Sporadic abortions around the beginning of the lambing period are expected by most shepherds and their incidence can increase with flock size. Some of these abortions can readily be attributed to external events. However, a few isolated abortions can sometimes precipitate quickly into an outbreak or ‘abortion storm’. Where the incidence rapidly surpasses 5 per cent of the ewe flock or where there are a number of abortions in a short time period, then a determination of the cause should be attempted.