Distichiasis is an ocular disorder characterized by the abnormal growth of eyelashes along the edges of the eyelids. These misplaced lashes may contact the cornea (the clear outer windshield of the eye responsible for focusing light that enters the eye), which can cause pain and disruption of the corneal surface. Distichia (aberrant lashes) grow from Meibomian glands, which are small glands that line the eyelid edges. Meibomian glands normally do not produce hair; their normal function is to produce a component of the tear film.
These aberrant lashes can cause a variety of clinical signs, including excessive tearing, squinting, corneal ulceration (disruption of the surface layer of cells of the cornea), or corneal inflammation and scarring. This can lead to chronic irritation and ocular pain, or if complications from corneal ulceration occur, damage to the eye may cause the horse to lose vision or necessitate removal of the eye. There are some individuals that have no signs accompanying the abnormal lash growth and thus this disorder may go undetected.
The genetic variant associated with distichiasis was recently discovered by an interdisciplinary team lead by researchers at the VGL. Researchers identified a 16kb deletion on the chromosome ECA13 (ECA13:g.178714_195130del), located in between the genes FAM20C and PDGFA, that was associated with the disease phenotype. It is hypothesized that this deletion likely impacts a regulatory region of the genome but the functional consequences of this deletion and the mechanisms by which it gives rise to distichiasis have not yet been elucidated.
Distichiasis is inherited as an autosomal recessive trait with incomplete penetrance. This means that 2 copies of the variant are required to cause disease, but that not all individuals that have 2 copies of the mutation will show the disease phenotype.
Horses who test homozygous for the associated variant (Dis/Dis) are recommended to have regular ocular exams performed by a veterinary ophthalmologist to determine if aberrant lashes are evident and monitor for other clinical sings of disease that may threaten vision.
The test is appropriate only for the Friesian breed. This DNA test can assist in mate selection to reduce the potential of producing an affected horse and can also identify which horses should be evaluated by an ophthalmologist for presence of disease.
This test can be ordered as a single test or as part of the Friesian Health panel, which also includes hydrocephalus and dwarfism, to determine if their breeding stock are carriers for any of these mutations or assist in confirming clinical diagnoses.