Biosecurity responses to incursions aim to achieve pest- or disease-free status as quickly as possible. One of the critical initial response activities involves tracing known movements (trace events) to and from an infected or infested property (IP) that could spread the pest or pathogen. During an incursion response, managers allocate surveillance resources to follow up trace events in order of priority. Prioritizing trace events is difficult and typically subjective. We present a simulation model where several dispersal mechanisms spread a pest between areas. We use model outputs to test different search strategies, using citrus canker (caused by Xanthomonas citri) as a case study. Model scenarios are based on an outbreak of citrus canker that occurred in Queensland in 2004.
Model parameters were extracted from published scientific reports and elicited from experts. We used model outputs to assess three search strategies to determine how best to monitor citrus canker spread. Parameters governing disease detectability and host susceptibility were varied in a sensitivity analysis.
In all simulation scenarios, the ‘adaptive radius’ rule performed best, whereby a circular search area was placed around the IP where the disease outbreak was first detected, with a radius proportional to the estimated number of months the property was infected. Importantly, none of the search rules tested detected all IPs without searching all areas with susceptible hosts in the region.
We identify a simple rule of thumb for searching during a citrus canker outbreak that is robust to uncertainty. We cannot generalize the results of this study for tracing other pests or pathogens. The model has created a framework that may be used to explore other contexts and disease dynamics, leading perhaps to more general rules for disease outbreak management.