Distichiasis / extra eyelash row (ECA13 marker) - Friesian horse

What does this test examine?

This genetic test examines the 16 kb deletion marker on ECA13 used for distichiasis in the Friesian horse. Distichiasis is also described as distichia, an extra row of eyelashes or abnormally positioned eyelashes along the eyelid margin.

What does distichiasis mean?

In distichiasis, eyelash-like hairs grow from an abnormal position at the eyelid margin. These hairs can irritate the cornea and conjunctiva. Horses may show tearing, squinting, chronic irritation, keratitis or corneal ulcers. Some horses show few obvious signs, while others develop clinically important eye problems.

Practical value of this test

• The test helps distinguish Friesian horses with the N/N, N/Dis or Dis/Dis genotype.
• Breeders can plan matings more deliberately and reduce the chance of foals with two copies of the marker.
• A Dis/Dis result makes targeted eye examination especially relevant, because incomplete penetrance is possible and not every horse shows signs to the same degree.
• The result supports breeding selection in lines where distichiasis occurs.

Inheritance and result

The marker is used within an autosomal recessive model with incomplete penetrance. Two copies of the distichiasis marker increase the chance of clinical distichiasis, but severity and visibility can differ between horses. A carrier with one copy usually does not show obvious signs, but can pass the marker to offspring.