Naturally-occurring Tyzzer's disease (Bacillus piliformis infection) in commercial rabbits: a clinical and pathological study.

Tyzzer's disease has been detected in nine unrelated, commercial rabbitries. During the acute stage of the disease, recently weaned rabbits showed profuse watery diarrhoea. Mortality was between 14.2 and 41.2% during the first three weeks of the outbreaks. In surviving animals, there was a chronic evolution with depression, anorexia, loss of weight and sometimes extreme cachexia. Reproduction animals were less badly affected. Multifocal hepatic necrosis, focal myocardial necrosis, patches of mucosal necrosis in ileum, caecum and colon and marked caecal oedema were most prominent at autopsy. In histological sections of the liver, bundles of slightly Gram-negative and Giemsa-, PAS- and silver-positive rod-shaped bacilli were established in apparently viable hepatocytes bordering foci of necrosis. They were also present in myocytes around necrotic foci in the heart and in enterocytes and smooth muscle cells of the muscularis mucosae of the intestinal mucosa. Transmission electron microscopy showed that these organisms had a similar ultrastructure as Bacillus piliformis. Most antibiotics used failed to combat the disease. Only oxytetracycline was active.