Control options for Johne’s disease (JD) are explored by computer simulation because intervention trials under field conditions would be too costly. The purpose of this review is to describe a simulation model developed in The Netherlands and adapted to dairy herds in Pennsylvania (USA). The paper consequently derives what information is required for the adaptation to New Zealand dairy herds and explains the methods by which that information is currently collected. The model ‘JohneSSim’ simulates the within herd development of JD based on time related changes. Time intervals are six months. JD is transmitted vertically through the placenta or from dam to calf around calving through direct contact, colostrum, rest milk or bulk milk, infected pasture, or through contact with adult replacement stock from other herds. Control options include test-and-cull, measures to prevent the infection of calves, vaccination, and grouping of animals by age. The simulation suggested that, in The Netherlands as well as in Pennsylvania, progressively improving calf-hygiene with little reliance on test-and-culling would decrease the prevalence considerably and was economically feasible (average benefit-costs-ratio excluding extra labor = 1.58). Test-and-cull was not economical even if a test with 80% sensitivity was available. Contract heifer rearing, colostrum management, and seasonal calving appear to be risk factors for transmission that have specific relevance for New Zealand dairy herds.
Proceedings of the New Zealand Society of Animal Production, Volume 62, Palmerston North, 294-298, 2002
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