A space-time cluster investigation of an outbreak of acute respiratory disease in Norwegian cattle herds.

Outbreaks of acute respiratory disease occurred in several cattle herds in the south-east regions of Norway during the winter and spring of 1995. The present study was conducted to investigate the temporal and spatial dynamics of the occurrence of acute respiratory disease between January and April 1995 in the affected cattle herds, in two neighbouring counties in Norway. This was done to assess the possible role of an infectious agent in these outbreaks, as well as to investigate the possibility of transmission between neighbouring herds. During the study period, a total of 190 (19.8%) outbreaks occurred in 955 cattle herds located in the defined study area. Descriptive maps of the spatial and spatio-temporal patterns of the outbreak were generated, and two different statistical tests of space-time interaction (the Knox test and k-nearest neighbour test) and a cluster-detection test (the space-time scan statistic) were used. The results of all the space-time interaction tests strongly suggested that the case herds were clustered in time as well as in space. In addition, the space-time scan statistic defined the most-likely cluster to be located within the second veterinary district (Toten where the disease occurred) during 9-15 weeks in 1995. The relative risk of a herd being infected inside the most-likely cluster compared with the surrounding study area was estimated to be 3.59 (p=0.001). The transmission rate between neighbouring farms therefore appeared to have been higher within this cluster than in the surrounding study area. This study supports the hypothesis that one common source of infection was involved in the outbreaks during the study period.