VARIATION BETWEEN HOSTS IN THE DEVELOPMENTAL SUCCESS OF THE FREE-LIVING STAGES OF TRICHOSTRONGYLE INFECTIONS OF SHEEP

Variation between hosts of different ages and genotypes in the develop mental success of trichostrongylid parasite eggs from sheep was invest igated in two trials, The percentage development to infective third-st age larvae of eggs collected from lambs and adult ewes infected with O stertagia circumcincta was compared in an indoor trial. In addition, s heep previously bred for either high or low faecal egg count and graze d outdoors on parasite contaminated pasture were sampled; egg developm ent, faecal egg count, generic profile and faecal dry matter were meas ured. In both trials, development to L3 was significantly lower in egg s derived from adult ewes than from lambs and, in the field trial, fro m animals selected for low faecal egg count. The observed differences could not be accounted for by variations in faecal egg count, faecal d ry matter content nor by differences in the generic composition of wor m egg output. Although not shown conclusively, the results are consist ent with an immune mechanism influencing the development of free-livin g stages of gastrointestinal nematodes outside the host. These results have significant implications for our understanding of parasite epide miology, particularly as it relates to sources of pasture contaminatio n, (C) 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd. All rights reserved.