Avian Influenza: The Environment and Biosecurity Affect Virus Transmission in Poultry Farms

Since 2021, the United States has been facing an outbreak of highly pathogenic avian influenza (H5N1 HPAI) on an unprecedented scale. More than 500 cases of direct transmission of the virus from wild birds to domestic poultry were confirmed between 2022 and 2023. Millions of poultry had to be culled, resulting in colossal economic losses and disruptions to the food chain. In response, American researchers have developed a model that integrates both environmental data (climate, habitat, wildlife abundance) and information from field surveys conducted at turkey farms, which are particularly vulnerable to contamination. Their goal: to identify the risk factors that increase or reduce the risk of transmission in order to better target biosecurity measures and health surveillance.  

Waterfowl and Climate: Two Risk Factors

The model confirms what surveillance had already suggested: migratory waterfowl, particularly so-called dabbling ducks, are the main reservoirs and spreaders of the HPAI virus. Their biology (feeding in surface waters, massive shedding of the virus via the cloaca) contributes to high levels of environmental contamination. Climatic conditions influence this risk: the researchers show that cold, wet winters promote the persistence of the virus in water and soil. Conversely, higher temperatures limit the survival of pathogens. The combination of a high abundance of waterfowl and harsh, rainy winters therefore creates a scenario particularly conducive to the transmission of the virus from wild birds to domestic flocks.  

Human Activities: An Aggravating or Protective Factor

While wild birds are the source of the risk, it is often human activities that contribute to the introduction of the virus into poultry farms. Researchers show that vehicle traffic, the sharing of equipment, and the comings and goings of people on farms play a key role. These findings underscore that contamination is not merely a matter of contact between wild birds and domestic poultry: it also depends on how access, hygiene, and logistics are managed on farms.  

The Importance of Managing Bedding and Carcasses

The study also highlights an often-overlooked factor: the management of litter and poultry carcasses. Farms that heat fresh bedding before use drastically reduce the risk, as heat inactivates the virus. Conversely, the use of shared disposal sites for carcasses increases the likelihood of cross-contamination between farms. Simply introducing newly delivered, untreated litter also increases the risk of transmission. These findings illustrate how biosecurity must be viewed as a coherent set of measures, one that also incorporates the management of waste and livestock byproducts.  

Turkeys Are More Vulnerable Than Chickens

Comparative analyses show that turkey farms pose a higher risk than chicken farms. There are several possible explanations: higher flock density, building layout, species-specific biological characteristics, etc. This finding warrants particular vigilance in turkey-producing states, such as Minnesota, where recurrent outbreaks have been observed.  

Surveillance and Prevention Tools

By integrating environmental, climatic, and field data, the developed model serves as an effective predictive tool. It helps identify the regions and times of year most prone to contamination. As a result, health surveillance can be strengthened in a targeted manner, for example during periods of heavy duck migration or during cold, rainy winters. The researchers also suggest using tools such as BirdCast (which models bird migrations in real time) or telemetry data to anticipate peaks in risk. This information could be cross-referenced with biosecurity plans to optimize practices and prevent “fatigue” among farmers caused by guidelines that are too burdensome to implement continuously.  

An Essential “One Health” Approach

Beyond protecting poultry farms, the authors point out that the transmission of the HPAI virus from wildlife to poultry can trigger the reverse mechanism (spillback ), fueling a cycle of cross-infections that sustains viral circulation. Furthermore, the recent spread of H5N1 to new species, including dairy cattle and certain mammals, illustrates the need to adopt a comprehensive “One Health” approach.   This study demonstrates that preventing risks associated with avian influenza cannot be reduced to a few general recommendations. It is the combination of targeted surveillance, consideration of environmental factors, and pragmatic biosecurity measures that will best protect livestock, wildlife, and, ultimately, human health.   A study shows that the spread of the avian influenza virus depends as much on migratory birds and climate as it does on human biosecurity practices.