Validity of indicators of dehydration in working horses: A longitudinal study of changes in skin tent duration, mucous membrane dryness and drinking behaviour

Reasons for performing study: Dehydration is a serious welfare concern in horses working in developing countries. Identification of a valid and practical indicator of dehydration would enable more rapid treatment and prevention. Objectives: To examine changes in bodyweight, clinical and blood parameters during rehydration of working horses, identify a 'gold standard' criterion for dehydration and use this to validate a standardised skin tent test, drinking behaviour and mucous membrane dryness as potential field indicators. Methods: Fifty horses with a positive skin tent test, working in environmental temperatures of 30-44°C in Pakistan, were rested and offered water to drink ad libitum. Bodyweight, clinical and blood parameters, mucous membrane dryness, drinking behaviour and skin tent duration at 6 anatomical locations were measured at 0, 30, 60, 120, 180, 240 and 300 min. Results: Skin tent duration was affected by side of animal (P = 0.008), anatomical location and coat moisture (both P<0.001). Younger animals had shorter skin tents at all time points (P = 0.007). There was no significant association between plasma osmolality (P_osm) or water intake and skin tent duration. Horses with a higher P_osm drank significantly more water (P<0.001), and had longer (P<0.001) and more frequent (P = 0.001) drinking bouts. Neither P _osm nor water intake affected qualitative and semi-quantitative measurements of mucous membrane dryness significantly. Conclusions and potential relevance: The standardised skin tent test and measures of mucous membrane dryness investigated in this study were not valid or repeatable indicators of dehydration when compared with P_osm as a 'gold standard' criterion. The volume of water consumed and the number and duration of drinking bouts were the most reliable guide to hydration status currently available for mature working horses. Offering palatable water to drink ad libitum provides both the diagnosis and the remedy for dehydration in working horses.