Development and survival of infective larvae of Haemonchus contortus and Trichostrongylus colubriformis on pasture in a tropical environment

A trial to determine the seasonal pattern of egg hatching and larval survival on pasture was carried out in representative wet and dry zones of Fiji. Fourteen plots were established on parasite-free pasture at each of two sites. One plot at each site was contaminated every month with faeces from naturally infected goats containing a known proportion of Haemonchus contortus and Trichostrongylus colubriformis eggs. Pasture was sampled at regular intervals after contamination and infective larvae identified and counted. Larvae of both species developed throughout the year in the wet zone but development was more sporadic in the dry zone. Larval counts generally declined to below detectable levels within 9 weeks of contamination between September and March but longevity increased during the cooler weather from April to August. The comparatively short larval survival times noted in this experiment may present opportunities for manipulation of parasite population dynamics in the wet tropics.