The ecology of Haemonchus contortus in a winter rainfall climate in Australia: the survival of infective larvae on pasture.

At sites near Albany, on the south coast of Western Australia, sheep faecal pellets containing either eggs or infective larvae (L3) of Haemonchus contortus were deposited onto natural pasture plots at intervals over 3 years. Faecal and pasture samples were collected periodically and processed to detect L3. On annual pasture plots, L3 were recovered for from 10 to over 20 weeks from depositions made between autumn and early spring, at which time temperatures were mild or low, rainfall was frequent and the pasture was visibly green. During the hot and dry summer, when the pasture was completely dry, L3 often failed to develop or were recovered on only a single occasion, and the mean larval survival period on positive plots was less than 5 weeks. In contrast, L3 deposited on green perennial pasture plots in summer were recovered for up to 4 months in faecal pellets and on pasture. The longest periods of larval survival were associated with the lowest temperature and highest rainfall recordings, and with the greatest quantity of green plant material in the pasture. It is suggested that the poor survival of L3 during the dry summer period could be used in a strategic treatment programme to interrupt the transmission cycle of H. contortus in winter rainfall climates. However, areas of green pasture which persist in summer may provide a sufficiently moist microclimate to permit the survival of H. contortus L3 despite otherwise unfavourable environmental conditions.