No major role for airborne transmission of bird flu virus in aerosols of wild waterfowl faeces in infecting poultry farms

In 2021, Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research) conducted a semi-quantitative risk assessment for the introduction of highly pathogenic avian influenza virus (HPAIv) into poultry farms via aerosolized faeces of infected wild birds. Based on that assessment using a relatively simple model, the researchers estimated that the risk of this introduction route is very low. With support from the Statutory Tasks program (WOT), the WBVR researchers conducted follow-up research. For this, a quantitative and comprehensive microbial risk assessment model was developed. “With this model, we can estimate the probability that the aerosolization of faeces from HPAIv-infected wild birds in the vicinity of poultry farms results in infection of indoor-housed poultry,” explains veterinary epidemiologist Armin Elbers.

Parameters

Model input parameters were retrieved from the scientific literature and experimental data. Data availability was diverse across input parameters. “Especially the parameters on aerosolization of wild bird droppings, HPAIv survival and aerosol dispersion are uncertain,” Elbers said.

Extremely low probability

“The daily probability of infection of a single poultry farm via aerosolization of contaminated faeces from wild waterfowl is extremely low,” concludes risk analyst Clazien de Vos, who conducted the study together with Elbers. Taking into account the total number of poultry farms and the length of the bird flu season, an HPAIv infection in poultry farms during the bird flu season in the Netherlands is to be expected once every 455 years via this introduction route. Even under a worst-case scenario, the probability of new infections is still very low: once every 17 years. This is a general estimate, averaged across different farm types, virus strains and wild bird species, and the results indicate that the uncertainty is relatively high. Based on these modelling results, the researchers conclude that this route of introduction is unlikely to play a significant role in the occurrence of HPAIv outbreaks in indoor-housed poultry.

Biosecurity

Other risk factors, such as (occasional) failure to operate in strict and consistent compliance with biosecurity measures on the poultry farm, may be of greater importance in the introduction of HPAIv on poultry farms.

Furthermore, the risk assessment provides tools for preventing possible wind-supported transmission of HPAIv via particles of faeces from infected wild birds. Drying of HPAIv-infected faeces from wild birds is a prerequisite for aerosolisation. “This practically only happens during the bird flu season when the droppings are deposited on concrete or stone-paved surfaces around poultry units. The probability of the occurrence of a chain of drying of HPAIv-contaminated bird droppings, subsequent aerosolisation and transport by wind of still infectious HPAIv via the air inlets of a poultry barn is very low,” Elbers said.

To make this probability negligible, the researcher advises poultry farmers to regularly check for the presence of wild bird droppings on the paved surfaces around poultry houses and safely remove them. “This will also reduce the likelihood of the incidental introduction of HPAIv-contaminated wild bird droppings into the poultry house by sticking to the boots of people walking on the property and entering the barn.”