SPATIAL AND TEMPORAL EPIDEMIOLOGY OF PSEUDORABIES VIRUS-INFECTION

Objective-To examine the pattern of pseudorabies virus (PRV) infection in Pennsylvania and identify the area factors associated with herd qu arantine status.Sample Population-123 PRV-quarantined commercial swine herds identified between 1986 and 1993 were selected as cases, and 16 2 uninfected herds were selected as controls. Procedure-Herd location, demographics, and temporal vaccination and quarantine data for a case -control study were obtained from producer questionnaires and state re cords, using a database of swine herds from 2 Pennsylvania counties. A ny herd that was on quarantine as of Jan 1, 1991, or quarantined subse quent to this date, was defined as a case. A herd was defined as a con trol if it had never been quarantined for PRV. Controls were group mat ched to cases by year. Study herds were centered in a circle, or buffe r zone, with a 1.61-km (1-mile), 3.22-km (2-mile), or 6.44-km (4-mile) radius, and densities of operation types, quarantined herds, nonquara ntined herds, and vaccinated herds in the buffer zone were compared. T he analytical outcome was the probability of a herd being quarantined, conditional on the buffer zone density of herds quarantined, herds no t quarantined, and herds in which a PRV vaccine Was used, These densit y variables were categorized into high, medium, and low, or just high and low categories. Confounding by year was assessed in the analysis. Analysis was performed, using unconditional logistic regression. Resul ts-Decreased density of PRV-quarantined herds in the study region was associated with reduced risk of a herd becoming quarantined, whereas i ncreased density of nonquarantined, presumably uninfected herds was as sociated with decreased probability of a herd becoming quarantined. De creased density of vaccinated herds was associated with increased prob ability of a herd becoming quarantined. In addition, being a farrow-to -finish study herd was associated with increased probability of becomi ng quarantined, compared with being a feeder pig producer study herd. Conclusions-Associations with quarantine status and area densities of vaccinated, nonquarantined, and quarantined herds indicate the importa nce of area spread in PRV control. These effects are seen most strongl y at a 3.22-km (2-mile) radius, but also are seen at a 6.44-km (4-mile ) radius.