Factors affecting the geographic distribution of pseudorabies (Aujeszky's disease) virus infection among swine herds in Illinois

Data on the geographic distribution of swine herds tested for pseudorabies virus (PRV) in the state of Illinois (USA) were analyzed to determine whether the prevalence of PRV-infected herds was clustered geographically at the county level. Second-order analysis of spatial dependence indicated there was a spatial clustering of counties of high PRV prevalence rates and that this clustering was greater than the observed clustering of counties with a large number of swine herds. The clustering of county PRV prevalence rates was most apparent within a radius of 120 km (on the average, approximately two couties away). The association of county PRV prevalence rates with average herd size, geographic density of swine herds in the country and regional (within 120 km) density of PRV-infected herds was analyzed using multiple linear regression. The primary factor affecting county PRV prevalence rates was the regional PRV density, which interacted with the other model variables. For counties with a low regional density of PRV infection, county PRV prevalence rates charged little with a change in county herd density or average herd size. In contrast, for counties with a high regional density of PRV infection, PRV prevalence within a county increased with increasing average herd size and increasing geographic density of swine herds in the county. The results of the current and previous studies implicate an important role for the geographic proximity of infected herds in the spread of PRV among swine herds.