Passive immunity due to PRRS in vaccinated and unvaccinated sow herds

If a sow has a high level of antibodies to the Porcine Reproductive and Respiratory Syndrome (PRRS) virus we assume that her piglets will get a high level of passive immunity from her colostrum. Although this measurable immunity is not directly related to the protection of pigs against PRRS, it may indicate a surrogate measure of immunity for the piglet. Recent work suggests that repeatedly vaccinated sows lose their measurable antibody titre to PRRS. However, in herds in which sows are not vaccinated, there may be various sub-populations of sows, some with high titres to PRRS and some with no titres to PRRS. The objectives of the current study were to determine (i) whether there is a relationship between the sow's PRRS titre and that of her piglets; (ii) if the transmission of passive immunity from a PRRS positive sow to her piglets is 100%; (iii) if sow vaccination increases the proportion of piglets with passive immunity. Sixteen PRRS positive herds were included in the study. Ten farms vaccinated the sows against PRRS at weaning, the other six farms did not vaccinate sows. The producers included in this study were those who were concurrently participating in other research at the University of Guelph and were not a random sample of herds. No herds were experiencing a clinical outbreak of PRRS at the time of sampling. Blood samples were taken from three to seven lactating sows per farm and from three to fourteen piglets per sow. These included all pigs nursing the sow that were born to that sow. On the average, eight piglets were sampled per litter at 2 to 10 days of age. The PRRS IDEXX ELISA test was used to determine each pig's level of immunity. The S/P ratios were categorized by group and both the protected and active infection groups were considered positive: the PRRS negative group had PRRS Elisa S/P ratios $\leq 0.4$; the PRRS protected group had PRRS Elisa S/P ratios between 0.41 and 2.50; the PRRS active infection group had PRRS Elisa S/P ratios $> 2.50$. Pig information was analysed at the pig level, whereas sow information was analysed at the sow level. Pig and sow titres were transformed into titre groups and then the data was analysed with a Chi-square using the Statistix software program. Differences in S/P ratios were determined with the Kruscal Wallace ANOVA for non-parametric data. Of the 491 pigs, 74% had a positive PRRS S/P ratio at 2-10 days of life. Most pigs (88%) nursing PRRS positive sows had a positive titre at 3 to 10 days of life. Pigs nursing positive sows were 6.2 times more likely to have a positive titre than pigs nursing negative sows $\rm (P < 0.0001)$. In both vaccinated and unvaccinated herds, the correlation between piglet and sow S/P ratios was 0.55. Passive immunity due to PRRS appears to have a short half-life. Pigs 2-8 days old were 2.8 times more likely to be PRRS positive than pigs 9-10 days old (P = 0.002). In vaccinated herds, 66% of the sows were positive, whereas in unvaccinated herds, 48% were positive. Sows from vaccinated herds were 2.9 times more likely to be positive than sows from unvaccinated herds (P = 0.04). In vaccinated herds, 81% of the pigs were positive, whereas 63% were positive in unvaccinated herds $\rm (P < 0.0001)$. Piglets from vaccinated herds were 2.3 times more likely to be positive than piglets from unvaccinated herds $\rm (P < 0.0001)$. In vaccinated herds, 91% of pigs nursing positive sows were positive, whereas in unvaccinated herds, 86% of pigs nursing positive sows were positive. In these data, 26% of the positive pigs came from negative sows. These represented 41% of the pigs from unvaccinated herds and 19% of pigs from vaccinated herds. In unvaccinated herds, 40% of the pigs nursing sero-negative sows were positive whereas in vaccinated herds, 51% of the pigs nursing sero-negative sows were positive $\rm (P > 0.05)$. We concluded that the antibody was concentrated in the colostrum. In unvaccinated herds, the negative sows may represent those with a rising or a declining antibody titre. Also, sows vaccinated repeatedly with the same commercial product have a lower titre than sows vaccinated only once. In our data set, parity one sows tended to have the highest average S/P titre (1.12 SD = 0.99), followed by parity 3+ sows (0.74 SD = 0.57). The lowest S/P titre was observed in parity 2 sows (0.48 SD = 0.34). These values were not statistically different (p = 0.12)