Johne's disease caused by Mycobacterium avium subsp paratuberculosis infection in red deer (Cervus elaphus): An histopathological grading system, and comparison of paucibacillary and multibacillary disease

AIM: To describe a grading system for evaluating lesions in the small intestine and mesenteric lymph nodes of red deer (Cervus elaphus) infected with Mycobacterium avium subsp. paratuberculosis (MAP), and report the distribution of granulomas and findings seen in paucibacillary and multibacillary forms of the disease. METHODS: Tissues were examined from red deer either experimentally (n=300) or naturally (n=131) infected with MAP. A disease severity score developed previously was expanded to help provide more sensitivity in assessing severity of disease. The distribution of granulomatous, paucibacillary and multibacillary lesions in sections of jejunum, ileocaecal valve and mesenteric lymph nodes was compared between sites and between animals with mild (severity score <= 7) and severe (severity score >= 8) forms of the disease. RESULTS: Based on the results of three published studies, the severity score related well with the clinical severity and gross lesions associated with the disease. Paucibacillary lesions tended to have smaller macrophages and increased numbers of Langhan's giant cells than multibacillary forms, but this was not a consistent finding. The multibacillary form of the disease had Langhan's giant cells, containing numerous acid-fast organisms (AFO), and in one form sheets of epithelioid-like macrophages with prominent vacuolated cytoplasm and few Langhan's giant cells. In deer experimentally infected with MAP, granulomatous lesions were more evident in mesenteric lymph nodes than in intestinal tissue, especially in the mild form of the disease. In mild cases, granulomas were significantly more evident in Peyer's patches than in the intestinal mucosa, but in severe cases, the difference was not significant. Paucibacillary forms of the disease were more evident in deer with the mild disease, and multibacillary forms were more evident in deer with the severe disease. CONCLUSIONS: The severity score provides an objective measure of the severity of Johne's disease, and is useful for comparing individuals and groups of deer in studies involving experimental or natural infection with MAP. In mild disease, lesions were more evident in mesenteric lymph nodes than in jejunum and ileocaecal tissue, and Langhan's giant cells were present in both paucibacillary and multibacillary forms of the disease. The posterior jejunum and ileocaecal-valve lymph nodes were the best sites for detecting mild lesions, while intestinal samples from the posterior jejunum and ileocaecal valve had a lesser but useful role.