Clinical expression appears to be principally associated with light infertile soils and their associated pastures and pasture improvement practices which may lead to micronutrient deficiencies or imbalances. Genetic effects are also important and are likely to be exacerbated by physiological stressors, such as winter shearing.

Both breeding of rabbits and associated high flea numbers may act together or independently in promoting outbreaks of RHD. Stresses involved with rabbit reproduction and low environmental temperatures also appear to influence the likelihood of outbreaks. The effects of proximity to cattle and water suggests that both flies and mosquitoes may have a minor role in local transmission.

Published information on aspects of the epidemiology of paratuberculosis was reviewed to examine the hypothesis that clinical expression of disease may be induced by deficiencies or imbalances of micronutrients. The progression of paratuberculosis and the development of clinical signs, although innately linked with the process of becoming infected with Mycobacterium avium subsp paratuberculosis, are mostly controlled by factors other than those predisposing to the acquisition of the organisms. There is strong evidence in the literature reviewed to suggest that soil acidification, excesses of iron and molybdenum, and deficiencies in copper and selenium and possibly other minerals and trace elements have a role in the disease process. The fact that no single nutritional supplement has been consistently found to produce clinical recovery suggests that the causal role of these factors is complex and may vary depending upon the geochemical environment. Altering the nutritional environment by appropriate trace element supplementation and liming, as well as decreasing the reliance on shallow-rooted pastures needing frequent fertiliser applications may assist in controlling the clinical expression of paratuberculosis. Such measures could produce a significant decrease in the incidence of clinical expression across flocks and herds.