Objective: To review and interpret aspects of the pathogenesis and epidemiology of paratuberculosis (Johne's disease) for veterinarians involved in current Johne's disease control programs.
Procedure: An electronic and manual search was undertaken to identify published information which, together with limited unpublished data, was interpreted and summarised.
Conclusions: Paratuberculosis, a chronic enteropathy of ruminants, is caused by Mycobacterium avium subsp paratuberculosis and is transmitted mainly in faeces to young animals by infected adults, some of which may not have clinical signs. The incubation period is inversely related to the size of the challenge dose but can be extremely prolonged. Clinical cases may not be seen within the economic lifespan of farm animals, particularly when stocking rates are low, pasture is spelled, or when animals are culled at a relatively young age. Other as yet unknown influences may determine the rate of progression or recovery from infection. Paratuberculosis appears in a range of forms from a disease with high prevalence and significant mortality through to one with very low prevalence and little obvious morbidity or mortality. Detection of infected flocks and herds relies on use of laboratory tests. Bacteriological culture of faeces is the most sensitive herd-level test. The passage of time and repeated testing are the greatest allies in detecting paratuberculosis because infected animals progress in the disease process and most tests are more effective in the later stages of the disease. These factors generally cause the prevalence of paratuberculosis to be underestimated at both herd or flock and regional level. Greater understanding of the epidemiology and pathogenesis of M a paratuberculosis infection is critical in order to design improved diagnostic strategies, assess the feasibility of eradication and develop control options, particularly in small ruminants.