H5N5 virus transmission is driven by repeated cross-Atlantic introductions and transmission among high latitude seabirds

In November 2025, the first recorded case of H5N5 virus infection in a human was reported in Washington State. Previously, these viruses had only been found in seabirds and shorebirds near the Atlantic Ocean, indicating a potentially different ecology compared to the more common H5N1 strains.

By analyzing 354 global H5N5 genomes, researchers have discovered that H5N5 viruses have been transported by high latitude seabirds and shorebirds across the Atlantic at least nine times. These introductions occurred more frequently in the fall and spring seasons. The team was able to reconstruct a likely scenario of cross-continental movement through Arctic or sub-Arctic regions, followed by a year of concealed transmission before reaching a human population.

These findings reveal that H5N5 viruses have experienced significantly more intercontinental movements than previously thought. Additionally, the viruses exhibit distinct host and geographic patterns that set them apart from the prevalent H5N1 lineages. Given the ability of H5N5 to infect humans, there is a strong case for increased surveillance measures to monitor and track the virus's spread.