Genetic Health Risks: Distichiasis (DIS)

When it comes to your horse's health, their eyelashes might not be the first thing on your mind, but that certainly doesn't mean they should be overlooked! A 2020 study identified a genetic variant associated with an inherited eyelash disorder, particularly in Friesian horses, known as distichiasis.1 In horses with distichiasis, extra eyelashes grow from where they shouldn't and can rub against the surface of the eye. This often causes irritation, discomfort, and, in some cases, vision problems. Discovering the genetic connection to this condition is a significant step toward a better understanding and long-term management of it.

Key Takeaways

• Distichiasis is a disorder where abnormal eyelashes grow from the eyelid margin, often irritating the cornea.
• Symptoms may include tearing, squinting, inflammation, or corneal ulcers.
• A genetic variant has been linked to the condition and is believed to affect regulatory elements between two genes involved in tissue growth and maintenance.
• Genetic testing can identify affected horses and carriers, supporting breeding decisions and facilitating early intervention.

What is Distichiasis?

Distichiasis is a congenital condition in which extra eyelashes, called distichia, grow from the openings of the meibomian glands along the eyelid margin. These glands normally secrete lipids that form the outermost layer of the tear film, helping to lubricate and protect the eye. In horses with distichiasis, eyelash follicles develop within the gland openings rather than below them. The resulting lashes may be fine and lightly pigmented or thick, coarse, and abrasive. Because they often point inward toward the eye, they can rub against the cornea, causing irritation or damage. While some horses may show no visible symptoms, others can develop chronic discomfort or secondary issues, such as corneal ulcers, if the condition is left untreated.

Symptoms of Distichiasis

The clinical signs of distichiasis depend on the number, stiffness, and position of the abnormal lashes.

**Symptoms may include: **

• Excessive tearing
• Red or irritated eyes
• Squinting or frequent blinking
• Light sensitivity
• Eye discharge
• Corneal ulcers or cloudiness in more advanced cases

Some horses may remain asymptomatic, especially if the lashes are sparse and soft, while others may require treatment to avoid chronic discomfort.

Diagnosis

A veterinarian can diagnose distichiasis through a thorough ophthalmic examination. Magnification and specialized lighting help identify the tiny lashes growing from the meibomian gland openings. Fluorescein staining may be used to detect corneal damage.

Treatment

Treatment options depend on the severity and may include:

• Manual removal of lashes (temporary relief)
• Cryotherapy to destroy the hair follicles
• Electroepilation (electric current to eliminate lash roots)
• Surgical removal in more severe or recurring cases

Early diagnosis and management help prevent secondary issues, such as infection or long-term corneal scarring.

Genetic Risk Factors for Distichiasis

The 2020 BMC Genomics study identified a 16-kilobase (kb) deletion on equine chromosome 13 (ECA13:g.178714_195130del) associated with distichiasis in Friesian horses. This deletion is located between the two genes FAM20C and PDGFA and is likely to affect a regulatory region of the genome. Although the precise biological mechanism is still under investigation, this deletion is thought to disrupt normal gene regulation during eyelid development.

Distichiasis has also been identified in other breeds, but the relationship between the same deletion and clinical symptoms remains unclear outside of Friesians. Further research is needed to determine both the prevalence and the expression of the trait in non-Friesian populations.

Genetic Testing for Distichiasis

Etalon Equine Genetics offers a genetic test to detect the distichiasis-associated variant. Understanding a horse's genetic status can help guide breeding decisions and help owners monitor their horses for any potential clinical signs.

**dis/dis = Homozygous for the deletion. Increased likelihood of expressing the trait. dis/n = Heterozygous. Carrier of the deletion but typically not symptomatic. n/n = No variant detected. Unlikely to pass on or express the condition. **

Testing is particularly valuable for Friesian breeders and anyone with a horse showing signs of eye discomfort.

Prognosis

With early diagnosis and appropriate treatment, the prognosis for horses with distichiasis is generally good. Many cases are manageable with regular veterinary care. Severe or recurring cases may require more complex procedures, but they still tend to have favorable outcomes with proper treatment.

Prevention

There is no known way to prevent distichiasis from developing in horses that are genetically predisposed. However, genetic testing is a powerful preventive tool in breeding programs. Ultimately, the only way to prevent distichiasis is by avoiding the mating of two carriers, which reduces the risk of producing foals homozygous for the deletion and potentially affected by the condition.

Conclusion

Distichiasis may be relatively rare, but for horses that have it, the consequences can be irritating at best and vision-threatening at worst. Thanks to recent genetic research, we now have a clearer understanding of the cause in Friesians and a reliable way to test for the associated variant.

Whether you're a breeder aiming to improve your herd's health or an owner seeking answers for your heart horse, our Pro Package 2.0 includes the distichiasis (DIS) DNA test, providing clarity, insight, and what we all want – peace of mind.

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References

1 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