Acid-fast stained tissue

This is a high power magnification photomicrograph of a Ziehl-Neelsen acid-fast stained histopathology section of the bovine ileum. Use of special stains allows the mycobacteria (red rod-shaped bacteria) to be seen. M. paratuberculosis grows as clumps inside macrophages. Although macrophages are specialized cells that normally kill bacteria, M. paratuberculosis has evolved to not only survive, but actually thrive inside macrophages. Seeing acid-fast bacteria together with the typical host cell reaction, diffuse granulomatous inflammation, is diagnostic for paratuberculosis. However, isolation of M. paratuberculosis from such tissues in culture is important for a definitive diagnosis. In many cases of Johne's disease, these bacteria may not be seen. Examples include the "paucibacillary" form of the disease in sheep or early stages of the infection in any species.