Bird flu variant D1.1 spread unnoticed in North American wild birds for about three months, study finds

A fast-spreading bird flu variant circulated among North American wild birds for roughly three months before officials detected it, according to a study led by Arizona State University professor Matthew Scotch together with the Arizona Game & Fish Department, announced by ASU on 23 September.

What was studied

The team focused on D1.1, a genetic variant of the H5N1 clade 2.3.4.4b virus that has driven the global bird flu outbreak of recent years. They compared virus genomes from sick and dead wild birds collected in Arizona with thousands of publicly available sequences from across the country. The analysis indicates that D1.1’s major expansion probably began around summer 2024 — before its first recognised detection in September 2024. Scotch describes this as a period of “cryptic, or unsampled, circulation”.

How it moved

The virus spread mainly from west to east, between neighbouring migratory flyways: from the Pacific into the Central flyway, then into the Mississippi and on to the Atlantic flyway. There was little evidence of direct movement between the two most distant corridors, the Pacific and the Atlantic. The flyways’ roles as “sources” or “sinks” of infection shifted over the course of the autumn migration.

Why it matters

D1.1 has infected people, including the first US death linked to H5N1, in Louisiana in early 2025. The Arizona viruses did not carry the Q226L mutation, which lab studies have linked to a shift toward binding human-type receptors, but the researchers stress that influenza keeps evolving and that officials should track which variants circulate, not just case counts.

Recommendations

Scotch calls for faster sequencing of unusual detections, stronger genomic surveillance in the weeks before and during migration, and linking wildlife, poultry and mammal monitoring in a “One Health” approach, with data shared quickly across regions. His message for the public is “awareness rather than alarm”: these viruses remain mainly an animal-health issue.