The production of PRRS negative pigs from multiple PRRS serologically stable herds over time using segregated early weaning (SEW)

Abstract - The objectives of this research were to determine if it is possible to produce PRRS negative pigs from multiple PRRS sero-positive herds over time using segregated early weaning (SEW) and to evaluate serological stability of participating breeding herds and the impact of sow vaccination on sow herd stability. The following definitions were established. A vaccinated herd was considered serologically stable if of the sow samples had PRRS ELISA S/P ratios < 2.0. A non-vaccinated herd was considered serologically stable if of the sow samples had PRRS ELISA S/P ratios < 1.0 and of sow samples had PRRS ELISA S/P ratios > 2.0 (modified from Dee. Proc AASP, 1998; 409-411). Serological stability of ten PRRS sero-positive herds was assessed by testing serum samples from 30 randomly selected sows per herd with an ELISA (IDEXX). Piglets were weaned between 8 and 14 days of age and isolated in an off-site nursery for 90 days. The procedure was repeated three times over a period of 14 months. Pigs originating from non-vaccinated herds were housed separately from pigs obtained from vaccinated herds. Serial blood tests of pigs were conducted at weaning and 30, 60 and 90 days of age. Serology was confirmed and clarified using PRRS RT-PCR and RFLP assays. Littermates remaining on the farm of origin were also tested at weaning and at 60 days of age. All closed vaccinated herds had of sow samples with PRRS ELISA S/P ratios < 2.0. All piglets from these herds became negative after 30-60 days and remained negative up to 90 days of age. The same results were obtained in the repeated trials. Littermates remaining on the farm of origin had an active PRRS virus (PRRSV) infection as demonstrated by rising S/P ratios in trial 1, but not in the repeated trials. Two closed non-vaccinated herds had of sow samples with PRRS ELISA S/P ratios < 1.0. One of these had of sow samples with PRRS ELISA S/P ratios >2.0. The number of sero-positive piglets and their S/P ratio values increased over time. In two non-vaccinated herds, piglets were PRRS virus positive at weaning. One field and one intermediate strain of PRRS virus were identified. PRRS negative pigs were produced from two stable, non-vaccinated herds. One open non-vaccinated herd had 56.7% and 10.0% of sow samples with PRRS ELISA S/P < 1.0 and PRRS ELISA S/P > 2.0, respectively. PRRSV positive piglets were detected at weaning in this herd. PRRS negative pigs were produced after dams were vaccinated. The production of PRRS virus negative pigs from multiple PRRS virus positive herds over time using SEW technology requires a stable breeding herd. Assessment of serological stability by the suggested criteria might be very useful, but should not be overestimated. The consistent stability of vaccinated herds observed in this study suggests that vaccinated herds are more likely to be stable than non-vaccinated herds. Results need to be validated in large commercial operations.