Surveillance on California dairy farms reveals multiple sources of H5N1

Transmission routes of highly pathogenic H5N1 between cows or to humans remainunclear due to limited data from affected dairy farms. We performed extensive air, farmwastewater, and milk sampling on 14 H5N1 positive dairy farms across two different Californiaregions. Virus was detected in the air in milking parlors and from exhaled breath of cows.Infectious H5N1 virus was detected in the air and water streams; sequence analysis revealed viralvariants on a farm in these locations. Longitudinal analysis of milk from the individual quarters ofcows revealed a high prevalence of subclinical H5N1 positive cows and a heterogeneousdistribution of infected quarters that maintained a consistent pattern over time. Our data highlightpotential modes of H5N1 transmission on dairy farms.