2. Pathogenesis of Paratuberculosis

The appearance of Johne’s disease in a sheep flock is a complex interplay between events at population, whole animal, tissue and cellular levels, as reviewed ^[14].

Population level events involve exposure to MAP with infection of a proportion of subjects at risk, with only some (5–15%, although rarely up to 25% in some flocks) developing clinical disease and dying. However, clinical cases are the “tip of the iceberg” due to high rates of subclinical infection. It has been shown that the presumed pathway from silent infection with no faecal shedding of MAP, through subclinical infection with light faecal shedding, to heavy faecal shedding and clinical disease, is incorrect. Surgical biopsies of infected animals examined over 3 years confirmed that a proportion of individuals avoided infection or resisted colonization by MAP, with some infected then recovering, whilst others progressed to clinical disease ^[15]. Understanding the pathways that determine these outcomes has progressed with work in an infection model in sheep ^[16][17][18]. Host dependency (age, genotype) and environment (stress, nutrition and dose of MAP) are considered factors of relevance, with Merinos and other breeds developing clinical disease, although some breeds may show earlier clinical presentation than others. This is accompanied by changes in early immunological pathways, including lymphocyte proliferation, apoptosis and cytokine activity, that are detected in blood cells of sheep following MAP exposure and lead to eventual disease expression in paratuberculosis ^[16][17][18].

Whole animal events involve colonisation of host tissues by MAP, and this occurs before inflammatory reactions are visible. The silent period of infection lasts for variable periods until granulomatous inflammatory foci arise that may become generalized, with shedding of MAP. There is a systemic cellular immune response that can be detected by the intradermal skin test, lymphocyte proliferation assay or whole blood IFN-gamma tests, indicating exposure to MAP and offering potential early detection of exposed individuals and farms, although they are still unable to distinguish between active infection and exposure. A switch from a cell mediated (Th1-like) response to a humoral antibody (Th2-like) response associated with progression of the infection to more severe forms has been proposed for paratuberculosis, although switches from Th1 to Th2 vary between sheep and are not observed at all in some animals ^[19][20].

Tissue events arise due to an interaction between MAP antigens and the antigen-specific CD4+ and CD8+ T cells in gut lymph nodes that are required for an effective immune response. Although B cells are activated to release specific antibody, this humoral response is ineffective as MAP is intracellular, although recently, gut antibodies have been suggested to have a role in protecting against recolonisation by MAP in sheep previously exposed to infection. It is the activated T cells returning to the intestine and interacting with infected phagocytes that produces the typical granulomatous inflammatory responses. Growing CD4+ cells and gamma-delta T cells are found in the Peyer’s patches in young sheep a few weeks after exposure to MAP, with granulomas eventually occupying the lamina propria. These enteric lesions appear to progress from paucibacillary to multibacillary, although mild paucibacillary lesions can regress ^[15]. Factors determining the switch from silent to active clinical disease are uncertain (although a novel regulatory pathway related to this disease switch has been identified in experimental sheep models), as are mechanisms enabling MAP to disseminate to other tissues, including within macrophages to the intestinal lumen, and then release to initiate faecal shedding.

Cellular events in MAP infection follow entry via epithelial cells, presumably involving numerous receptors on cell membranes of the intestinal mucosa. Infection occurs within 30 mins of MAP contact with M cells and goblet cells, with mycobacteria passed rapidly to macrophages to which they attach and enter, mediated by the fibronectin receptor and components of the MAP cell wall. The host responds when pathogen-associated molecular patterns (PAMP) such as cell wall lipoproteins engage the host’s pathogen recognition receptors (PRR) such as the toll-like receptors (TLR), leading to the induction of cytokines and activation of innate immune mechanisms. TLR2/TLR6 heterodimers are involved in the recognition of mycobacterial antigens. The fact that pathogenic mycobacteria survive phagocytosis, acquire nutrients for growth within the phagolysosome, spread between macrophages, disseminate within them and finally escape the confines of the macrophage to infect additional animals is fundamental to MAP pathogenesis. Mycobacteria are passed from infected macrophages to uninfected macrophages, presumably by apoptosis, and MAP alters its gene expression within macrophages, using numerous metabolic pathways and dozens of altered proteins to withstand stresses such as hypoxia, nitrosative and oxidative conditions, and nutrient depletion compared to optimal growth conditions. Gene expression studies in paratuberculosis have identified that MAP may utilise lipids as an energy source once within the macrophage, with several gene families differentially regulated, including S100 calcium binding, lysozyme function, MHC class I and class II, T cell receptor and transcription factors ^[21]. These differentially regulated genes are considered as putative biomarkers of MAP exposure, or of the specified disease or resilience outcomes.

3. Ovine Paratuberculosis Control Strategies

Control strategies for paratuberculosis include the following:

• Test and cull programs;
• Management interventions to reduce faecal–oral transmission;
• Vaccination to limit and suppress infection ^[1].

In extensive sheep flocks in Australia, it is the latter strategy that has proven to be most successful. However, further research solutions may provide more effective JD control programs, a priority being the development of tests that can detect late subclinical infections to enable removal of future sources of infectious material from flocks/herds and the food chain, plus predict the likely outcomes of animals exposed to the organism at an early age. The recent developments in OJD control include an improved understanding of the immune and cellular profiles of sheep with varying paratuberculosis outcomes (as above), offering potential improved testing regimes, plus increasing recognition of the importance of ongoing annual lamb vaccination and improved biosecurity to reduce losses and risks of disease ^[1][4]. However, improving national paratuberculosis control programs should also be a priority to manage disease risk from trade. Strong leadership and communication from livestock health authorities is required to build trust within rural communities confused by the difficulties in managing this insidious disease.

Change management considerations provide a framework for reflection on animal health strategies, potentially enhancing our understanding of the strengths and weaknesses of national programs that are driven by livestock industry stakeholders keen to enhance the welfare of animals and quality of ruminant-derived products ^[1][4]. However, control of paratuberculosis in Australia by government regulation, de-stocking and other approaches generally failed to prevent or control the disease or build the trust required for producer compliance with these approaches ^[1][4]. The OJD situation in NSW in particular, the state with the largest sheep numbers where the disease was first established and recognized, precipitated into a rural community crisis until the introduction of Gudair™ vaccination in 2002. Vaccination was then supplemented by the introduction of the “sheep health statement”, a vendor declaration system to facilitate disease risk awareness during the trading of sheep, plus eventually a more effective program of general biosecurity risk awareness, supported by legislation.