Climate and the distribution of intestinal Trichostrongylus spp. of sheep.

The relative prevalences of Trichostrongylus vitrinus, T. colubriformis and T. rugatus in sheep in south-west Western Australia were examined for evidence of associations with climatic conditions in different localities. The region has a mediterranean-type climate with hot, dry summers and cool, wet winters. The intestinal Trichostrongylus spp. populations in sheep sampled comprised 50% T. vitrinus, 38% T. colubriformis and 12% T. rugatus. The prevalence of T. vitrinus was negatively correlated with mean autumn, winter and spring temperatures of a locality. The prevalence of T. colubriformis was positively correlated with mean autumn, winter and spring temperatures. The prevalence of T. rugatus was not correlated with temperature of any season, but was negatively correlated with mean annual rainfall and length of growing season of a locality. There were suggestions of association of amount of rainfall of a locality and prevalence of T. rugatus was not correlated with temperature of any season, but was negatively correlated with mean annual rainfall and length of growing season of a locality. There were suggestions of association of amount of rainfall of a locality and prevalence of T. colubriformis and of differences in seasonal prevalence of T. vitrinus, but the significance of these was obscured by confounding of some climatic factors. The differences in prevalence of the three Trichostrongylus spp. at different locations was attributed to differences in preferred ambient conditions for development and survival of the free-living stages.