THE VALUE OF CIRCULATING EOSINOPHIL COUNT AS A SELECTION CRITERION FOR RESISTANCE OF SHEEP TO TRICHOSTRONGYLE PARASITES

In Merino sheep bred for either increased or decreased resistance to H aemonchus contortus, faecal worm egg counts (FEC) were lower in the re sistant line (6,831 vs 17,645 epg, P<0.01), and circulating eosinophil s CEOS) were higher, but not significantly so (3.40 x 10(4) ml(-1) vs 1.40 x 10(4) ml(-1), P = 0.11). Another flock was artificially infecte d with Trichostrongylus colubriformis and significant genetic variatio n was found in both FEC (heritability 0.40 +/- 0.11) and EOS (0.19 +/- 0.08). In a third flock comprising tropical sheep breeds, a natural c hallenge with T. colubriformis resulted in significant sire effects on FEC (heritability 0.20 +/- 0.10) but not EOS (heritability inestimabl e). We conclude that EOS offers no advantage over FEC as a selection c riterion for resistance.