Clostridium piliforme (formerly known as Bacillus piliformis) is the causative agent of Tyzzer's disease. In mice,16 hamsters,17-19 gerbils,20,21 and rabbits,22 this is a fatal disease characterized by multiple focal necrosis of the liver (Figure 11.1). Long slender bacteria are found in the cytoplasm of the hepatocytes at the periphery of the necrotic foci. These bacteria are also found in huge numbers in the alimentary tract, especially in the ileum and caecum, and especially in association with ileitis, caecitis, and colitis. It has long been known that different mutants infect different animal species23,24 and that infection between species, therefore, is a rare event, with the exception that Mongolian gerbils under some circumstances may be sensitive to infection from other species, such as rats, mice, and rabbits.25 In rats, it is a mild disease of weanlings connected with megaloileitis (Figure 11.2), multiple focal necrosis of the livers, and single necroses in the myocardium.26 Resistance to development of Tyzzer's disease may be due to genetic traits.27,28 The organisms probably persist in the intestinal epithelium of healthy animals. The prevalences of infected individuals in rat and mouse colonies vary, but it is often more than 50%.29 The agent may cross the placenta.30,31
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