Airborne transmission’s role in spreading US 2015 bird flu

Airborne transmission is likely to have played a role in spreading the highly pathogenic avian influenza strain that devastated the US poultry industry in 2015, resulting in over $3.2bn in economic impacts. In less than 7 months, 232 farms in 15 states and over 50 million birds were infected.

Anecdotal evidence at the time suggested that in some cases the AI virus was aerially introduced into poultry houses, as abnormal bird mortality started near air inlets of infected houses.

A federal assessment of the outbreak found no one clear factor, bust said the virus was likely to have spread by multiple routes, including biosecurity gaps and possibly airborne transmission.

Scientists led by Dr Yang Zhao at the Department of Agricultural and Biosystems Engineering at Iowa State University, modelled air movement trajectories and virus concentrations that were used to assess the probability or risk of airborne transmission for the 77 HPAI cases in Iowa.

Airborne virus

The results shows that the majority of the positive cases in the state might have received airborne virus, carried by fine particulate matter, from infected farms within the state (ie, intrastate) and infected farms from the neighbouring state (ie, interstate).

The modelled airborne virus concentrations at the Iowa recipient sites never exceeded the minimal infective doses for poultry; however, the continuous exposure might have increased airborne infection risks.

In the worst-case scenario, (ie., maximum virus shedding rate, highest emission rate and longest half-life) 33 Iowa cases had >10% (three cases >50%) infection probability, indicating a medium to high risk of airborne transmission for these cases.

Probability of airborne HPAI infection could be affected by farm type, flock size and distance to previously infected farms; and more importantly, it can be markedly reduced by swift depopulation and inlet air filtration.

The research results, say the scientists, provide insights into the risk of airborne transmission of HPAI virus via fine dust particles and the importance of preventative and containment strategies such as air filtration and quick depopulation of infected flocks.

• The study has been published in Scientific Reports.