Evaluation of temporal and spatial clustering of horses with Corynebacterium pseudotuberculosis infection

Patterns of temporal or spatial clustering of C. pseudotuberculosis infections were investigated in 134 infected horses compared with 800 control horses, randomly selected from all non-case horses examined at a veterinary medical teaching hospital, between July 1, 1992, and June 30, 1994. Admission date and geographical location were recorded and results were examined by Scan, Cuzick & Edwards', and Knox tests. For all windows ≥3 days (134 case horses) and ≥7 days (subset of 69 case horses), results of the Scan test were significant. Results of Cuzick & Edwards' test were significant for all data sets. A significant spatial cluster of case horses was observed for October, November, and December 1992. Results of the Knox test were significant for temporal intervals between 7 and 56 days and spatial intervals between 4.3 and 6.5 km. Higher Knox(x) proportions were observed for temporal intervals of 0 to 7, 8 to 14, 22 to 28, and 29 to 35 days. Significant spatial and temporal clustering of horses with C. pseudotuberculosis infection was detected. Analysis of the results strongly indicates that this disease is directly or indirectly (short distance and time) transmitted. Data analyses also indicated an incubation period of 3 to 4 weeks. The disease could be transmitted through horse-to-horse contact or from infected to susceptible horses by insects, other vectors, or contaminated soil.