SEROLOGICAL RESPONSES AGAINST PSEUDOMONAS-AERUGINOSA IN MERINO SHEEP BRED FOR RESISTANCE OR SUSCEPTIBILITY TO FLEECE ROT AND BODY STRIKE

Lines of medium-wool Peppin Merino sheep selected for resistance (R) o r susceptibility (S) to fleece rot and body strike were compared serol ogically following 4 weekly intradermal inoculations of live Pseudomon as aeruginosa, the predominant bacterial species associated with fleec e rot. There was essentially no evidence of fleece rot or body strike in these animals prior to experimentation. In Exp. 1, the 1989-born ew e progeny of the R and S lines were studied (n(R) = 52; n(S) = 49). Ov er 12 weeks, average serum antibody responses against P. aeruginosa we re greater in R sheep but the differences were generally non-significa nt at P = 0.05. The apparent differences in antibody response between R and S lines were largely attributable to the effects of some individ ual sires in the R line. In a second experiment, the 1990-born ewe pro geny of the R and S lines (n(R) = 75; n(S) = 76), and the ewe progeny of a line unselected on fleece rot and body strike resistance (F line; n(F) = 53), were studied. Following inoculation with live P. aerugino sa, the mean serum antibody responses of the R and S sheep, which were not different before inoculation, were significantly higher for R she ep for the first 8 weeks of a 10-week observation period. The mean res ponses 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 muc h greater than for the S and F lines, but thereafter sire effects were similar among lines. Taken together, these results suggest that direc t selection for the R or S genotype produced lines of sheep with diver gent serological responsiveness following intradermal inoculation with P, aeruginosa; however, in one generation the effects of some individ ual sires appeared to be largely responsible for these differences.