Evaluation of methods for measuring coverage and representativeness of an early-warning disease surveillance system

Early-warning surveillance provides an essential component of the evidence required to protect animal health. Assessing the proportion of the population included in surveillance systems (coverage) provides a measure of the effectiveness of early-warning surveillance, and contributes to ensuring that these systems are efficient. This paper describes an investigation of methods used for assessing the coverage and representativeness of the 'FarmFile' early-warning surveillance system. This system uses information collected with samples submitted to diagnostic laboratories by private veterinary practitioners in England and Wales. Available data on pig holdings and veterinary practices in four English counties, selected to represent a range of diverse population characteristics, were supplemented using surveys of veterinary practices. Coverage assessments were based on submissions made to FarmFile in 2009. The proportion of holdings covered varied from 5-62 per cent in Devon and Cumbria, and 16-97 per cent in Norfolk and East Riding of Yorkshire. The results suggest that while the proportion of individual pigs covered by the current early-warning surveillance system is high, small and breeding-only holdings in some regions may be poorly covered. Coverage assessments vary depending on the methods used for their assessment, and multiple assessment methods can provide a 'range' within which coverage lies.