A controlled field experiment to explain amplification of avian influenza viruses along poultry marketing chains in Bangladesh

Although avian influenza viruses (AIVs) are highly prevalent in Bangladesh's live bird markets (LBMs), they are rarely detected in poultry farms. This study aimed to identify the stage of poultry marketing chains at which viral amplification occurs.

We implemented a controlled field experiment to assess whether reducing the risk of infection for chickens during transport and trade decreases their likelihood of shedding AIVs in LBMs. In Chattogram, Bangladesh, 64 batches of 10 chickens (340 backyard and 300 broiler chickens) were caged together in market stalls and followed over a period of 84 hours. For each batch, 5 chickens were purchased from farmers and subject to stringent biosecurity measures during storage and transport (intervention group), while another 5 chickens were purchased from traders and underwent conventional marketing chains (control group). We collected oropharyngeal swabs from chickens upon arrival at the LBM as well as 12, 36 and 84 hours later. Conditional logistic regression and discrete-time proportional hazard models were developed to analyse RT-qPCR results.

The effect of the intervention on the proportion of chickens shedding AIVs was only subtle upon arrival at the LBM but significant at the following time points. Backyard chickens subject to the intervention had lower odds of testing positive after 12 (OR=0.26[0.06-1.03, P=0.05) and 84 hours (OR=0.29[0.16-0.51], P<0.001). The similar was observed after 12 hours (OR=0.42[0.20-0.87], P=0.02) for backyard chickens, of which more than 99% were already shedding after 36 hours. Overall, transmission and infection events occured at a slower rate for backyard chickens (HR=0.09, P<0.001).

This study indicates that a substantial proportion of viral shedding in Bangladesh's LBMs results from infection of chickens during transport and trade. We demonstrate that interventions targeting the collection and dissemination of chickens along marketing chains are needed to reduce the prevalence of AIVs in LBMs.