Epidemiologic and economic risk analysis of Johne's Disease control

Johne's disease or paratuberculosis in cattle is a chronic, progressive intestinal disease caused by infection with Mycobacterium avium subsp. paratuberculosis (Map). There is a growing concern about the apparent increase in the prevalence of Johne's disease and the resulting economic and possible trade implications. In addition, although there has not been any definitive proof, Johne's disease may be associated with some forms of Crohn's disease in humans. As a result, there is an increased need for effective and economically attractive control programs against Johne's disease. The main objective of the research described in this thesis was to support decision-making in the design and development of control and certification-and-monitoring programs for Johne's disease by providing insight into the epidemiologic and economic effects of different strategies. To meet this objective, a stochastic simulation model, the 'JohneSSim' model was developed and used to evaluate control and certification-and-monitoring strategies on Dutch and mid-size US cattle herds. According to the model when applied to Dutch dairy farms, test-and-cull strategies alone using the current tests available do not considerably reduce the prevalence of Johne's disease and are economically unattractive. As a consequence, the focus of policy-makers changed to management measures to prevent the spread of Map within herds. A new Dutch Johne's disease program was designed, called Paratuberculosis Program Netherlands (PPN), and evaluated with the JohneSSim model. It was found that under PPN, a low true prevalence could be reached within 20 years and that PPN was on average economically attractive. AIso, a number of certification-and-monitoring schemes for Johne's disease test-negative dairy herds were evaluated on their costs and effectiveness. Furthermore, control strategies on Dutch beef herds were evaluated, and it was concluded that under current practical circumstances no control strategy was economically attractive and realistic. For US mid-size dairy herds, similar results were obtained as for Dutch dairy herds. Vaccination was found to be economically attractive, but not able to reduce the prevalence. Measures to prevent spread of Map within herds and contract heifer rearing were found to be better control strategies that both decrease the prevalence and have economic benefits. Both in The Netherlands and in the US, this study greatly supported the decision making process in the development and improvement of Johne's disease control and certification-and-monitoring strategies.