Distichiasis is a rare genetic condition that occurs in Friesians in which unorganized and additional eyelashes grow from the eyelid margin. The most common symptom of Distichiasis is excessive blinking, excessive tearing/watery eyes, chronic corneal inflammation and corneal ulceration. Distichiasis should be considered as a possible cause that is important to rule out when diagnosing eye conditions in Friesian horses.
If left untreated, Distichiasis can cause serious corneal trauma and ulceration. The most common treatment for Distichiasis involves the removal of unnecessary eyelashes, typically 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. The number of treatments needed is often relative to the degree the horse is affected with Distichiasis.
Recently, UC Davis Veterinary Genetics Laboratory conducted a Genome Wide Association Study (GWAS) which 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. Through their research, the team identified a deleted portion of a specific chromosome that was associated with Friesian horses that presented with positive clinical signs of Distichiasis. The researchers believe this “deletion” likely impacts how this area of the genome is regulated.
UC Davis’s study determined that Distichiasis is inherited as an autosomal recessive trait with incomplete penetrance. This means that two copies of the variant (one from the father and one from the mother) are required to cause disease. What is especially interesting is that not all Friesian horses that have two copies of the mutation show clinical signs of the disease. This is important to note as homozygous carriers for Distichiasis may go unidentified without testing, yet they will pass the gene variant on to 100% of their offspring. The study also determined the allele frequency (how often it appears in the population) was much higher for Distichiasis (32%) compared to hydrocephalus (8%) and dwarfism (6%).
Potential Test Results
N/N: Horses without the genotype (N/N) cannot develop distichiasis and cannot transmit the variant to their offspring.
N/Dis: Carriers (N/Dis) with one copy of the variant will not show signs of the disease but can transmit the distichiasis variant to 50% of their offspring.
Dis/Dis: Horses that are homozygous and have two copies of the variant (Dis/Dis) and will transmit the distichiasis variant to 100% their offspring but may be clinically negative showing no signs of the disease. Homozygous horses can develop distichiasis at any point so they should have regular eye exams to check for signs of the disease.
The Next Steps
The University of Kentucky, FHANA’s official genetics lab, now has a test available for Distichiasis. The KFPS is also currently working to develop a test for distichiasis in the Netherlands. At this time there have not been enough horses tested for distichiasis to allow the KFPS to issue guidelines for how to use the test when making breeding selections. While the research conducted by UC Davis indicates there are a relatively high number of Friesian horses in the population with the gene variant (greater than 30%), there is not enough data to determine how often the variant expresses itself.
Once a larger number of Friesian horses have been tested and examined for clinical signs of the disease, the KFPS will be able to issue recommendations on how best to use the test results as it relates to breeding. In the interim, horses that test homozygous for the associated variant (Dis/Dis), even if they show no current signs of the disease, are recommended to have regular ocular exams performed by a veterinary ophthalmologist to determine if aberrant eye lashes are evident and monitor for other clinical sings of Distichiasis that may threaten vision.
Do I need to test my horse for distichiasis? Currently that is a decision each owner should make personally. The KFPS has not issued any requirements to test breeding horses at this time. If your horse has been experiencing issues with its eyes testing may be appropriate. You should consult with your veterinarian and consider whether the test is advisable.
How can I order this test? For more information on how to order this test through FHANA’s official lab at the University of Kentucky, please contact the FHANA office.
Will distichias test results appear under my horse’s profile on the FHANA or KFPS website? No, distichiasis test results will not appear on your horse’s profile on the FHANA or KFPS website. In the future, if the KFPS requires testing of any horses, it is possible for your results to be recorded in the database. Please save a copy of your test results from the University of Kentucky in the event they are needed.
What should I do if my horse tests as a carrier for distichiasis? Carriers have one copy of the variant and will not show signs of the disease but can transmit the distichiasis variant to 50% of their offspring. At this time the KFPS has not made any recommendations on how to use test results from carrier horses to make breeding decisions as more data is needed.
What should I do if my horse tests homozygous for distichiasis? Homozygous horses have two copies of the variant and will transmit the distichiasis variant to 100% their offspring. It is very important that homozygous horses receive regular eye examinations check for signs of distichiasis as they may develop the disease at any point. At this time the KFPS has not made any recommendations on how to use test results from homozygous horses to make breeding decisions as more data is needed.
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