DISEASE RESISTANCE IN MERINO SHEEP .5. GENETIC-HETEROGENEITY IN RESPONSE TO VACCINATION WITH DICHELOBACTER-NODOSUS AND CLOSTRIDIAL ANTIGENS

Genetic aspects of antibody responses in two commercial, multivalent v accines were examined in 2974 Merino sheep, representing the progeny f rom 129 sires in four major bloodlines. All lambs were vaccinated twic e, between 2 and 4 months of age, with a clostridial vaccine containin g antigens against five clostridial species. Approximately half (1239) the progeny from all sires were also vaccinated at 16 months of age, with a Dichelobacter nodosus vaccine containing whole-cell antigens fr om all nine major serogroups (A-I). ELISA antibody titres to Clostridi um tetani and Clostridium chauvoei were measured 2 weeks after booster vaccination. K-agglutinating antibody titres, against all serogroups of D. nodosus, were measured on three occasions-following primary vacc ination (V1), 3 and 8 weeks following booster vaccination (V2). Mean a ntibody titre on all three occasions (MEAN POST-VAC), and changes in a ntibody titre between V1 and V2, and after V2, were analysed as additi onal antibody traits for D. nodosus. For both clostridial and D. nodos us antibody traits, year of birth and flock were major fixed effects. Sex of the lamb was not important in the case of clostridial antibody titres, but ewe hoggets had consistently higher D. nodosus antibody ti tres compared to those for ram hoggets from the same cohort. Based on restricted maximum likelihood (REML) analyses, heritability estimates for C. tetani and C. chauvoei were 0.12 (+/- 0.04) and 0.24 (+/- 0.07) . Preliminary estimates for maternal effects showed additive maternal genetic variance for C. tetani and C. chauvoei (0.09 +/- 0.05 and 0.17 +/- 0.07). Maternal environmental effects appeared negligible. Herita bility estimates of D. nodosus antibody titre for all nine serogroups ranged from 0.22 to 0.66 following V1, and were approximately 40 % hig her than those following V2 (range 0.17-0.44), which in turn were appr oximately 50 % higher than estimates for persistence of titre (range 0 .08-0.32). Genetic correlations tor antibody titre at consecutive samp lings were high, and the heritability estimates for MEAN POST-VAC tit re for each serogroup of D. nodosus were: A (0.51), B (0.58), C (0.41) , D (0.24), E (0.34), F (0.33), G (0.26), H (0.34), and I(0.35). Genet ic correlations for antibody titres among the nine D. nodosus serogrou ps ranged from moderately negative (- 0.40) to strongly positive (0.87 ). Similarly, genetic correlations between clostridial and D. nodosus antibody titres ranged from -0.08 to +0.41. C. tetani antibody titre s howed a genetic correlation of +0.50 with C. chauvoei titre. It was co ncluded that significant additive genetic variation exists in Merino s heep for vaccine response to commercial vaccines. However, there was n o strong evidence for common genetic control of immune response to dif ferent antigens.