Faecal egg counts in Angora goats following natural or experimental challenge with nematode parasites: within-flock variability and repeatabilities

Phenotypic variation in faecal worm-egg count (FEC) was studied in 100 Ango ra goats between 6 months and 2 years old as part of a wider investigation into the potential use of FEC as a selection trait in breeding goats for im proved natural resistance to nematode infection. Shortly after weaning at 4 -5 months old the goats were subjected to a procedure involving successive natural and experimental challenges, which was continued at 6-monthly inter vals until the goats were 2 years old. Experimental challenges consisted of Trichostrongylus colubriformis infective larvae administered orally at a d ose rate of 1000 larvae/kg liveweight. FECs resulting from both natural and experimental infections varied considerably between individual goats as we ll as between different sampling times. Mixed model analysis of variance of natural infections confirmed that a significant component of this variatio n was attributable to differences between individual goats (P < 0.01) as op posed to other potential sources of variability, indicating that although g enerally deemed to be more susceptible than sheep, Angora goats do neverthe less vary in their ability to resist nematode infection. Rank correlations between pairs of FECs derived from the same infection were relatively high for both natural and experimental infections (r = 0.63-0.70 and 0.54-0.79, respectively; all P < 0.01). However, those between mean FECs measured on s uccessive 6-monthly sampling occasions were considerably lower and more var iable (r = 0.01-0.41 and 0.19-0.62 for natural and experimental infections, respectively), Nevertheless, FECs resulting from natural infections showed moderate correlations with those from corresponding experimental infection s (P < 0.01) on all except the first sampling occasion (r = 0.23-0.61). Cor relations were marginally higher when FECs from experimental infections wer e adjusted for infecting dose size fr = 0.32-0.63). FECs resulting from exp erimental infections were significantly higher on the first sampling occasi on (at 6 months) than on subsequent occasions (P < 0.01) suggesting that so me degree of resistance to nematode establishment had developed in the goat s by 12 months old. On the basis of unadjusted FECs there appeared to be no marked increase in resistance beyond that age, although analyses based on FECs adjusted for infecting dose size suggested an increase in acquired res istance up to 18 months of age. FECs following natural challenge in goats w hich had also received experimental infections were not significantly diffe rent from those exposed only to natural challenge, indicating that the dren ch-abbreviated experimental infections had had no immunizing effect. Does t hat had given birth and were lactating at 24 months had significantly highe r FECs than 'dry' does in the flock (P < 0.01), suggesting that like sheep, goats exhibit a post-parturient relaxation of immunity. The use of FECs as potential indicators of resistance status in goats and as a criterion for the selection of breeding animals is discussed. (C)1999 Elsevier Science B. V. All rights reserved.