Introduction

Johne's disease (JD), caused by Mycobacterium avium subspecies paratuberculosis (MAP) is thought to occur on about 70% of New Zealand dairy farms and causes economic loss due to premature culling and reduced milk production. Widespread speculation about the possible role of MAP in Crohn's disease in humans has led to concern within the dairy industry. New Zealand has no national JD control strategy, unlike competing dairy producers.The JohneSSim computer model is a stochastic, dynamic model that simulates JD at the cow level over 20 years. For each control strategy it simulates true prevalence, costs and benefits for a population of dairy farms. The objective of this study was to use JohneSSim to model JD and possible control strategies for the New Zealand dairy industry.

Material and methods

Recent case/control and longitudinal studies in New Zealand, industry data, expert opinion and international data provided model input. Control strategies tested included Improved Farm Management (IFM), Vaccination, Genetic Resistance and Annual Test-and-Slaughter based on faecal culture or ELISA.

Results

Simulated on-farm prevalence rose from 15% to 49% and annual losses from $2300 to $23400 in the absence of control. All strategies slowed the rate at which prevalence increased, but only IFM reversed this rate resulting in prevalence of <1% after 11 years. IFM and Vaccination were the only economically attractive strategies.

Conclusion

The likely effects of each control strategy must be clearly understood, as must the aim of a control program. IFM reduced JD to very low levels and would probably be a low cost strategy. Combining IFM with vaccination may further increase economic attractiveness. All other strategies provided degrees of control but only slowed the rate at which prevalence increased. Uncertainty inherent in many parameters indicates focus should be on trends observed, not specific values.