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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
Lordosis (L1, L2, L3, L4), also known as "Swayback", is a curvature or dip in the spine that is often seen in older horses. However, in the American Saddlebred, this condition also affects younger horses. These animals do not appear to experience pain from their condition and are still able to be used under saddle.
Malignant hyperthermia (MH) is a muscle disorder in which anesthesia, stress, or extreme exercise trigger a hyperthermic state. Symptoms include high temperature, increased heart rate, high blood pressure, sweating, acidosis, and muscle rigidity. If symptoms are not immediately resolved, death is likely to occur.
Myosin-Heavy Chain Myopathy (MY; previously "IMM") is a genetic muscle disease found most commonly in stock-type horses that can result in two different presentations, Immune-Mediated Myositis (IMM) and Nonexertional Rhabdomyolysis “tying up,” which are both characterized by muscle damage or loss. Both presentations are associated with the same genetic variant. Horses with MYHM may exhibit one or both presentations at different times in their lives, although some horses carrying the variant might not display any symptoms at all.
Myotonia (MYT) is a rare disorder involving a slowed relaxation of muscles after contraction. The most well-known example of myotonia is “fainting goats,” a breed that is characterized by sudden rigidy and/or falling over when startled. In the single documented horse, this resulted in a protruding third eyelid when excited, as well as problems with muscle stiffness.
