Caroline Jacobson

Diarrhoea ("scouring") in sheep increases the risk of faecal soiling of the breech ("dag") that in turn causes significant production losses for sheep producers and increases susceptibility of sheep to breech blowfly strike. The common causes of scouring in sheep of post-weaning age in Western Australia have not been well described. In a written questionnaire sent to sheep producers in the south west of Western Australia, about half of the respondents reported lamb, hogget and ewe flocks with moderate or severe dag. Flocks with moderate or severe dag were reported more commonly in the winter and spring months in all age groups. Young sheep and mated ewes were most susceptible to moderate or severe dag. The utilisation of professional worm control advice and parasite management tools did not reduce the risk of moderate/severe dag. A study conducted at an abattoir showed that large strongyle worm egg counts (WEC) were frequently identified in lamb lines but were much less common in adult lines. The relationship between WEC and scouring was not clear, suggesting that factors other than large strongyle infections were important, particularly in adult sheep. The low WEC and seasonal scouring pattern observed in adult sheep was consistent with the larval hypersensitivity scouring syndrome and/or factors related to green pasture as potential common causes of scouring in adult sheep. This observation was consistent with detailed investigations of flocks with "low WEC scouring" that found larval hypersensitivity scouring syndrome or factors associated with green pasture were the most likely causes of scouring in eight of the nine flocks examined. Large immature worm burdens were common and the scouring sheep had more fourth stage larvae than normal sheep. Treatment with a fully effective drench and an ivermectin controlled-release rumen capsule did not result in a reduction of faecal moisture content between three and seven weeks after treatment. The effects of dietary soluble non-starch polysaccharides were studied using carboxymethylcellulose (CMC) as a model. The CMC model was used to demonstrate that significant interactions between diet and strongyle larvae determined both faecal consistency and strongyle establishment. Sheep fed CMC had softer, looser and wetter faeces, but the factors that determined faecal consistency were complex. Establishment of T. colubriformis was significantly greater in sheep fed CMC suggesting that the environment within the gut may affect establishment of T. colubriformis in the small intestine. The findings suggested that dietary factors may interact with strongyle larvae to determine both worm establishment and severity of scouring. The results of the studies described in this thesis suggested that factors related to immature strongyle larvae, diet and the immune response interact to determine the severity of the scouring observed in sheep of post-weaning age in the south west of Western Australia.