Necropsy
Paratuberculosis cannot be diagnosed on superficial examination of the intestines for signs of thickening.
The intestines should be opened from the duodenum to the rectum to expose the mucosa. There is not
always a close correlation between the severity of clinical signs and the extent of intestinal lesions. The
mucosa, especially of the terminal ileum, is inspected for pathognomonic thickening and corrugation. Early
lesions are seen by holding the intestine up to the light, when discrete plaques can be visualised. Mucosal
hyperaemia, erosions and petechiation have been observed in deer with paratubeculosis. The earliest
lesions are thickening and cording of lymphatic. The mesenteric lymph nodes are usually enlarged and
oedematous. Smears from the affected mucosa and cut surfaces of lymph nodes should be stained by
Ziehl–Neelsen’s method and examined microscopically for acid-fast organisms that have the morphological
characteristics of M. paratuberculosis. However, acid-fast organisms are not present in all cases. Diagnosis
is therefore best confirmed by the collection of multiple intestinal wall and mesenteric lymph node samples
into fixative (10% formol saline) for subsequent histology. Both haematoxylin-and-eosin-stained sections and
Ziehl–Neelsen-stained sections should be examined. The typical lesions of paratuberculosis consist of
infiltration of the intestinal mucosa, submucosa, Peyer’s patches and the cortex of the mesenteric lymph
nodes with large macrophages, also known as epithelioid cells, and multinucleate giant cells, in both of which
clumps or singly disposed acid-fast bacilli are usually, but not invariably, found.