Friesians
Some Friesian horses inherit a variant for misplaced eyelashes that grow from an abnormal position, often toward the eye itself, rather than outward. If these misplaced lashes contact the cornea of the eye, they can cause pain, discomfort, impaired vision, and corneal ulceration. Researchers have recently identified the DNA change responsible for the condition, known as distichiasis, and hope Friesian breeders will aim to avoid it in their production plans.

Now Friesian owners know which genetic variant to look for in their mares and stallions before breeding them to prevent these health and welfare problems, said Rebecca Bellone, PhD, Genetics Laboratory director at the University of California, Davis.

“Distichiasis is the third genetic disorder in Friesian horses for which a genetic basis has been identified,” Bellone told The Horse. “I am hoping that Friesian breeders will use this test along with that for hydrocephalus and dwarfism as a tool to select mate pairs to lower the incidence of these genetic disorders in the breed.”

Eyelashes usually grow out of follicles situated just under the Meibomian glands, which secrete lipids that help keep the eye from drying out, Bellone said. With distichiasis, the eyelash follicles are situated within the Meibomian gland orifice rather than under it—a problem that develops in the embryonic stage. Distichiasis-associated eyelashes are sometimes thin and less pigmented, but they can also be thick, stiff, and abrasive, and they can grow straight up or angle slightly back, coming into direct contact with the cornea.

While distichiasis can occur in other breeds—as well as humans and other animals—it has been reported most frequently in Friesians, said Bellone. She said higher levels of inbreeding in these horses might have allowed for increased frequency of recessive disorders. If both parents have the recessive variant, their foal has a chance of developing physical signs of the condition, she explained.

Bellone and her fellow researchers determined that 30% of known distichiasis cases in Friesians could be traced back to a single stallion, which was either the sire or grandsire of all those cases, she said.

Through a study using genomic tools and 14 affected horses and 38 Friesian controls, the team narrowed the condition down to a single mutation—a 16 kilobase deletion on horse chromosome 13. Then they looked for that mutation in 955 more horses representing 54 other breeds; Bellone said they only found it in 11 horses. All those horses had only one copy of the gene, meaning they were carriers of the recessive trait, which would not result in physical signs of distichiasis.

While this work identifies the variant responsible for distichiasis, further research would need to investigate how and why the mutation affects eyelash development, she said. In the meantime, current and prospective owners of Friesians can test horses to see if they carry copies of the gene.

Horses with a single copy of the gene should not be bred to each other, Bellone advised. Horses with both copies of the gene merit careful veterinary examination to manage any eye issues as early as possible.

“The test can be used by Friesian owners to identify which horses should be examined by a veterinary ophthalmologist for signs of aberrant lashes that, if left untreated, could lead to corneal ulcers or corneal scaring,” Bellone said.

 

Hisey, E.A., Hermans, H., Lounsberry, Z.T. et al. Whole genome sequencing identified a 16 kilobase deletion on ECA13 associated with distichiasis in Friesian horses. BMC Genomics 21, 848 (2020). https://doi.org/10.1186/s12864-020-07265-8