What is distichiasis?
- Distichiasis is an eye condition in which eyelashes grow from an abnormal position along the eyelid.
- Distichiasis can cause irritation and ulceration of the corneas, which can affect vision.
- Distichiasis is rare in horses, with most cases reported in the Friesian breed.
- An associated genetic variant has been identified in Friesians and a genetic test is available to assist in mate selection and avoid producing affected offspring.
- Horses with two copies of the identified genetic variant should have regular eye exams to monitor for signs of clinical disease that could affect vision.
Distichiasis is a disorder of the eye in which the eyelashes grow from small glands that line the edges of the eyelids. These glands - the Meibomian glands - usually have a role in tear production, and do not normally produce hair. The abnormal positioning of these eyelashes can cause them to contact the cornea (the clear ‘window’ at the front of the eye), leading to irritation and corneal ulcers, which can affect vision. Distichiasis can occur in one or both eyes. The majority of reported cases have occurred in Friesian horses, with 30% of cases tracing back to a single common ancestor within two generations.
Distichiasis in Friesian horses is inherited as an autosomal recessive condition with incomplete penetrance. This means that affected horses have two copies of the associated genetic variant, but not all horses with two copies will show clinical signs. A genetic test is available for distichiasis in Friesian horses. To date, this genetic variant has not been associated with disease in other breeds.
What are the clinical signs of distichiasis?
Clinical signs of distichiasis in horses can include squinting, abnormal contraction of the eyelid muscles, excessive tearing, corneal ulcers (disruption of the surface layer of cells of the cornea), corneal inflammation, and scarring. Secondary infections may also occur. In some cases, these signs can lead to pain and vision loss. Some affected individuals may show no clinical signs.
How is distichiasis diagnosed?
Distichiasis is diagnosed based on the presence of abnormal lashes and clinical signs consistent with the disease. In Friesian horses, diagnosis may be confirmed through genetic testing.
How is distichiasis treated?
The abnormal eyelashes may be removed under general anesthesia and the follicle destroyed by electroepilation or cryotherapy to try to prevent regrowth. Since some of the abnormal eyelashes may be too short to be visualized at a given time, repeated surgeries may be required. Removal of a section of tissue containing affected glands and hairs may provide a more permanent solution in some cases. Infections secondary to ulceration can necessitate removal of the eye. In cases with multiple aberrant lashes that cause irritation or ulceration, the recurrence rate is high, despite treatment.
What is the prognosis for distichiasis?
The prognosis for horses diagnosed with distichiasis is good. Distichiasis is not a life-threatening disease.
How can distichiasis be prevented?
The genetic test for distichiasis can be used in Friesian horses to assist in mate selection and avoid producing affected offspring.
Horses with two copies of the identified genetic variant should have regular eye exams performed by a veterinary ophthalmologist to determine if abnormal lashes are present and monitor for signs of clinical disease that could affect vision.
For more information
UC Davis Veterinary Genetics Laboratory Distichiasis genetic testing
Hisey, E.A., Hermans, H., Lounsberry, Z.T., Avila, F., Grahn, R.A., Knickelbein, K.E., Duward-Akhurst, S.A., McCue, M.E., Kalbfleisch, T., Lassaline, M.E., Back, W., & Bellone, R.R. (2020). Whole genome sequencing identified a 16 kilobase deletion on ECA13 associated with distichiasis in Friesian horses. BMC Genomics, 21, 848. doi: 10.1186/s12864-020-07265-8
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