Determining the Environmental and Ecological Factors Associated With Poultry Farm Spillover of Highly Pathogenic Avian Influenza (H5N1) in British Columbia, Canada.

Highly pathogenic avian influenza (HPAI) H5N1, clade 2.3.4.4b is a global threat causing widespread outbreaks on poultry farms and high mortality among wild bird populations in Canada. Wild aquatic birds are generally recognised as key reservoir hosts for avian influenza viruses (AIV's). Furthermore, environmental weather variables have been hypothesised to play a critical role in the spread and persistence of HPAI in the external environment. During the 2022 outbreak of HPAI (H5N1) in Canada, many positive farm cases were identified via epidemiological investigations as independent introductions, likely a result of virus spillover from the external environment into the poultry farm. We investigated the association between recently occurring meteorological and wild bird occurrence factors and the likelihood of poultry farms in British Columbia testing positive for HPAI (H5N1) due to independent viral introductions. Using a retrospective case-crossover analysis, this study produced a multivariable conditional logistic regression model examining weekly-averaged meteorological factors (n = 49) and a series of univariable models to examine wild bird occurrence across case and control periods for independent introductions of HPAI onto poultry farms in British Columbia (n = 48). During case periods, we found significant associations between the following variables: our multivariable analysis found elevated levels of precipitation and decreasing maximum relative humidity increased odds of independent introduction events. Additionally, the increasing occurrence of dabbling duck species groups, snow geese (Anser caerulescens) and short-billed gulls (Larus brachyrhynchus) during case periods was associated with increased odds of independent introduction events. This research identifies key weather variables and increased occurrence of specific waterbird species groups in the weeks directly prior to case occurrence as predictive factors for the occurrence of independent introductions of HPAI (H5N1) onto British Columbia poultry farms. These findings help us better understand the weather and wild bird factors likely associated with the risk of spillover of HPAI to poultry farms in this region.