Ovine Johne's disease

Abstract

Ovine Johne's disease (OJD) is commonly considered to be an ‘iceberg disease’ whereby the majority of infected individuals and the majority of the production losses associated with infection are subclinical, and only a small proportion of the infected animals develop overt, easily identifiable clinical signs. This small number of clinical cases represent the ‘tip’ of a much larger OJD subclinical iceberg, the economic impact of which is chronically underestimated. Several other diseases have also been described in this way, such as maedi visna, caseous lymphadenitis, ovine pulmonary adenomatosis and border disease. However, OJD is the most important of these diseases because of the high prevalence of flocks infected and economic impact of production losses that result from sub-clinical infections. This article reviews disease transmission, diagnosis, control and prevention as well as recent work providing estimates of the prevalence of OJD within the national flock.