Moisture requirements for the migration of Haemonchus contortus third stage larvae out of faeces

The abomasal nematode Haemonchus contortus causes severe disease and production loss in small ruminants in warmer regions and is also an emerging threat in many temperate climates. Specific knowledge of the effects of climate on the epidemiology of H. contortus is needed to effectively apply sustainable control strategies, which rely on prediction of infection risk. Although the effects of temperature and rainfall on larval development in this species have been characterised, much less is known about migration out of faeces and onto herbage. This is an important deficit in our understanding of the epidemiology of haemonchosis in regions with relatively low and particularly erratic rainfall. Methods were developed to assess the migration of third stage larvae (L3) out of faeces under simulated rainfall in the laboratory. These were applied in a series of experiments, which showed that rainfall is required for migration. However, a single rainfall event was not sufficient for migration from faeces of which the crust has hardened after having been kept in dry conditions. Light and regular rainfall resulted in rapid emergence from moist faeces kept in humid conditions, but much slower emergence from dry faeces in dry conditions. Ambient relative humidity therefore appears to act through faecal moisture content to modify the effect of rainfall on larval migration. Larvae survived well in dry faeces for a number of days, but did not migrate in the absence of rainfall, so sheep faeces could potentially act as a larval reservoir in dry conditions, with peaks of infection following rainfall. Rates of faecal desiccation and rehydration on pasture could therefore be highly relevant to temporal patterns of larval availability.