The diagnostic value and interpretation of faecal egg counts in sheep

Abstract

The relationship between the strongyle egg count and (a) the total strongyle worm burden and (b) the “total pathogenic index” (T.P.I.) of the worm burden in 190 “young” (up to 12 months of age) and 131 “old” (over 12 months of age) sheep was examined.

In “young” sheep, mean egg counts were found to provide a useful indicator of the seasonal changes in the level of infection (r = 0.99) while a correlation coefficient of 0.74 between individual egg and worm counts (Nematodirus excluded) was recorded. In “old” sheep the corresponding correlation coefficients were 0.16 and 0.23, respectively. When egg and worm counts were categorised according to the concept of “low”, “moderate” and “high”, however, the association between them was found to be almost equally consistent in both age classes of stock. Consideration of these findings and the fact that the association between the egg count and the T.P.I, was, in individual “young” sheep, almost as good as (r = 0.69), and in “old” sheep, notably better (r = 0.69) than, that between the corresponding egg and worm counts, suggests that faecal egg counts are of considerable diagnostic value. In the case of Nematodirus infections, however, where the correlation coefficient between egg and worm counts in “young” sheep was 0.35 and where much of the damage to the host may occur before egg laying has commenced, only very limited reliance should be placed on egg counts for diagnostic purposes.