Detecting bovine digital dermatitis in the milking parlour: To wash or not to wash, a Bayesian superpopulation approach.

Visual assessment in the milking parlour is a commonly used method to determine the prevalence and severity of bovine digital dermatitis (BDD). It is generally suggested that cows' feet are washed prior to examination to maximise the sensitivity of the assessment, but concern has been expressed that washing cows' feet could contaminate the teats and lead to intramammary infection. Furthermore, the evidence for washing cows' feet is equivocal, as some studies have reported similar sensitivities for detecting BDD without washing as that reported by studies which used washing. Furthermore most of these studies have used data from housed cattle. The findings from these studies may not be applicable to cattle at pasture where feet are often contaminated with mud rather than faeces and lesions may be smaller and less severe. The aim of this study was to compare, in cattle at pasture, the sensitivities of BDD examinations before and after washing. Two herds known to have BDD were enrolled and approximately half of each herd was screened for BDD by examining the cows' hind feet before and after washing. The sensitivities of these examinations were estimated using a Bayesian superpopulation approach, and were found to be 0.34 (95% credible interval [CrI]: 0.088-0.69) and 0.63 (95%CrI: 0.46-0.78) for pre- and post-washing, respectively. There was a 93.95% probability that the sensitivity of examination post-washing was greater than that pre-washing. These results suggest that in pasture-based herds, many BDD lesions will be missed if cattle are examined without their feet being washed.