Availability of gastro-intestinal nematode larvae to sheep following winter contamination of pasture with six nematode species on the Northern Tablelands of New South Wales

This experiment was designed to determine the infective consequences of winter pasture contamination by sheep infected with 6 species of trichostrongylid gastro-intestinal nematode (GIN) at Armidale in the New England region of NSW. Ten helminthologically inert pasture plots were contaminated using ‘donor’ sheep harbouring artificial infections of Haemonchus contortus, Trichostrongylus colubriformis, Trichostrongylus vitrinus, Trichostrongylus axei, Nematodirus spathiger and Teladorsagia circumcincta. A single plot was contaminated in March to serve as a positive control and three plots in each of the winter months of June, July or August of 2007. Weekly assessments of 24 h faecal output and worm egg count of all donor sheep, combined with differentiation of larval cultures enabled calculation of total egg deposition per plot for each GIN species for each month of contamination. All plots were provided with supplemental spray irrigation sufficient to remove desiccation as a cause of non-development. The infective consequence of each monthly contamination was assessed by grazing each plot with two tracer sheep for the last 14 days of each month following contamination until a final grazing in November. Translation for each GIN species was expressed as the percentage of eggs deposited translating to worms counted in tracer sheep.The GIN parasites fell into two distinct groups differing greatly in translational success. Translation of egg to establishment in the tracer host was ineffective for H. contortus, T. axei and T. colubriformis but effective for T. vitrinus, T. circumcincta and N. spathiger. Overall mean total translation for each of these species throughout the experiment was respectively 0.00%, 0.00%, 0.00%, 0.34%, 0.45% and 0.48%. We conclude that winter contamination of pastures in this region with eggs of the latter 3 species is capable of impacting on the health of sheep subsequently grazing these pastures. Where H. contortus and T. colubriformis predominate, paddocks selected for spring lambing may be grazed safely over the winter months without contributing significantly to pasture contamination. These findings are particularly relevant to the preparation of clean spring lambing paddocks in this region using grazing management.