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Serum levels of an antibody (Ab) and immunoglobulin G, (IgG,) to the larval (L3) stage of the internal parasite, Trichostrongylus colubrifomis, were determined in 3 years in Romney lambs. Animals from two pairs of selection flocks, bred for increased or reduced resistance to internal parasites, were compared for Ab or IgG, levels, and a progeny test flock was recorded to provide data for genetic parameters (n = 64 sires). Lambs were exposed to a natural parasite challenge on pasture, following an anthelmintic drench treatment at weaning. Blood and faecal samples from 4- to 6-month-old lambs (January samples) were taken when the mean FEZ of a monitor group reached 800 to 1500 eggs/g. Lambs were again drenched with anthelmintic and a further cycle of the protocol was carried out (March samples). Selection flock differences in March (High flock minus Low flock means) averaged - 0.73, - 0.93 and 1.27 phenotypic standard deviations for log,Ab, log, IgG, and log, (FEC + 100) at one site, and -0.89, - 0.94 and 1.89 phenotypic standard deviations at the other. Repeatabilities and he&abilities of log,Ab, log, IgG,, and log, (FEC + 100) were estimated. For data collected in March, the genetic correlations between log,Ab or log IgG, and log, (FEC + 100) were - 0.56 f 0.18 and - 0.35 f. 0.19 respectively, whilst they were -0.48 f 0.17 and -0.47 + 0.15 between log,Ab or log, IgG, and the average of log, (FEC + 100) in January, March and May. The results suggest that ram selection based on serum antibody levels in March would result in 51-67s of the genetic gain in reducing FEC that could be achieved by using FEC directly. |
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