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

Objective - To determine whether horses with Corynebacterium pseudotuberculosis infections that were examined at a veterinary medical teaching hospital between July 1, 1992, and June 30, 1994 had patterns of temporal or spatial clustering. Animals - 134 case and 800 control horses randomly selected from all non-case horses admitted during the study period. Procedures - Admission date and geographic location were determined. Scan, Cuzick & Edwards', and Knox tests were applied to determine whether case horses had patterns of temporal or spatial clustering. Results - 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. Conclusions - Significant spatial and temporal clustering of horses with C pseudotuberculosis infection was detected. Clinical Relevance - Analysis of the results strongly indicates that this disease is directly or indirectly (ie, short distance and time) transmitted. In addition, data analyses 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 via insects, other vectors, or contaminated soil.