Temporal clusters of bovine Salmonella cases at a veterinary medical teaching hospital, 1996-2007.

The objectives of this study were to identify and characterize temporal clusters of bovine Salmonella cases at a veterinary medical teaching hospital and to determine which clusters were likely to have involved nosocomial transmission. Data on fecal Salmonella shedding status, serotype, and antimicrobial resistance were collected retrospectively for all cattle admitted to the Cornell University Equine and Farm Animal Hospital between January 1, 1996, and June 1, 2007. Pulsed-field gel electrophoresis (PFGE) was performed on all available isolates. Cluster analysis was used to identify temporal clusters of cases. A total of 5398 cattle were admitted during the study period; the prevalence of fecal Salmonella shedding among clinical suspects was 6.5%, whereas that among nonsuspects tested through routine surveillance was 2.5%. Eight temporal clusters (including 57 cattle) were investigated as possible outbreaks involving nosocomial transmission, ranging in size from 4 to 10 cases. All but one cluster were centered over the month of August or September. A total of 15 Salmonella serotypes were represented, with the most common being Typhimurium (33%), Newport (23%), and Agona (12%). Among the isolates available for PFGE analysis, there were 19 PFGE types represented. The majority of temporal clusters during the study period were not nosocomial in origin. However, two of the clusters were outbreaks directly resulting from nosocomial Salmonella transmission, based on case histories, serotype data, antimicrobial resistance patterns, and PFGE analysis. The clear seasonal pattern exhibited by these clusters underscores the need for heightened Salmonella vigilance during the late summer and early fall. The combination of statistical methods, routine bacteriologic data, and PFGE analysis is an effective means of conducting surveillance and outbreak investigations in a hospital setting.