Microclimate has a greater influence than macroclimate on the availability of infective Haemonchus contortus larvae on herbage in a warmed temperate environment

Global climate change is altering epidemiological patterns of gastrointestinal nematode infections in grazing livestock, including through effects of temperature and moisture on the availability of infective third-stage larvae (L3). While considerable experimental effort has been devoted to the influences of climate on L3 development and survival in major nematode species, knowledge of effects on L3 migration out of faeces and onto herbage is more limited. In this study, we examined elements of this process for Haemonchus contortus in controlled and natural climates. The effect of temperature on migration rate from faeces was quantified and found to peak at 15 °C. In glasshouses, a 3 °C difference in mean temperature failed to produce a statistically significant difference in the number of L3 reaching herbage after a single rainfall event, and faecal moisture content (FMC) did not decline significantly more rapidly at the higher temperatures. Most larvae left the faeces and reached the grass within 3 h after simulated rainfall. On natural pasture in temperate summer, FMC was strongly affected by microclimate, with shade and long grass both significantly slowing drying. Results suggest that microclimate is important in determining FMC and larval migration, and that its effects can be greater than those of macroclimate, e.g. moderate differences in average ambient temperature. More work is needed to develop a full predictive understanding of larval availability in natural settings, which is the product of interacting factors acting on overlapping parasite cohorts.