The scenario tree epidemiological model in estimation effects of B. melitensis Rev 1 vaccination on disease prevalence

Abstract

The aim of this study was to evaluate the effects of the vaccination program in Bosnia and Herzegovina on annual prevalence of small ruminant brucellosis. The stochastic scenario tree model was used to simulate vaccination and surveillance implementation during 8 years. Outputs were annual proportions of true positive, true negative, false positive, and false negative flocks. Vaccination coverage was described by the Pert distribution with average 70% and min and max 50% and 80%, respectively. Effects of risk factors (high prevalence-areas and transhumance) were considered. The model was separately simulated for three levels of initial average flock prevalence (2%, 5%, and 10%). In the following years, flock prevalence arising from the fitted distribution of the false negative flocks from the previous year increased by the estimated reproductive number. Average within flock prevalence was provided separately for the vaccinated and unvaccinated flocks, indirectly accounting for the vaccine efficacy. Specificity of the diagnostic tests (Rose Bengal plate and Complement fixation) was reduced by 5% for the vaccinated flocks to represent the increased occurrence of false positive results. Each simulation was iterated 1000 times using @Risk, providing average prediction and 5th and 95th percentile of outputs. According to our estimates, only consistent vaccination combined with systematic removal of diseased animals can result in significant reduction of disease prevalence.