In cervids, some cases of M. paratuberculosis infection have
been reported to be indistinguishable from M. avium
infection, with lymph node caseation and necrosis a not
uncommon finding (87). The pathological findings in the
recently reported cases in foxes and weasels were consistent
with early phase infections. The lesions were mild and
restricted to the mesenteric lymph nodes. Small numbers of
intracellular acid-fast bacilli were present in single
‘macrophage-like cells’ and discrete granulomata had formed
in the cortex and paracortex of the nodes in six of nine foxes
and in one stoat (9).
In summary, microscopic lesions such as histiocytic or
granulomatous inflammation including acid-fast bacilli in any
tissue of the gastrointestinal tract or mesenteric lymph nodes
are compatible with Johne's disease. Multinucleated giant cells
or epithelioid (Langhans' type) macrophages (without the
intracytoplasmic crystalline material, pigment or debris that
indicates the cell is simply draining inert material from the
tissue) in the lamina propria at any level of the gastrointestinal tract should raise suspicions of M. paratuberculosis infection,
even in the absence of acid-fast bacilli.