Passive immunity due to PRRS in vaccinated and unvaccinated sow herds

\nIf a sow has a high level of antibodies to the Porcine Reproductive and Respiratory \nSyndrome (PRRS) virus we assume that her piglets will get a high level of passive \nimmunity from her colostrum. Although this measurable immunity is not directly \nrelated to the protection of pigs against PRRS, it may indicate a surrogate measure of \nimmunity for the piglet. Recent work suggests that repeatedly vaccinated sows lose their \nmeasurable antibody titre to PRRS. However, in herds in which sows are not vaccinated, \nthere may be various sub-populations of sows, some with high titres to PRRS and some with \nno titres to PRRS. The objectives of the current study were to determine (i) whether there \nis a relationship between the sow's PRRS titre and that of her piglets; (ii) if the \ntransmission of passive immunity from a PRRS positive sow to her piglets is 100%; (iii) \nif sow vaccination increases the proportion of piglets with passive immunity. Sixteen \nPRRS positive herds were included in the study. Ten farms vaccinated the sows against \nPRRS at weaning, the other six farms did not vaccinate sows. The producers included in \nthis study were those who were concurrently participating in other research at the \nUniversity of Guelph and were not a random sample of herds. No herds were experiencing \na clinical outbreak of PRRS at the time of sampling. Blood samples were taken from three \nto seven lactating sows per farm and from three to fourteen piglets per sow. These \nincluded all pigs nursing the sow that were born to that sow. On the average, eight\n piglets were sampled per litter at 2 to 10 days of age. The PRRS IDEXX ELISA test \nwas used to determine each pig's level of immunity. The S/P ratios were categorized \nby group and both the protected and active infection groups were considered positive: \nthe PRRS negative group had PRRS Elisa S/P ratios $\\leq 0.4$; the PRRS protected group had\n PRRS Elisa S/P ratios between 0.41 and 2.50; the PRRS active infection group had PRRS \nElisa S/P ratios $> 2.50$. Pig information was analysed at the pig level, whereas sow \ninformation was analysed at the sow level. Pig and sow titres were transformed into \ntitre groups and then the data was analysed with a Chi-square using the Statistix \nsoftware program. Differences in S/P ratios were determined with the Kruscal Wallace \nANOVA for non-parametric data. Of the 491 pigs, 74% had a positive PRRS S/P ratio at\n 2-10 days of life. Most pigs (88%) nursing PRRS positive sows had a positive titre at\n 3 to 10 days of life. Pigs nursing positive sows were 6.2 times more likely to have a\n positive titre than pigs nursing negative sows $\\rm (P < 0.0001)$. In both vaccinated and \nunvaccinated herds, the correlation between piglet and sow S/P ratios was 0.55. Passive \nimmunity due to PRRS appears to have a short half-life. Pigs 2-8 days old were 2.8 times\n more likely to be PRRS positive than pigs 9-10 days old (P = 0.002). In vaccinated herds,\n 66% of the sows were positive, whereas in unvaccinated herds, 48% were positive. Sows \nfrom vaccinated herds were 2.9 times more likely to be positive than sows from unvaccinated \nherds (P = 0.04). In vaccinated herds, 81% of the pigs were positive, whereas 63% were \npositive in unvaccinated herds $\\rm (P < 0.0001)$. Piglets from vaccinated herds were 2.3 \ntimes more likely to be positive than piglets from \nunvaccinated herds $\\rm (P < 0.0001)$. \nIn vaccinated herds, 91% of pigs nursing positive sows were positive, whereas in\n unvaccinated herds, 86% of pigs nursing positive sows were positive. In these \ndata, 26% of the positive pigs came from negative sows. These represented 41% of \nthe pigs from unvaccinated herds and 19% of pigs from vaccinated herds. In unvaccinated \nherds, 40% of the pigs nursing sero-negative sows were positive whereas in vaccinated herds, \n51% of the pigs nursing sero-negative sows were positive \n$\\rm (P > 0.05)$. We concluded that the \nantibody was concentrated in the colostrum. In unvaccinated herds, the negative sows may \nrepresent those with a rising or a declining antibody titre. Also, sows vaccinated \nrepeatedly with the same commercial product have a lower titre than sows vaccinated \nonly once. In our data set, parity one sows tended to have the highest average S/P \ntitre (1.12 SD = 0.99), followed by parity 3+ sows (0.74 SD = 0.57). The lowest S/P \ntitre was observed in parity 2 sows (0.48 SD = 0.34). These values were not statistically \ndifferent \n(p = 0.12). \n

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