Trichostrongylid infections of sheep in a winter rainfall region. II. Epizootiological studies in the western district of Victoria, 1967-68

Abstract

Mean counts of O. circumcincta and Trichostrongylus spp. from groups of 'tracer' sheep ranged between 220 and 2150 per fortnight between June and October and were less than 100 per fortnight from November to February inclusive. O. circumcincta was more numerous than Trichostrongylus spp. The relatively low numbers of available larvae were attributed to the dry weather conditions in autumn which did not favour development and survival of the free-living stages. Between May and December 1967, worm numbers increased linearly in weaner sheep to reach mean levels of 14,000 without any detectable resistance to reinfection or significant decrease in liveweight gain or wool production. Correlations between mean plasma pepsinogen concentrations from weaner and adult sheep and 'tracer' worm burdens were significantly different; r = 0.37 and 0.47 for weaners and r = 0.8 for adult sheep. Thus the relationship between larval intake and abomasal damage as reflected by plasma pepsinogen concentrations is not a simple one. Mucosal damage is probably preceded by a hypersensitive state. Anthelmintic treatment, given in October and January, reduced the worm egg output of treated sheep to one-tenth that of untreated sheep for a period of 18 weeks between October and March.