Veterinary Science - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemThe pathogenesis and control of diarrhoea and breech soiling ('winter scours') in adult merino sheep( 1997)Diarrhoea and soiling of the breech with faeces (‘dag’) is one of the most prevalent problems of adult Merino sheep grazing improved pastures in south-eastern Australia during winter and early spring. This syndrome (‘winter scours’) occurs even on farms using programs which effectively control gastro-intestinal nematodes. The causes and costs of winter scours have not been well defined. Comparisons were made among groups of ewes, with and without treatment with controlled-release capsules containing albendazole. Adjusted odds ratios indicated that untreated ewes were 12 to 16 times more likely to be affected with severe dag than ewes treated with a capsule. From this, it was concluded that trichostrongylid parasites were a necessary cause of winter scours. Subsequently, comparisons were made, between affected and unaffected ewes, for faecal worm egg counts and total worm counts. There were no relationships between these indicators of adult parasitism and the occurrence of diarrhoea. The sum of these observations provided strong evidence that winter scours is associated with the ingestion of trichostrongylid larvae. Further, it was concluded that selection for decreased faecal worm egg count would not reduce the prevalence of winter scours. The importance of host factors was examined. Significant differences were detected in the mean dag scores of different strains of Merino sheep, and winter scours was highly repeatable between years (r = 0.44 to 0.61), thus demonstrating that host factors were important in determining susceptibility to winter scours. Within sheep selected as being not susceptible to winter scours, high doses of infective larvae (20,000 Ostertagia circumcincta and Trichostrongylus vitrinus L3/week) did not induce diarrhoea. In contrast, within sheep selected as being susceptible, even small doses of larvae (2000/week) initiated scouring. Sheep selected as being susceptible also had persistently higher faecal moisture levels than non-susceptible sheep, even during times when accumulation of dag did not occur. The immune responses of affected and unaffected sheep were compared. No significant differences were detected between mast cell and globule leucocyte counts, antibody levels in serum, gut tissue and mesenteric lymph nodes, circulating eosinophil counts or the dermal eosinophil response to the intra-dermal injection of mitogens and larval antigens. From this, it appeared that there was no difference in the protective immune response to gastro-intestinal nematodes of affected and unaffected sheep. A feature of the immune response of affected sheep was a hypersensitive inflammatory reaction, characterised by the infiltration of significantly more eosinophils, and changed lymphocyte populations, in the pylorus and upper jejunum. The changes in the lymphocyte populations included a reduced CD4+:CD8+ T-cell ratio, due mainly to significantly reduced numbers of CD8+ T-cells, and significantly reduced numbers of cells reacting to interferon-gamma (IFN-y) monoclonal antibody. Overall, winter scours was estimated to cost the Victorian sheep industry at least $10 million per annum. Improved strategic worm control programs are unlikely to prevent winter scours, because only low doses of infective larvae initiated diarrhoea in susceptible sheep. Phenotypic culling and genetic selection was proposed as the most suitable long-term control strategy to remove sheep susceptible to the hypersensitivity inflammatory response, and thereby reduce the prevalence of severe dag in adult Merino sheep.