Modelling of prevalence development in a paratuberculosis control program in a dairy herd

A modelling approach to calculate the success of a paratuberculosis control programme in dairy herds is presented. The essential parameters of the model are the prevalence at the beginning of the programme, diagnostic sensitivity and specificity of the tests used, discipline in culling test-positive animals, turnover in the herd, percentage of replacement with own stock and paratuberculosis prevalence in animals bought into the herd from outside, and a general hygiene-based factor. Diagnostic measures and time schedule used in the modelling approach are given by the paratuberculosis-control-programme of the local board for infectious disease control in food animals in the state of Lower Saxony. It was found by the model-calculations that in case of a high initial prevalence the anticipated six-year duration of the control programme is justified in order to ensure a lasting improvement of herd health. If hygienic measures are strictly obeyed and all test positive animals are culled a clear reduction on paratuberculosis prevalence can be achieved within the first year. According to the model in the second and third year the prevalence will increase again despite ongoing diagnostic measures in order to decrease again continuously with the beginning of the fourth year. Given an initial prevalence of 10%, 20% or 30% the prevalence after six years is calculated to be at 3%, 5% or 8% when all measures are followed as given in the control programme. The presented programme seems to be appropriate to predict prevalence development in paratuberculosis infected dairy herds if the herds are managed according to the guidelines of the "Tierseuchenkasse Niedersachsen", the local board for infectious disease control in food animals in the state of Lower Saxony, Germany. It becomes apparent that within six years a high decrease of the prevalence in the herds, but not a complete eradication of disease can be achieved by consistently complying with the rules given in these guidelines.