Temporal and spatial analysis of the 1999 outbreak of acute clinical infectious bursal disease in broiler flocks in Denmark

The objective of this study was to describe the spatial and temporal dynamics of the 1999 outbreak of acute clinical infectious bursal disease (IBD) in broiler chicken farms in Denmark. The analysis was performed using data from all broiler farms located in the Jutland peninsula and the island of Funen (168 municipalities). The Moran's index I, K-functions and scan statistics were used to describe the dynamics of the epidemic. In addition, spatially correlated survival analyses were performed and the posterior frailties were mapped to identify areas with high or low survival times. From January to October 1999, a total of 43 farms (81 flocks) out of a total of 299 farms (2970 flocks) in 110 municipalities were infected with IBD. The outbreak developed more or less simultaneously in two regions, one in the north and the other in the center of the study area. The space-time descriptive methods suggested that the cases were more likely to occur during a short period of time and over relatively short distances, indicating that local factors facilitated the spread of the virus. For instance, the highest risk, as estimated by the K-function, was within 20 km and 20 days. The risk of transmission peaked in July and remained high until the end of the study. The spatially correlated survival analysis identified municipalities with low survival times that roughly corresponded to the clusters found using scan statistics. The analysis for the full hierarchical structure of the dataset identified variation (i.e. clustering) at the farm and municipality levels.