The curvilinear relationship between worm length and fecundity of Teladorsagia circumcincta

The variation among sheep in fecundity of Teladorsagia (Ostertagia) circumc incta was estimated by dividing the faecal egg count by the worm number fol lowing deliberate infection of mature Scottish Blackface lambs. Fecundity w as skewed and ranged from 0 to 350 eggs per worm per day. Most animals had relatively low worm fecundities, but a small number of individuals had rela tively high worm fecundities. However, as fecundity is a ratio of two impre cise estimates, extreme values may be statistical artefacts. Following both deliberate and natural infection, differences in worm fecundity were assoc iated with differences in adult female worm length. In both infections, fec undity varied with worm length to the power 0.4. This relationship should a ssist the measurement of fecundity in studies of host immunity, in epidemio logical modelling and in estimating the influence of density-dependent rela tionships. (C) 1999 Australian Society for Parasitology. Published by Elsev ier Science Ltd. All rights reserved.