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 prevalenceof small ruminant brucellosis. The stochastic scenario tree model was used to simulate vaccination and surveillance implementationduring 8 years. Outputs were annual proportions of true positive, true negative, false positive, and false negative flocks. Vaccinationcoverage 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 flockprevalence (2%, 5%, and 10%). In the following years, flock prevalence arising from the fitted distribution of the false negative flocksfrom the previous year increased by the estimated reproductive number. Average within flock prevalence was provided separately for thevaccinated and unvaccinated flocks, indirectly accounting for the vaccine efficacy. Specificity of the diagnostic tests (Rose Bengal plateand 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. Accordingto our estimates, only consistent vaccination combined with systematic removal of diseased animals can result in significant reductionof disease prevalence.