Risk factors identified as source of new infections (area spread) with pseudorabies (Aujeszky's disease) virus in 50 swine herds in a contiguous geographical area of Ohio

A retrospective cohort study was done using 50 swine herds in western Ohio for a 15-month period from December 1991 through February 1993. The objective of the study was to determine risk factors for 'area spread' of newly infected herds in an area with pseudorabies virus (Aujeszky's disease) (PRV) - especially in relation to factors associated with biosecurity. Herds were operated as either farrow-to-finish, feeder-pig producer or feedlots. The study area was a high-density swine area endemic for PRV since the late 1970s. Twelve of 14 (86%) quarantined herds delivered finishing hogs to the same swine market during the study period. Herds were stratified into two groups with the division between groups at the mean number of shipments of swine delivered to markets per year (mean, 34 per year). The odds of the herds delivering swine to one specific market in the study area were 21 times higher for quarantined than for non-quarantined herds (P < 0.05), four times higher for those herds selling swine ≥ 34 times to market per year compared with those selling swine to market once per year (P < 0.05), and 14 times higher if grower/finishers were not vaccinated in the quarantined herds compared with being non-vaccinated in the non-quarantined herds. These stepwise logistic regression results support the concept of 'area spread' - which is defined as the source of new PRV infections being other than animal contact. Area spread here was associated with which swine market was used and the frequency of swine delivery to markets per year.