104. Distichiasis

Distichiasis, a rare condition in which extra or “double” eyelashes grow from eyelids, should be considered as a possible cause when diagnosing certain eye conditions in Friesian horses such as excessive blinking, excessive tearing/watery eyes or chronic corneal ulceration.

Distichiasis is rare in horses and is a result of unorganized and additional eyelashes that emerge from the eyelid margin. Distichiasis can cause serious corneal trauma and ulceration if left untreated. The most common treatment for distichiasis involves the removal of unnecessary eyelashes under general anesthesia. This is accomplished by destroying the lash follicle, through electrolysis or cryotherapy, to prevent further regrowth. Often multiple cycles of treatment are required to allow time for potential regrowth in order to ensure effective follicular destruction has been accomplished.

Studies conducted by Utter and Wottman in 2012 and Hermans and Ensink in 2014 have demonstrated a potential predisposition for distichiasis in the Friesian horse. In the Hermans and Ensink study, 17 of 18 cases were Friesian horses and 45% of horses affected with distichiasis had damage to the cornea. The study concluded the lower eyelid was more likely to be affected. Horses in the study received from one to six treatments before achieving resolution. Herman’s and Ensink assert distichiasis should be considered as a differential diagnosis in Friesian horses with blepharospasm (abnormal contraction of the eyelid muscles), epiphora (excessive tearing) or chronic corneal ulceration, and a closes examination of the eyelid margin should be performed with magnification to identify or rule out distichiasis.

Recently, UC Davis Veterinary Genetics Laboratory’s Genome Wide Association Study identified two loci for Distichiasis in the Friesian horse. The term loci refers to a specific, fixed position on a chromosome where a particular gene or genetic marker is located. The ordered list of loci known for a particular genome is called a gene map. Gene mapping is the process of determining the specific locus or loci responsible for producing a particular phenotype or biological trait.

References:
Distichiasis causing recurrent corneal ulceration in two Friesian horses. M. E. Utter K. L. Wotman, 2012.
Treatment and long-term follow-up of distichiasis, with special reference to the Friesian horse: a case series. Hermans H., Ensink J., 2014.
Adnexal Surgery. Freya M. Mowat, Joshua T. Bartoe, in Equine Surgery (Fifth Edition), 2019.

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