Serological responses against Pseudomonas aeruginosa in Merino sheep bred for resistance or susceptibility to fleece rot and body strike

Abstract

Lines of medium-wool Peppin Merino sheep selected for resistance (R) or susceptibility (S) to fleece rot and body strike were compared serologically following 4 weekly intradermal inoculations of live I, the predominant bacterial species associated with fleece rot. There was essentially no evidence of fleece rot or body strike in these animals prior to experimentation. In Exp. 1, the 1989-born ewe progeny of the R and S lines were studied (nR = 52; nS = 49). Over 12 weeks, average serum antibody responses against P. aeruginosa were greater in R sheep but the differences were generally non-significant at P = 0.05. The apparent differences in antibody response between R and S lines were largely attributable to the effects of some individual sires in the R line. In a second experiment, the 1990-born ewe progeny of the R and S lines (nR = 75; nS = 76), and the ewe progeny of a line unselected on fleece rot and body strike resistance (F line; nF = 53), were studied. Following inoculation with live I, the mean serum antibody responses of the R and S sheep, which were not different before inoculation, were significantly higher for R sheep for the first 8 weeks of a 10-week observation period. The mean responses of the F line generally fell between the R and S responses. One week after the first inoculation, sire effects in the R line were much greater than for the S and F lines, but thereafter sire effects were similar among lines. Taken together, these results suggest that direct selection for the R or S genotype produced lines of sheep with divergent serological responsiveness following intradermal inoculation with P. aeruginosa; however, in one generation the effects of some individual sires appeared to be largely responsible for these differences.