Economic Consequences of Paratuberculosis Control in Dairy Cattle Herds

The PTB-Simherd is a dynamic, stochastic, and mechanistic Monte-Carlo model simulating adairy herd including young stock. Paratuberculosis and relevant control strategies against thisdisease was built into an existing simulation model. The model is able to simulate bothepidemiological, technical and economic consequences of paratuberculosis and controlstrategies against it in the herd. It also reflects indirect effects of paratuberculosis and controlstrategies through effect on replacement and herd demographics. A cow is specified withbiological parameters (updated in weekly time-steps), including paratuberculosis stated byinfection status and latest results of two different diagnostic tests. Management is specified atherd level with 353 parameters of which 78 are related to paratuberculosis. Risk of infectionis specified for five separate periods. Infected animals are given three risks of diseaseprogression to next state of paratuberculosis: A weekly base risk, risk from calving andfeeding stress. Production effects of paratuberculosis are specified (depending on infectionstate) by reduced milk production, body weight loss, reduced fertility, and an increased deathrisk. Two groups of control strategies can be combined or used separately: 1. Optimisation ofmanagement (adjusting risks of infection), and 2. Test-&-cull strategies with the use of twodifferent diagnostic tests. Sensitivity of these tests can be specified according to parity,lactation-stage and paratuberculosis status, and different strategies for using the test result canbe specified. To demonstrate model behaviour, scenarios with varying infection risks (sensitivity analysis)plus scenarios with seven different control strategies in two herds with good and poorreproduction were simulated for ten years. A sensitivity analysis showed that there was alimited effect of changing the infection risk in one risk period, if the risks in other periodswere high, whereas there was a substantial effect, if the risks in other periods were very small.Simulation of control strategies in herds with good and poor reproduction managementdemonstrated that interactions between reproduction and control strategies affected especiallyherd size, replacement rate, cases of other diseases and the need for purchasing pregnantheifers. By comparing simulated control strategies, breaking of infection routes turned out tobe the only strategy that reduced the true prevalence of paratuberculosis in a herd.Supplementing this strategy with test-&-cull strategies had limited effect on prevalence and