Surveillance of Mycobacterium avium subsp. paratuberculosis in dairy herds

Abstract

In this thesis, the potential for improvements in surveillance of Mycobacterium avium subsp. paratuberculosis (Map) infection and paratuberculosis in dairy herds was investigated, leading to a reduction in surveillance costs whilst continuing to meet specific quality targets. In particular, differentiation of surveillance strategies to accommodate the aims and needs of various groups of dairy farmers was studied. A stochastic simulation model (JohneSSim) was used to identify cost-effective test schemes for a surveillance programme for certified ‘Map-free’ dairy herds and a bulk milk quality assurance programme for paratuberculosis (BMQAP) in dairy herds. In addition, field data were analysed to obtain information useful to the improvement of surveillance systems for Map. This included analyses of cattle transfers between certified ‘Map-free’ herds, a comparison of a serum-ELISA and Ziehl-Neelsen stained smears (ZN-test) as tools in the surveillance of clinical paratuberculosis, and an analysis of the age at which cattle become infectious. Based on the results of the simulations as well as the analyses of field data, recommendations were made on the design of the Dutch surveillance programme for ‘Map-free’ herds and a BMQAP. The analyses of field data showed that the contact structure of cattle transfers between ‘Map-free’ herds was not random but underdispersed, which adds substantially to the guarantees provided by the ‘Map-free’ status. Furthermore, it was shown that the serum-ELISA is preferred to the ZN test in the surveillance of clinical paratuberculosis. Finally, it was shown that a substantial proportion of cattle in infected herds become infectious before two years of age. The thesis is concluded by a discussion on the application of the study results, effects of surveillance of Map and paratuberculosis on the interests of consumers, dairy processing industry and dairy farmers, and recommendations on future developments and research regarding the surveillance of Map in dairy herds