Spatial Distribution of Free-of-Charge Pathology Submissions to the California Animal Health and Food Safety Laboratories During the Exotic Newcastle Outbreak in 2002-2003

After the 1971-1973 outbreak of exotic Newcastle disease (END) in California, a free-of-charge diagnostic submission program was created for backyard poultry flocks. This program was implemented to improve disease surveillance in small poultry flocks. The aim of this study was to evaluate the spatial distribution of free-of-charge pathology submissions to the California Animal Health and Food Safety laboratories during the END outbreak in 2002-2003. Cases and controls were selected from within a 100-mile (161-km) radius of each of three laboratories, and their geographic distributions were evaluated. Global clustering of cases was significant around all three laboratories, with mixed results at the local clustering level and the only significant clustering at the focal level around the Davis laboratory with an observed to expected ratio of approximately 5. The area of influence for all three laboratories was about 20 miles (32 km). The significant clustering of cases around the laboratories indicates that more public information about the free-of-charge program could result in coverage of a larger portion of the population; however, the value of the information resulting from increased sampling should be considered relative to the additional cost of obtaining it.