The University of California has developed a DNA test for distichiasis. This test makes it possible to phase out this eye condition, which involves aberrant lashes on the edges of the eyelids that can cause damage to the cornea, via breeding. The Dr Van Haeringen Laboratory is working towards making the test available in the Netherlands and the KFPS is investigating how to implement the test in breeding.
Ruling out carrier matches
Distichiasis is relatively more common in the Friesian breed than in other horse breeds. This eye condition can cause a variety of complaints, from excessive discharge to corneal ulcers, but is rather easy to treat by lasering of the lashes. The hereditary disorder is single recessive and therefore has the same hereditary pattern as hydrocephaly, dwarfism and the chestnut factor. Carriers (Aa) show no deviation. In 25% of the cases a match between two carriers leads to an affected (aa) horse. When combining two carriers the chance of a double carrier, an affected horse, is 25%.
Not always resulting in eye complaints
Distichiasis does however, differ from other hereditary disorders in one aspect. Horses that possess both unfavourable gene varieties (aa), the ‘double carriers’, don’t have and don’t develop eye complaints. The American research has shown that only in a few cases these affected individuals develop distichiasis. The KFPS is investigating the extent of carriers in the Netherlands and what percentage of the affected horses do indeed develop the condition. This research will be followed up with policy, for example potential testing of stallions.