Relationship between the pathology of bovine intestinal tissue and current diagnostic tests for Johne's disease

Johne's disease is an enteric disease caused by the intracellular pathogen Mycobacterium avium subsp. paratuberculosis (MAP). Upon translocation from the lumen of the small intestine, mycobacteria have the ability to thwart innate defense mechanisms and persist within the macrophage in the lamina propria. In an effort to understand how the pathology of disease is reflected in current diagnostic tests, immunofluorescent (IFA) labeling was performed to quantitate macrophage and MAP numbers in the ileum of infected cattle and correlate results with common methods for diagnosis of MAP infection; including ELISA, IFN-γ assay, RT-PCR, culture of MAP, and histological classification of tissue sections. Predictive models for clinical and subclinical disease states, histopathology acid-fast (AF), MAP location, granulomatous inflammation and type classifications, as well as macrophage, MAP and macrophages with intracellular MAP IFA labeling were successfully developed. The combination of macrophage number and ELISA were the best predictors of clinical disease state, while macrophage number was the best and only significant predictor of subclinical disease state. Fecal culture and number of MAP were the best predictors of granulomatous inflammation, and of combined AF, MAP location and granuloma type, respectively. Additionally, fecal culture and tissue culture were the best predictors of numbers of macrophages and MAP, respectively, while both ELISA and tissue culture were the best predictors of number of macrophages with intracellular MAP.