Risk factors for equine recurrent uveitis in a population of Appaloosa horses in western Canada

Objective: To characterize clinical manifestations, measure frequency, and evaluate risk factors for equine recurrent uveitis (ERU) in Appaloosa horses in western Canada. Animals: 145 Appaloosa horses. Procedures: Ophthalmic examinations were completed and eyes were classified as having no or mild clinical signs, or moderate, or severe damage from ERU. Clinical signs, age, sex, base coat color, and pattern were recorded. Whole blood and/or mane hair follicles were collected for DNA extraction, and all horses were tested for the leopard complex (LP) spotting pattern allele. Pedigree analysis was completed on affected and unaffected horses, and coefficients of coancestry (CC) and inbreeding (COI) were determined. Results: Equine recurrent uveitis was confirmed in 20 (14%) horses. The mean age of affected horses was 12.3 years (±5.3; range 3-25). Age was a significant risk factor for ERU diagnosis (OR_year = 1.15) and classification (OR_year = 1.19). The fewspot coat pattern was significantly associated with increased risk for ERU compared to horses that were minimally patterned or true solids. The LP/LP genotype was at a significantly greater risk for ERU compared to lp/lp (OR = 19.4) and LP/lp (OR = 6.37). Classification of ERU was greater in the LP/LP genotype compared to LP/lp. Affected horses had an average CC of 0.066, and there was a significant difference in the distribution of CC for affected horses versus the control group (P =.021). One affected horse was the sire or grandsire of nine other affected. Conclusions: Age, coat pattern, and genetics are major risk factors for the diagnosis and classification of ERU in the Appaloosa.