ABSTRACT. Soft feces and a decreased delivery rate were observed in a specific-pathogen-free (SPF) C3H-scid mouse breeding colony. Grossly, the ceca were shrunken and edematous in the affected mice. Histopathologically, severe edema in the cecal submucosa as well as infiltration of inflammatory cells in the lamina propria and submucosa of the ceca and colon were observed. No pathogenic microorganisms were detected by the routine microbiological tests. By anaerobic bacterial-examination, Clostridium (C.) difficile with toxin A was isolated from the cecal contents of the affected mice. The mice were diagnosed with C. difficile-associated colitis. This case appears to be the first report of natural infection with C. difficile in SPF mice with clinical signs. KEY WORDS: C3H-scid mouse, Clostridium difficile, colitis.J. Vet. Med. Sci. 69(9): 973-975, 2007 Clostridium difficile is known to be an agent that causes antibiotic-associated diarrhea and pseudomembranous colitis in humans [2,5]. In laboratory animals, antibiotic-associated colitis has been reported in hamsters [1,11,16], guinea pigs [9,10] and rabbits [7]. At post-administration of antibiotics such as clindamycin or penicillin, fatal enterocolitis with diarrhea, cecal dilation and hemorrhages in hamsters [11], lethal hemorrhagic cecitis with cecal dilation in guinea pigs [10] and fatal colitis with or without cecal dilation and diarrhea in hamsters [1,16] , rabbits [7] and guinea pigs [9] have been observed. It has been reported that C. difficile could be isolated from germ-free rats showing diarrhea, reduction of cecal size, extensive submucosal edema and slight mononuclear cell infiltration in the cecal mucosa [4]. In contrast, conventional mice with a high isolation rate of C. difficile from their feces did not show pathological lesions, even after administration of antibiotics [6]. In experimental infection, germ-free mice developed diarrhea, severe decreased cecal size, and acute colitis [15]. In our SPF facility, soft feces and decreased delivery rate were clinically observed in a C3SnSmn.CB17-Prkdc scid /J (C3H-scid) mouse breeding colony without any experimental treatment including administration of antibiotics. In this paper we describe the first natural case of colitis associated with the presence of C. difficile in SPF mice with clinical signs and suggest that C. difficile may be an etiological agent of colitis in SPF-immunodeficient mice.C3H-scid mice were maintained as a breeding colony in our SPF facility. They were housed in sterilized aluminum cages with wood chip bedding at 24-26°C under a 12-hr light-dark cycle. Commercial food pellets (MBR-1, Funabashi Farm Co., Chiba, Japan) sterilized with gamma-rays, and chlorinated (8-13 ppm) and acidified (pH 2.5-3.0) drinking water were provided ad libitum. Routine bacteriological and serological examinations of retired mice, including C3H-scid mice, in this SPF facility were negative for Salmonella spp., Citrobacter rodentium, Pasteurella pneumotropica, Pseudomonas aeruginosa, Corynebacterium kut...