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

\n The objectives of this research were \nto determine if it is possible to\nproduce PRRS negative pigs from multiple PRRS sero-positive herds over time using\nsegregated early weaning (SEW) and to evaluate serological stability of participating\nbreeding herds and the impact of sow vaccination on sow herd stability. The following\ndefinitions were established. A vaccinated herd was considered serologically stable if \n$\\geq 90.0\\%$ of the sow samples had PRRS ELISA S/P ratios \n$< 2.0$. A non-vaccinated herd was \nconsidered serologically stable if $\\geq 90.0\\%$\n of the sow samples had PRRS ELISA S/P ratios $< 1.0$\nand $< 10.0\\%$\n of sow samples had PRRS ELISA S/P ratios $> 2.0$ (modified from Dee. \nProc AASP, 1998; 409-411). Serological stability of ten PRRS sero-positive herds was assessed\nby testing serum samples from 30 randomly selected sows per herd with an ELISA \n(IDEXX). Piglets were weaned between 8 and 14 days of age and isolated in an off-site nursery\nfor 90 days. The procedure was repeated three times over a period of 14 months. \nPigs originating from non-vaccinated herds were housed separately from pigs obtained from\nvaccinated herds. Serial blood tests of pigs were conducted at weaning and 30, \n60 and 90 days of age. Serology was confirmed and clarified using PRRS RT-PCR and RFLP\nassays. Littermates remaining on the farm of origin were also tested at weaning\nand at 60 days of age. All closed vaccinated herds had \n$\\geq 90.0\\%$ of sow samples with PRRS ELISA\nS/P ratios $< 2.0$. All piglets from these herds became negative after 30-60 days\nand remained negative up to 90 days of age. The same results were obtained in the repeated\ntrials. Littermates remaining on the farm of origin had an active PRRS virus \n(PRRSV) infection as demonstrated by rising S/P ratios in trial 1, but not in the repeated\ntrials. Two closed non-vaccinated herds had $\\leq 90.0\\%$ of sow samples with PRRS \nELISA S/P ratios $< 1.0$. One of these had $\\geq 10.0\\%$ \nof sow samples with PRRS ELISA S/P ratios $>2.0$. \nThe number of sero-positive piglets and their S/P ratio values increased \nover time. In two non-vaccinated herds, piglets were PRRS virus positive at weaning. One\nfield and one intermediate strain of PRRS virus were identified. PRRS negative \npigs were produced from two stable, non-vaccinated herds. One open non-vaccinated herd had\n56.7% and 10.0% of sow samples with PRRS ELISA S/P $< 1.0$\n and PRRS ELISA S/P $> 2.0$, \nrespectively. PRRSV positive \npiglets were detected at weaning in this herd. PRRS negative\npigs were produced after dams were vaccinated. The production of PRRS virus negative\npigs from multiple PRRS virus positive herds over time using SEW technology requires a\nstable\nbreeding herd. Assessment of serological stability by the suggested criteria might be\nvery useful, but should not be overestimated. The consistent stability of vaccinated herds\nobserved in this study suggests that vaccinated herds are more likely to be stable \nthan non-vaccinated herds. Results need to be validated in large commercial operations.\n

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