Clade 2.3.4.4b of the highly pathogenic avian influenza (HPAI) virus causing outbreaks in wild and domestic birds around the world, continues to spread in dairy cows, poultry and other animals across the United States. Since April, the U.S. CDC confirmed 52 human cases through genome sequence analysis. In three of these cases, the amino acid substitution NA-S247N was identified that may slightly reduce susceptibility to the neuraminidase inhibitor oseltamivir in laboratory tests. The CDC also identified a different change in the polymerase acidic (PA) protein of a virus collected from a recently confirmed human case of H5N1 bird flu in California.

The current outbreak validates the unpredictability of HPAI viruses, as the route of exposure in dairy cows and mode of virus transmission remains unknown. The virus RNA was found at high concentrations in raw milk. Several animal species at dairy and poultry farms, as well as a growing number of farm workers are affected. Data from the USDA’s Animal and Plant Health Inspection Service show that the viruses in infected cows, other animals and the farm workers are closely related. A mammalian adaptation marker (E627K) was noted in only one farm worker so far, with nearly all farm workers developing mild eye symptoms, alongside respiratory symptoms.

On 15. November, specimen data of British Columbia´s first human case, a teenager who remains in critical condition, with an unknown route of exposure to H5N1 bird flu, has been shared via GISAID’s EpiFlu by the British Columbia Centre for Disease Control (EPI_ISL_19548836). Today, specimen data from poultry, collected on 7. November was shared by the Canadian Food Inspection Agency. The latest trees, based on representative subsamples, are dated to 20 November 2024.