Preparation for a potential outbreak of bluetongue virus in Ireland: surveillance design to estimate local prevalence after an initial case detection

Bluetongue virus serotype 3, emerged in northern Europe in 2023 and 2024. As of June 2025, Ireland is bluetongue free. However, to inform control decisions in the event of a possible incursion, a surveillance plan to detect cases and estimate prevalence is required. We created an active surveillance plan for 20km radius temporary control zones (TCZs) after initial case detection. Potential TCZs (n = 1062) covering Ireland were generated, and surveillance sample sizes were estimated based on cattle data in each TCZ. A two-stage (herd and animal level) design accounted for within-herd clustering. We simulated implementation of the surveillance plan in each TCZ to understand surveillance performance in the Irish cattle population. With simulated between-herd prevalence of 5%, within-herd prevalence of 30%, and using reverse transcriptase polymerase chain reaction with perfect specificity and 99% sensitivity, surveillance to estimate prevalence was adequate when between 39 and 61 herds per TCZ were sampled. With simulated between-herd prevalence of 30%, 10-11 herds per TCZ needed to be sampled to estimate prevalence. Within herds, sampling 10–11 cattle was sufficient for prevalence estimation. We integrated Shiny and ArcGIS web applications to allow users to simulate different scenarios under different settings. These include different test sensitivity and specificity, and different within- and between- herd prevalence contexts. This interface presents infected herds sampled, and true- and false- positives and negatives in a variety of conditions. Evidence from this scenario analysis can be integrated into a multi-pronged early warning and potential follow-up surveillance programme to facilitate decision making in the event of an incursion of BTV into Ireland. The URL for the web application associated with this paper is: https://www.arcgis.com/apps/dashboards/16722dde78d240f4a96303173bc6da2c