The effect of a porcine reproductive and respiratory syndrome outbreak on genetic parameters and reaction norms for reproductive performance in pigs1.

The objective of this study was to estimate genetic parameters of antibody response and reproductive traits after exposure to porcine reproductive and respiratory syndrome virus. Blood samples were taken approximately 60 d after the outbreak. Antibody levels were quantified as the sample-to-positive ratio (S/P ratio) using a fluorescent microsphere assay. Reproductive traits included total number born (TNB), number born alive (NBA), number stillborn (NSB), number mummified (NBM), and number born dead (NBD). Mortality traits were log transformed for genetic analyses. Data were split into prior, during, and after the disease outbreak phases using visual appraisal of the estimates of farm-year-week effects for each reproductive trait. For NBA, data from all phases were combined into a reaction norm analysis with regression on estimates of farm-year-week effects for NBA. Heritability for S/P ratio was estimated at 0.17 ± 0.05. Heritability estimates for reproduction traits were all low and were lower during the outbreak for NBA but greater for mortality traits. TNB was not greatly affected during the outbreak, as many sows that farrowed during the outbreak were mated prior to the outbreak. Heritability for TNB decreased from 0.13 (prior) to 0.08 (after). Genetic correlation estimates between prior to and during the outbreak were high for TNB (0.86 ± 0.23) and NBA (0.98 ± 0.38) but lower for mortality traits: 0.65 ± 0.43, -0.42 ± 0.55, and 0.29 ± 1.39 for LNSB, LNBM, and LNBD, respectively. TNB prior to and after the outbreak had a lower genetic correlation (0.32 ± 0.33). In general, genetic correlation estimates of S/P ratio with reproductive performance during the outbreak were below 0.20 in absolute value, except for LNSB (-0.73 ± 0.29). Based on the reaction norm model, estimates of genetic correlations between the intercept and slope terms ranged from 0.24 ± 0.50 to 0.54 ± 0.35 depending on the parameterization used, indicating that selection for the intercept may result in indirect selection for steeper slopes, and thus, less resilient animals. In general, estimates of genetic correlations between farm-year-week effect classes based on the reaction norm model resembled estimates of genetic correlations from the multivariate analysis. Overall, compared to previous studies, antibody S/P ratios showed a lower heritability (0.17 ± 0.05) and low genetic correlations with reproductive performance during a porcine reproductive and respiratory syndrome outbreak, except for the LNSB.

Antibody response to porcine reproductive and respiratory syndrome virus (PRRSV) nonstructural proteins and implications for diagnostic detection and differentiation of PRRSV types I and II.

To further characterize the humoral immune response of pigs to porcine reproductive and respiratory syndrome virus (PRRSV), direct enzyme-linked immunosorbent assays (ELISA) were used to study the kinetics of antibody responses directed against PRRSV nonstructural proteins in pigs experimentally exposed to the virus. The highest immunoreactivities were against nsp1, nsp2, and nsp7. Using the recombinant nsp7 as an antigen, we validated a dual ELISA for the simultaneous detection and differentiation of serum antibodies against type I and type II PRRSV. Receiver operating characteristic analysis based on 1,334 known-positive and 1,357 known-negative samples showed good specificity (98.3% to type I and 99.3% to type II) and sensitivity (97.4% for type I and 99.8% for type II). To differentiate type I and type II PRRSV, 470 sera originating from experimentally inoculated pigs were tested, and positive sera were correctly differentiated in 469 of 470 samples. The capability of the nsp7 dual ELISA to detect serum antibody responses from pigs infected with various genetically different field strains was determined. The nsp7 dual ELISA possessed 97.6% agreement with the Idexx HerdChek PRRS 2XR ELISA. In further testing of Idexx ELISA suspected false-positive samples, the nsp7 dual ELISA resolved 98% of the samples as negative. Taken together, these results indicate that the nsp7 dual ELISA can be used as a differential test for PRRSV serology with high levels of sensitivity and specificity. This ELISA offers an additional tool for routine or follow-up diagnostics, as well as having substantial value in epidemiological surveys and outbreak investigations.