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Gene or Region: chr13:178714-195130
Reference Variant: No deletion
Mutant Variant: 16kb deletion on chromosome 13
Affected Breeds: Friesian
Research Confidence: High - Strong association in studied population
Explanation of Results: dis/dis = homozygous for Distichiasis, increased chance of trait expressed dis/n = heterozygous for Distichiasis, carrier n/n = no variant detected
Distichiasis is a condition in which abnormal eyelashes—called distichia—emerge from the Meibomian glands along the eyelid margin. These glands normally produce a part of the tear film, not hair. When distichia are present, they may come into contact with the surface of the eye (the cornea), leading to irritation, excessive tearing, squinting, or, in more severe cases, corneal ulcers and vision loss. While some horses show clear signs of discomfort, others may have no symptoms at all, making the condition easy to overlook without a thorough exam.
Distichiasis is inherited as an autosomal recessive trait with incomplete penetrance—meaning a horse must have two copies of the variant (Dis/Dis) to be at risk, but not all will show signs of disease. For Friesians with this genotype, regular eye exams by a veterinary ophthalmologist are recommended to monitor for aberrant lashes or vision-threatening changes.
Researchers have identified a 16-kilobase (kb) deletion on chromosome 13 (ECA13:g.178714_195130del) associated with this disorder. This deletion is located between the FAM20C and PDGFA genes and is believed to affect a regulatory region of the genome, though the exact mechanism leading to Distichiasis remains under investigation. Genetic testing is available to detect this variant, aiding in breeding decisions and early clinical interventions to manage or prevent the condition. Distichiasis has also been found in other breeds not related to Friesians. Further research is needed to determine the frequency and effect in other breeds.
NOTE: This variant has been seen in other breeds at very low levels, and all horses were carriers. In breeds other than Friesians it is currently unclear if this variant is causing Distichiasis. More research is needed for these breeds.
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
"Warmblood" fragile foal syndrome (FFS) is a connective tissue disorder resulting in joint laxity and extremely thin skin that is only loosely connected to the body. The skin is easily torn, resulting in lacerations, hematomas, and seromas across the foal. Affected foals are euthanized shortly after birth.
Androgen Insensitivity Syndrome (AIS) is an X-linked disorder of sexual development resulting in a female horse with XY chromosomes. Horses with AIS exhibit stallion-like behavior such as agression toward other horses, Flehman response and vocalization toward cycling mares.
Cerebellar Abiotrophy (CA) is a degenerative neurological disorder, due to the death of neurons in the brain. Symptoms (head tremors, lack of coordination, wide stances, exaggerated gain, difficulty rising and startling easily), typically appear in foals between six weeks and four months of age.
Chronic Idiopathic Anhidrosis Risk (CIA) is the inability to sweat in response to increased body temperature, (AKA "non-sweater"). This condition is dangerous and sometimes life-threatening for horses, who rely on sweating for 65-70% of their temperature regulation.
