Appaloosas sport spotting patterns that can be as subtle as a half-halt or as wild as the freckles on Little Orphan Annie’s face. The gene responsible for these patterns is the Leopard complex spotting, dubbed LP and located on equine chromosome 1 (ECA1). And because Appaloosas are eight times more likely to develop equine recurrent uveitis (ERU) and four times more likely to go blind than other breeds, researchers have long suspected a genetic predisposition for these spotted horses to develop these conditions.

“Equine recurrent uveitis occurs when there is recurrent inflammation along the uveal tract in the eye and is a leading cause of blindness in horses, especially Appaloosas,” explained Molly McCue, DVM, MS, PhD, Dipl. ACVIM, associate professor at the University of Minnesota's College of Veterinary Medicine. “This breed is commonly affected with a particular type of ERU, referred to as 'insidious ERU' because it causes a persistent, low-grade inflammation with a gradual and cumulative destructive effect rather than painful episodes of uveitis.”

Although there is some evidence that ERU is an autoimmune disorder, there is also evidence to support an underlying genetic basis for ERU in Appaloosa horses. To investigate this latter theory more closely, McCue and colleagues collected DNA from 53 Appaloosas with ERU and 43 healthy Appaloosa controls.

The team evaluated the horses' genetic markers (regions of DNA previously identified as potentially related to ERU), specifically scanning their DNA to see if those markers were present in horses with ERU but absent in ERU-free ones