The ecology of Haemonchus contortus in a winter rainfall region in Australia: the development of eggs to infective larvae.

The seasonal pattern of development of Haemonchus contortus eggs to infective larvae was investigated on the south coast of Western Australia. Sheep faeces containing H. contortus eggs were deposited on pasture plots at intervals over a 3-year period, and pasture grass and faeces were sampled periodically for infective larvae (L3). The maximum recoveries occurred in late autumn and in late spring, when adequate moisture coincided with warm temperatures. Larval development was low and sporadic over the hot and dry summer period, and depressed during winter, although most egg depositions in winter yielded L3 at some time. The proportion of L3 recovered was related to temperature and moisture parameters, and the major constraint appeared to be the availability of moisture at pasture level. The best indicator of the suitability of conditions for the development of H. contortus was the proportion of green pasture material present, based on a visual assessment.