Relationship between coat color and response to a standardized dose of detomidine in adult warmblood horses

Abstract

Introduction: The melanocortin-1 receptor (MC1R) determines coat color in horses. Chestnut-colored horses carry a non-functional MC1R gene due to a mutation. The equine MC1R gene shows a high degree of similarity to the human MC1R gene. Human studies comparing red-haired and non-red-haired individuals have produced inconsistent results and comparable equine studies are lacking. At the Equine University Clinic in Utrecht there is an anecdotal impression that chestnut-colored horses require higher doses of detomidine. This study aimed to evaluate whether this impression could be scientifically substantiated.

Materials and methods: Forty warmblood horses (11 chestnut-colored horses and 29 horses with other coat colors) were included. Each horse received 5 micrograms of detomidine (Domosedan®) per kilogram body weight intravenously. Five minutes after administration, the horses were filmed for 30 seconds. Three veterinarians independently scored the videos and the mean of their scores represented each horse’s final sedation score.

Results: Inter-observer agreement was excellent. There were no significant differences in gender, age or body weight between the two groups. Chestnut-colored horses had a mean sedation score of 3.58 and horses with other coat colors had a mean score of 4.23, with higher scores indicating greater sedation depth. The p-value (0.38) indicated no statistically significant difference between the two groups.

Discussion: The findings suggest no statistically significant difference in sedation depth between chestnut-colored horses and horses with other coat colors following intravenous administration of a standardized dose of detomidine. Human studies have reported variations in hypnotic response, analgesic sensitivity and pain sensitivity between individuals with red hair and those without, but comparable research in horses is lacking. Future studies should include larger sample sizes or include only one gender, use standardized low-stress filming conditions and ensure adequate video quality. Genotyping all horses is also recommended.

Conclusion: This study found no evidence that chestnut-colored horses respond differently to intravenously administered detomidine.