Spatial and temporal epidemiology of infectious laryngotracheitis in central California: 2000-2012.

In October of 2005 an outbreak of a vaccine-like strain of infectious laryngotracheitis (ILT), indistinguishable from the chicken embryo origin (CEO)-like vaccine strains, was detected by routine passive surveillance in the Central Valley of California, U. S. A. In response, a highly coordinated industry effort by two companies led to a significant decrease in the incidence of ILT over the same geographic region between 2008-2012. In order to understand the geographic and temporal spread of ILT in California before and after the outbreak, Global Information Systems (GIS) mapping coupled with spatial, temporal, and spatial- temporal statistics were used to identify retrospective and prospective low-rate clustering (i.e., less ILT than statistically expected) and high-rate clustering (i.e., more ILT than statistically expected) of ILT spatially and temporally. Results showed two high-rate retrospective spatial-temporal clusters and one low-rate prospective spatial-temporal cluster which were all statistically significant (P < 0.05). Overall, spatial-temporal clustering accounted for 36.9% of the positive ILT cases, while temporal clustering and spatial clustering done separately each accounted for 0% of the ILT cases, respectively. This demonstrates the utility of combining spatial and temporal clustering for ILT surveillance. Due to the risk of reversion to virulence and spread to immunologically naive broilers, future application of the CEO-based vaccine in the identified high rate spatial-temporal clusters should be avoided and other vaccine alternatives considered in order to avoid repeat outbreaks in those areas. This should especially be followed during the winter months of December, January, and February, which were found to have the highest prevalence of ILT (P < 0.05). Analysis of GIS data within the high-rate clusters showed that wind direction and farm density were minor factors in the spread of ILT. Shared roads may have played a role in the spread of ILT in one of the two high rate spatial-temporal clusters.