This is a
high power magnification photomicrograph of a Ziehl-Neelsen acid-fast
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.