The relationship between the rate of intake of trichostrongylid larvae and the occurrence of diarrhoea and breech soiling in adult Merino sheep

Abstract

A hypersensitive inflammatory response, associated with the ingestion of trichostrongylid larvae, is a major cause of diarrhoea and soiling of the breech with faeces in adult Merino sheep ('hypersensitivity scours'). This response is characterised by an infiltration of eosinophils and altered lymphocyte populations in the gut of affected sheep. The objective of this study was to investigate the numbers of Ostertagia circumcincta and Trichostrongylus vitrinus infective larvae that were needed to induce diarrhoea in adult Merino sheep grazing improved pastures. Sheep from three farms were categorised as either being susceptible, or not susceptible, to hypersensitivity scours, then transported to an experimental farm and grazed on pastures with low numbers of trichostrongylid infective larvae. They were ranked by farm of origin and previous worm egg count, then systematically allocated to one of five dose-rate groups and infected with 0, 2, 4, 10 or 20 x 10(3) trichostrongylid infective larvae per week for 13 weeks. In sheep selected as being susceptible to hypersensitivity scours, the lowest dose rate of infective larvae (2000 per week) was as effective in inducing diarrhoea as the highest dose rate (20,000 per week). In contrast, even the highest larval dose did not induce diarrhoea in sheep selected as not susceptible to hypersensitivity scours. Sheep selected with an increased susceptibility to hypersensitivity scours also had a significantly higher amount of moisture in their faeces at all times except during summer. The apparent lack of a threshold dose of trichostrongylid infective larvae needed to induce hypersensitivity scours suggests that even improved worm control programs, which substantially decrease the ingestion of larvae, may not be able to reduce the prevalence of hypersensitivity scours or the severity of breech soiling. Controlled-release anthelmintic capsules will control breech soiling but are often not a cost-effective strategy. Thus, genetic selection of less susceptible sheep is probably the best long-term option for the control of hypersensitivity scours, and the assessment of faecal moisture may be a convenient marker of susceptibility to this syndrome.