A stochastic model simulating paratuberculosis in a dairy herd.

Paratuberculosis (PTB) causes severe economic losses to farmers and the infection has very complex effects (many indirect) on the production of a dairy herd. These indirect effects have not or only briefly been described by earlier PTB-simulation models, and therefore they were included in a new model called PTB-Simherd. Our aim was to develop the basis for a decision-support tool which can predict herd-specific production-related effects from introduction of different control strategies against PTB. The PTB-Simherd is a dynamic, stochastic, and mechanistic Monte-Carlo model simulating a dairy herd including young stock. Paratuberculosis and relevant control strategies against this infection were built into an existing herd simulation model. The model simulates epidemiological and production related consequences of PTB and control strategies against it in the herd. It also reflects indirect effects of PTB and control strategies through effects on replacements and herd demographics. Every animal in the herd is specified with biological parameters (including PTB state and test results) and it is updated in weekly time-steps. Management is specified at herd level with 353 parameters of which 78 are related to PTB. To demonstrate the basic characteristics of the model, scenarios with varying infection risks (sensitivity analyses) plus scenarios with seven different control strategies in two herds with good and poor reproduction were simulated for 10 years. Breaking of infection routes turned out to be the only strategy predicted to reduce the true prevalence of PTB in a herd. Supplementing this strategy with test-&-cull strategies had limited effect on prevalence and using test-&-cull alone just delayed the increase in prevalence. The effects of different PTB-control strategies on the production (especially sale/purchase of heifers, feed consumption and prevalences of other diseases) were predicted to be affected by other conditions like heat-detection success, replacement% and herd demographics--which were again affected by PTB infection of the herd. These links and indirect effects of control strategies thus seem important to include when modeling and predicting effects of PTB control in dairy herds.