Naturally-occurring Tyzzer's disease (Bacillus piliformis infection) in horse foals.

Spontaneous Tyzzer's disease is described in quarter horse foals which died suddenly with no clinical history of apparent illness. Significant gross findings included icterus, focal paletan areas in the liver and catarrhal entercolitis. Focal dark red lesions were present in the small intestine of one foal, and the mesenteric lymph nodes of another were enlarged and hyperemic. Histopathologically, the liver showed multiple discrete and confluent foci of necrosis, fatty change, sinusoid congestion and haemorrhage. Bundles of intracytoplasmic bacilli were demonstrated in hepatocytes at the margin of liver lesions using special stains. Infiltration of the portal triads with inflammatory cells and biliary duct hyperplasia and degeneration were also observed. Lesions in the intestine were characterised by mucosal necrosis with inflammatory cell infiltration, increased mucus production, submucosal lymphoid hyperplasia and, in one foal, submucosal haemorrhage. Necrosis of lymph follicles, congestion and haemorrhage were also present in the spleen and mesenteric lymph nodes.