In a research letter published this week in Emerging Microbes & Infections, researchers at the Canada Food Inspection Agency (CFIA) describe their discovery of a mutated H5N1 avian flu strain resistant to the antiviral drug oseltamivir (Tamiflu) on eight chicken farms in British Columbia in October 2024.

When investigating a widespread and ongoing H5N1 outbreak at 45 poultry farms, the CFIA National Centre for Foreign Animal Disease team sequenced the virus, identifying it as a clade 2.3.4.4b A(H5N1) strain. The virus had a neuraminidase surface protein derived from a low-pathogenic flu virus from a North American lineage. 

Despite evidence to suggest this substitution reduces viral fitness, viruses harboring this substitution spread rapidly across 8 farms in the 15 days following its initial detection.

"Isolates from 8 farms reveal a mutation in the neuraminidase protein (H275Y) that is exceptionally rare among clade 2.3.4.4b viruses (present in 0.045% of publicly available clade 2.3.4.4b isolates)," the researchers wrote. "NA-H275Y is a well-known marker of resistance to the neuraminidase inhibitor oseltamivir."

The virus likely emerged in Canada in September 2024, the authors said. The US Department of Agriculture later updated its North American A(H5N1) genotyping tool GenoFlu to designate the virus as genotype D1.1. 

"Despite evidence to suggest this substitution reduces viral fitness, viruses harboring this substitution spread rapidly across 8 farms in the 15 days following its initial detection," the researchers wrote. "As oseltamivir is the most widely used therapeutic and prophylactic against IAV [influenza A virus], the continued circulation of viruses harboring NA-H275Y may necessitate a re-evaluation of influenza treatment strategies in Canada."

It's unclear whether the mutated virus is still circulating.

Four more H5N1 detections in US

Yesterday, the US Department of Agriculture Animal and Plant Health Inspection Service confirmed four more highly pathogenic avian flu detections, including a commercial turkey farm in Sac County, Iowa, affecting nearly 30,000 birds and three backyard flocks in Maine and Pennsylvania totaling nearly 500 birds.

In the last 30 days, 101 commercial and 55 backyard flocks have been infected, at a loss of 21.7 million birds.