Using scenario tree modelling for targeted herd sampling to substantiate freedom from disease

Background: In order to optimise the cost-effectiveness of active surveillance to substantiate freedom from disease, a new approach using targeted sampling of farms was developed and applied on the example of infectious bovine rhinotracheitis (IBR) and enzootic bovine leucosis (EBL) in Switzerland. Relevant risk factors (RF) for the introduction of IBR and EBL into Swiss cattle farms were identified and their relative risks defined based on literature review and expert opinions. A quantitative model based on the scenario tree method was subsequently used to calculate the required sample size of a targeted sampling approach (TS) for a given sensitivity. We compared the sample size with that of a stratified random sample (sRS) with regard to efficiency. Results: The required sample sizes to substantiate disease freedom were 1,241 farms for IBR and 1,750 farms for EBL to detect 0.2% herd prevalence with 99% sensitivity. Using conventional sRS, the required sample sizes were 2,259 farms for IBR and 2,243 for EBL. Considering the additional administrative expenses required for the planning of TS, the risk-based approach was still more cost-effective than a sRS (40% reduction on the full survey costs for IBR and 8% for EBL) due to the considerable reduction in sample size. Conclusions: As the model depends on RF selected through literature review and was parameterised with values estimated by experts, it is subject to some degree of uncertainty. Nevertheless, this approach provides the veterinary authorities with a promising tool for future cost-effective sampling designs

Simulation Based Evaluation of Time Series for Syndromic Surveillance of Cattle in Switzerland

Choosing the syndrome time series to monitor in a syndromic surveillance system is not a straight forward process. Defining which syndromes to monitor in order to maximize detection performance has been recently identified as one of the research priorities in Syndromic surveillance. Estimating the minimum size of an epidemic that could potentially be detected in a specific syndrome could be used as a criteria for comparing the performance of different syndrome time series, and could provide some guidance for syndrome selection. The aim of our study was to estimate the potential value of different time series for building a national syndromic surveillance system for cattle in Switzerland. Simulations were used to produce outbreaks of different size and shape and to estimate the ability of each time series and aberration detection algorithm to detect them with high sensitivity, specificity and timeliness. Two temporal aberration detection algorithms were also compared: Holt-Winters generalized exponential smoothing (HW) and Exponential Weighted Moving Average (EWMA). Our results indicated that a specific aberration detection algorithm should be used for each time series. In addition, time series with high counts per unit of time had good overall detection performance, but poor detection performance for small epidemics making them of limited use for an early detection system. Estimating the minimum size of simulated epidemics that could potentially be detected in syndrome TS-event detection pairs can help surveillance system designers choosing the most appropriate syndrome TS to include in their early epidemic surveillance system

Evaluation of the benefit of emergency vaccination in a foot-and-mouth disease free country with low livestock density

Foot-and-mouth disease (FMD) is highly contagious and one of the most economically devastating diseases of cloven-hoofed animals. Scientific-based preparedness about how to best control the disease in a previously FMD-free country is therefore essential for veterinary services. The present study used a spatial, stochastic epidemic simulation model to compare the effectiveness of emergency vaccination with conventional (non-vaccination) control measures in Switzerland, a low-livestock density country. Model results revealed that emergency vaccination with a radius of 3 km or 10 km around infected premises (IP) did not significantly reduce either the cumulative herd incidence or epidemic duration if started in a small epidemic situation where the number of IPs is still low. However, in a situation where the epidemic has become extensive, both the cumulative herd incidence and epidemic duration are reduced significantly if vaccination were implemented with a radius of 10 km around IPs. The effect of different levels of conventional strategy measures was also explored for the non-vaccination strategy. It was found that a lower compliance level of farmers for movement restrictions and delayed culling of IPs significantly increased both the cumulative IP incidence and epidemic duration. Contingency management should therefore focus mainly on improving conventional strategies, by increasing disease awareness and communication with stakeholders and preparedness of culling teams in countries with a livestock structure similar to Switzerland; however, emergency vaccination should be considered if there are reasons to believe that the epidemic may become extensive, such as when disease detection has been delayed and many IPs are discovered at the beginning of the epidemic