Assessment of Oral Mucosal Colour in Horses: Initial Application of the Uldahl Standard Across Physiological States

Assessment of oral mucosal colour is widely used in equine clinical and welfare evaluations, but inconsistent terminology limits comparison between clinical observations and research studies. This study evaluates, as an initial proof-of-concept, the Uldahl Standard for systematically describing mucosal colour and saturation. Gingival and lingual mucosae were assessed in clinically normal horses, healthy horses sedated for routine dental procedures, and horses with severe systemic circulatory compromise. Photographic colour samples validated against in vivo observations were classified using Munsell notation, ISCC-NBS terminology and the Uldahl Standard. Three illustrative cases of tongue discolouration were additionally assessed. Descriptive analyses characterised colour and saturation across physiological states, while clinically normal and sedated horses were compared using exact chi-square tests and logistic regression. Overall, 42 unique colour-saturation combinations were identified. Chromatic diversity increased with physiological compromise. Primary colour differed significantly between clinically normal and sedated horses for both gingival (p = 0.05) and lingual (p = 0.004) mucosae, whereas secondary colour and saturation did not. Grey was significantly associated with physiological state in both gingival (p = 0.05) and lingual (p = 0.03) mucosae. It showed high specificity but low sensitivity, including 100% specificity for lingual grey. Purple occurred only in sedated and systemically compromised horses, while blue occurred only as a secondary colour in systemically compromised horses. The findings demonstrate the feasibility of applying the Uldahl Standard to equine oral mucosal assessment and suggest that primary colour, particularly grey, may provide greater discriminatory information than secondary colour or saturation. Further studies are required to establish the diagnostic relevance of broader colour-saturation patterns.