Irrigation, faecal water content and development rate of free-living stages of sheep trichostrongyles.

The effect of irrigation by flooding the pastures on the ability of the eggs of sheep Trichostrongyles to develop into infective larvae (L3) was estimated in outdoor conditions by depositing parasitised faeces on plots, without irrigation or submerged, at different times and durations. The rates of development of Teladorsagia circumcincta, Trichostrongylus vitrinus and Chabertia ovina were very low in spring and summer in dry plots but proportional to the duration of submersion in the irrigated plots. T circumcincta was mainly favoured in spring, T vitrinus in summer, but higher rates were observed in autumn. The action of water had more effect on freshly deposited faeces. In laboratory experiments on T circumcincta, T vitrinus and Haemonchus contortus, the submersion of cultures was unfavourable to egg development but favoured L1 development. Cultures with different faecal water contents (FWC) simulating a submersion (7 or 16 h) or an alteration of spraying and dehydration, or at constant values of 60 and 50% enabled us to conclude that high FWC favoured the development of T vitrinus, had some negative effect on T circumcincta, but H contortus was more susceptible to variations of FWC. The FWC had an effect on the size of L3 and, for T circumcincta only, the survival rate of the small L3 obtained at low FWC seemed decreased. These data could explain the absence of H contortus, as well as the abundance of T vitrinus on the studied pastures.