Page 116 - August 2022

EQUINE HEALTH
types of overo Paints, particularly the splashed overo, and the frame/splashed combinations, and occasionally in frame overos. It is associated with those color patterns, particularly when
the horses have blue eyes and white on the face and legs. This is a common phenotype in Paint horses,” he says.
“The melanocytes—pigment-producing cells in the skin—originate from the neural crest cells in the embryo, and so do the melanocytes that play a role in hearing. The inner ear needs melanocytes to help it develop. Without them, the hearing-sensing cells degenerate. This is the link between coat color and hearing, since those cells originate from the same place,” he says.
“In overos, there is abnormal migration of some of the melanocytes and this is why there are white spots (lack of color) on the body—because the melanocytes did not migrate to those areas.
If that individual did not have melanocytes migrate to the inner ear, that horse will be deaf
if it happens in both ears.” Color-associated deaf horses are born deaf, whether in one ear or both.
Magdesian’s first experience with equine deafness was a Paint mare he raised when he
was a boy. She was calm as a foal and did not respond to loud noises. He owned her for 26 years until her death, and no one else could tell anything was wrong with her. She was shown at halter and ridden. He has examined other deaf Paints and began researching this syndrome, performing the BAER test on a number of horses, starting with his own mare. Magdesian ruled out other causes of deafness through careful ear and neurological examinations, blood work, and radiographs of her head.
In his study he looked at 14 deaf horses - confirmed deaf by BAER tests - and an additional 20 horses believed by their owners to be deaf. He documented color patterns and eye color. All the deaf Paints were overos or toveros and the most common pattern (65%) was splashed white— either by itself or in combination with frame overo. There were a few frame overos, solid white,
tovero and frame-sabino blends, but none of the horses were only tobiano or sabino. Even if the affected horses had
only small white areas,
such as a belly spot
and white socks, all of
them had extensive white on the head and at least a partial blue eye. Most had two blue eyes—a trait associated with deafness in other species.
To compare deaf Paint
horses with hearing horses,
Magdesian looked at Paint
and Pinto horses that could
hear with some had similar coat
patterns. “Phenotype alone cannot
predict deafness. Not all splash
overos are deaf, but the prevalence
of deafness is highest among horses with this pattern,” he says.
He also tested frame overo horses for the gene mutation that causes lethal white overo syndrome (LWOS). This gene is found in frame overos, and if doubled up (one from each parent), will result in a foal that cannot survive. The majority of deaf horses in his study (91%) were positive for this mutation, while only 43% of the hearing Paints he tested carried this gene. “This does not imply that the LWOS gene alone is responsible for the deafness but does show an association. The presence of more than one overo gene may increase the chances of deafness, but this requires further study,” says Magdesian.
Many of the deaf horses he examined that had classic splash overo - and no frame characteristics - also carried the LWOS gene. “It should also be emphasized that the LWOS gene is not necessary for a horse to be deaf, as 9% of the deaf horses did not carry the mutation. Interestingly, three lethal white foals were also hearing-tested prior to being humanely euthanized. All three were
homozygous for the LWOS gene and all were completely deaf,” he says.
Since deafness is an inherited defect in blue-eyed Paint horses, efforts should be made
to avoid this defect when selecting and breeding Paints. “Ideally, deaf horses should not be used in breeding programs,” says Stewart. She also recommends that pre-purchase examinations for Paints should include an assessment of hearing ability, and that judges at breed shows should assess hearing ability.
“Out of all the possible congenital conformational or temperamental defects a horse can be born with, deafness is a minor blemish,” says Stewart. Deaf horses can sometimes give their riders or handlers a great advantage in the show ring because they don’t get upset by noise of crowds, cars backfiring, passing trains, gun shots or sirens.
It is important that their owners or prospective buyers are aware of this disability; however, and realize they should never approach the horse from a position where
they can’t be seen. “These horses cannot be trained using voice commands but are just as intelligent and talented as horses with normal hearing. For instance, there was a famous white-faced sorrel reining stallion that was deaf. Although he passed this defect to several progeny, he has also passed on his athletic abilities. Prospective breeders and buyers of his foals are often eager to obtain one of his white- face blue-eyed deaf offspring,” says Stewart.
Lack of hearing has been found in certain types of overo Paints, particularly the splashed overo, and the frame/splashed combinations, and occasionally in frame overos.
Magdesian tested frame overo horses for the gene mutation that causes lethal white overo syndrome (LWOS), which is found in frame overos. If there is one gene from each parent, the foal cannot survive.
“Phenotype alone cannot predict deafness. Not all splash overos are deaf, but the prevalence of deafness is highest among horses with this pattern,” – Magdesian
114 SPEEDHORSE August 2022