Diarrhoea in merino ewes during winter: association with trichostrongylid larvae

A study on 3 farms, each having a preventive strategy for the control of nematode infections, determined the risk factors associated with diarrhoea among Merino ewes in winter. Comparisons were made among groups of mated and unmated ewes with and without treatment with controlled-release capsules containing albendazole. Diarrhoea was assessed from the accumulations of faeces around the breech (dag score) and related to worm egg counts, total worm counts and histopathological findings. The overall prevalence of severe dag in ewes not treated with a capsule was 26% (221/844). In comparison, only 2.7% of treated ewes had severe dag. The adjusted odds ratio on each farm indicated that untreated ewes were between 12 and 16 times more likely to be affected with severe dag than ewes treated with a capsule. The effect of lactation was significant on only one farm whereas initial body weight had no significant effect. There was no significant association between worm egg counts and the occurrence of severe dag. There were significantly more eosinophils in the small intestine of ewes affected with severe dag compared with unaffected ewes. There were no significant differences in mast cell and globule leucocyte numbers between affected and unaffected ewes. This study provides strong evidence that the main cause of diarrhoea among Merino ewes grazing winter and early spring pastures is the ingestion of trichostrongylid larvae even by sheep that have a well-developed protective immune response to these parasites. New strategies for the control of nematode infections are needed to prevent diarrhoea and dag formation in adult sheep.