as zanamivir, which remains effective against the resistant strain. Additionally, ongoing research into new influenza treatments and vaccines will be crucial in preparing for future outbreaks.
 
The study findings of the discovery were published as a research letter in the peer reviewed journal: Emerging Microbes and Infections.
https://www.tandfonline.com/doi/full/10.1080/22221751.2025.2469643
 
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