Moisture requirements for the free-living development of 'Haemonchus contortus': Quantitative and temporal effects under conditions of low evaporation

Abstract

The key influence of moisture availability on development of the free-living stages of 'Haemonchus contortus' is well-documented, although quantitative relationships between moisture variables and development are poorly defined. A factorial experiment (3 x 2 x 2) was conducted in programmable incubators to determine the effects of amount and distribution of simulated rainfall on 'H. contortus' development at low evaporation rates (approximately 2mm/day) under temperatures typical of summers on the Northern Tablelands of NSW, Australia. Sheep faeces containing 'H. contortus' eggs were placed in experimental units containing sterile soil and had one of the three amounts (12, 24 and 32 mm) of simulated rainfall applied, in either a single event on the day after deposition (d 1) or three split events over 6 days (d 1, 3 and 6). Treatments were applied either in week 1 only, or in weeks 1 and 2. Recovery of infective larvae (L3) at 4, 7 and 14 days post-contamination increased with each incremental amount of simulated rainfall over the range of 12-32 mm and was significantly higher under the single 'rain' event (2.8%), compared with the three smaller, split events (1.9%). The second application of simulated rainfall in week 2 had only a very small influence on L3 recovery, suggesting that the majority of development to L3 occurred in response to simulated rainfall events in the first 7 days. Both faecal moisture content and the cumulative ratio of precipitation and evaporation (P/E) were strongly and positively correlated with recovery of L3. Recovery of L3 from treatments, which received simulated rainfall in week 1 only was best described by P/E at d 5. Whether the relationships observed in this study hold under field conditions, where macroclimactic conditions such as evaporation rate are substantially more limiting to free-living development, is yet to be determined.