Targeted surveillance for high pathogenicity avian influenza in vaccinated birds

Abstract

The continued global spread of high pathogenicity avian influenza (HPAI) virus poses serious threats to poultry production, trade and food security. Vaccination is gaining attention as a strategic complement to traditional control measures. Vaccination reduces clinical disease and often slows the spread of viral infection within and among flocks due to decreased virus shedding and susceptibility, which can in turn affect virus detection, necessitating enhanced surveillance. Recent studies suggest that targeted sampling of sick and dead birds remains a sensitive approach in vaccinated populations. However, uncertainty persists regarding transmission dynamics, disease mortality and other factors influencing within-flock spread. To address these uncertainties, a stochastic within-flock model incorporating multiple immunity classes was developed to simulate HPAI spread in vaccinated birds. Model scenarios with varying transmission parameters and surveillance strategies were evaluated to examine sampling design factors including targeted sampling, number of samples per sampling event, sampling event frequency and split sampling over consecutive days. Results indicate that increased sampling frequency substantially reduces time to detection, thereby lowering the risk of lateral spread. Vaccinated flocks require increased sampling varying with flock size, transmission rate and mortality level. Splitting the sampling effort across two consecutive days improved detection performance under slower spread scenarios in smaller flock sizes. Overall, findings demonstrate that targeted sampling of dead birds remains an effective surveillance strategy for HPAI detection in vaccinated flocks. Optimising surveillance design to balance detection timeliness and resource efficiency and to minimise risks associated with undetected transmission is essential to achieve specific surveillance goals.

Keywords

High pathogenicity avian influenza, Outbreak response, Surveillance design, Transmission, Vaccination.