Distichiasis is an ocular disorder which causes eyelashes to grow through the Meibomian glands of the eyelid. Therefore, a second row of eyelashes is formed, called distichiae.

Distichiae eyelashes differ from normal ones, they can be shorter, thinner, and less pigmented or thick and stiff. This can cause tearing, corneal irritation, keratitis, and corneal erosions or ulcers, which can impact ocular comfort and vision, and may lead to secondary infection. This may even lead to loss of the eye.

This test is based on a genome wide association study. This study described a genetic mutation that is associated with distichiasis in Friesian horses. The inheritance of this mutation is not yet fully understood. A recessive mode of inheritance is proposed, the penetrance of the mutation has yet to be further researched.