The 2.3.4.4b clade of the highly pathogenic avian influenza (HPAI) virus causing outbreaks in wild and domestic birds around the world, continues to spread in dairy farms across the United States since March 2024, with 51 human cases reported thus far. These HPAI viruses cause morbidity and mortality in over 60 mammalian species, mostly carnivores, after consuming infected carcasses. Although cows had not been considered to be at risk of infection, the current outbreak in the U.S. demonstrates influenza remains unpredictable.

The route of exposure of 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.

Over a thousand full virus genome sequences from this outbreak, collected from mammalian and avian specimens in at least 18 States are shared via GISAID EpiFlu. Since the last update on 14. November, specimen data of British Columbia´s first human case, a teenager who is currently hospitalized, with an unknown route of exposure (EPI_ISL_19548836) has been made available by British Columbia Centre for Disease Control (BCCDC). Additionally, specimen data from poultry, collected between 20. and 23. October, was made available by the Canadian Food Inspection Agency. The latest trees, based on representative subsamples, are dated to 15 November 2024.