Effect of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) on reproductive performance of gilts and sows in PRRSV-positive herbs with special reference to vaccination and management strategies

Abstract

The aims of this thesis were to determine the effect of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) on reproductive performance of gilts and sows in PRRSV-positive herds with special reference to vaccination and management strategies. In the first study, whole-herd PRRS modified-live virus (MLV) vaccination was performed in a 1,200-sow herd following a PRRSV outbreak. Reproductive performance data over a three-year period was available for analysis. Vaccination was associated with a lower farrowing rate (FR, 83.8% vs. 90.0%, P<0.001), number of total piglets born per litter (TB, 10.6 vs. 11.4 TB, P<0.001), number of piglets born alive per litter (BA, 10.0 vs. 10.3 BA, P=0.012), percentage of stillborn piglets per litter (SB, 4.6% vs. 7.0%, P<0.001), and percentage of mummified fetuses per litter (MM, 0.7% vs. 1.6%, P<0.001), and a higher return rate (RR, 11.3% vs. 5.9%, P<0.001) when compared with the period before the PRRSV outbreak. Pregnant females vaccinated during early gestation farrowed fewer BA and more MM than the comparison groups, whereas females vaccinated during late gestation had a lower FR. In this herd, PRRS MLV whole-herd vaccination had neutral, positive, and negative effects on reproductive performance. In the second study, the reproductive performance data contained of 211,009 mating and 180,935 farrowing records from 27,042 PRRS-MLV-vaccinated sows and 45,816 non-vaccinated sows from 20 PRRSV sero-positive herds were analyzed. FR, RR, and AR in non-PRRS-MLV-vaccinated and vaccinated sows were 85.0% and 89.7% (P<0.001), 6.9% and 3.7% (P<0.001), and 1.6% and 2.0% (P=0.964), respectively. TB, BA, SB, MM, and number of piglets weaned per litter (WP) differed significantly (P<0.001) between non-vaccinated and vaccinated sows (11.2 and 11.5 TB, 10.0 and 10.6 BA, 6.9% and 5.1% SB, 3.2% and 2.2% MM, and 9.2 and 9.6 WP, respectively). It could be concluded that PRRS MLV vaccination improved some reproductive performances of sows in PRRSV sero-positive herds. In the third study, uterine tissues of 100 culled gilts from six herds were collected for PRRSV detection using immunohistochemistry. PRRSV was detected in the cytoplasm of the macrophages in the subepithelial connective tissue layers of the endometrium in 33.0% of the culled gilts. The percentage of the gilts’ uterine tissues containing PRRSV did not differ between herds with the gilts vaccinated with the EU-strain (24.5%) and the US-strain (24.1%) MLV PRRS vaccines but tended to be lower than the non-vaccinated gilts (50.0%). PRRSV could be found even in the gilts older than 11 months of age. It can be concluded that PRRSV remains in the uterine tissue of the infected gilts for several months in both vaccinated and non-vaccinated gilts. In the fourth study, aborted fetuses, mummified fetuses, and stillborn piglets from 89 sows from 10 herds were collected for PRRSV detection using quantitative polymerase chain reaction. The results showed that 67.4% of the samples contained PRRSV. The virus was found in 65.6% of the aborted fetuses, 63.3% of the mummified fetuses, and 74.1% of stillborn piglets (P=0.664). PRRSV antigen was retrieved from both non-PRRS-MLV-vaccinated herds (68.2%) and the vaccinated herds (65.2%) (P=0.794). It could be concluded that PRRSV was frequently detected in dead fetuses in swine commercial herds in Thailand regardless of vaccination.