The availability of Trichostrongylid larvae to grazing sheep after seasonal contamination of pastures

Abstract

Separate paddocks of improved pasture were contaminated at different rimes with worm eggs from grazing sheep with naturally acquired infections of the genera Ostertagia, Trichostrongylus and Nernatodirus. Worm counts from 'tracer' sheep on each paddock were used to estimate the availability of larvae for up to 9 months after contamination. Typical hot and dry weather prevailed during the summer months of 1970-71, and few Ostertagia and Trichostrongylus larvae were available on any paddocks. However, larvae of these species survived in faeces over summer and their availability increased to reach peak levels on all paddocks by 8 weeks after the onset of autumn rains. Infection persisted at moderate levels until midwinter. Rainfall in the summer months of 1971-72 exceeded the average by about 50% while in autumn it was reduced by a similar amount. Compared with the previous year, greater numbers of Ostertagia and Trichostrongylus spp. were available in summer on paddocks contaminated before and during summer, but fewer persisted through autumn and winter. In both years larvae from eggs deposited in late summer and autumn made the greatest contribution to worm burdens of 'tracer' sheep slaughtered in winter months. By mid-October, when mean maximum temperatures and evaporation rates were rising, the numbers of larvae decreased rapidly to low levels on all paddocks contaminated from late summer onwards. Availability of Nematodirus spp. was similar on all paddocks contaminated before or during summer. Peak numbers occurred in summer months of 1970-71, but not in 1971-72, and in both years a secondary peak was observed in March-April before the increase in availability of Ostertagia and Trichostrongylus spp. Some implications for the control of helminthiasis in sheep are discussed.