DIARRHEA 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 di arrhoea among Merino ewes in winter. Comparisons were made among group s 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 ewes not treated with a capsule w as 26% (221/844). In comparison, only 2.7% of treated ewes had severe dag. The adjusted odds ratio on each farm indicated that untreated ewe s 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 sign ificant on only one farm whereas initial body weight had no significan t effect. There was no significant association worm egg counts and the occurrence of severe dag. There were significantly more eosinophils i n the small intestine of ewes affected with severe dag compared with u naffected 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 pr otective immune response to these parasites. New strategies for the co ntrol of nematode infections are needed to prevent diarrhoea and dag f ormation in adult sheep.