American Paint Horses with certain coat patterns and blue eyes appear to be at particular risk for deafness, reported researchers from the School of Veterinary Medicine, University of California-Davis.
"Deafness is infrequently recognized in horses. While there are a variety of mechanisms by which a horse can become deaf, there is little information regarding the possibility that a genetic mutation that causes spotted coat colors may also result in hearing impairment," explained study co-author K. Gary Magdesian, DVM, Dipl. ACVIM, ACVECC (emergency and critical care).
Genetic mutations that cause specific coat colors and hearing impairment have been identified in dogs, cats, and other species.
"To investigate whether coat color patterns were associated with deafness in American Paint Horses, we compared clinical findings, hearing test results (a brain auditory-evoked response test), and genotype of the endothelin B receptor gene (linked to a specific overo coat pattern) in both confirmed and suspected deaf American Paint Horses and nondeaf American Paint Horses and Pintos," relayed Magdesian.
Key findings in this study were that:
All 14 deaf American Paint Horses had abnormal brain auditory-revoked response tests;
All nondeaf horses had normal brain auditory-revoked response tests;
Most but not all deaf American Paint Horses had splashed white or splashed white-frame blend coat patterns;
Other coat patterns noted in deaf horses included frame overo and tovero;
Extensive head and limb white markings were observed in deaf horses;
Most deaf horses had two blue eyes;
Most deaf and suspected deaf horses (31 out of 34, or 91%) had the endothelin B receptor gene mutation, and
Deaf and suspected deaf horses could be used successfully in performance events.
"This study is important for owners and veterinarians to be aware that some American Paint Horses are more at risk for deafness than others and should be considered when performing prepurchase examinations, when handling deaf horses, and the potential for passing this trait on to offspring," concluded Magdesian.
The study, "Evaluation of deafness in American Paint Horses by phenotype, brainstem auditory-evoked responses, and endothelin receptor B genotype," was published in the November 15, 2009, edition of the Journal of the American Veterinary Medical Association.The abstract is currently available on PubMed.
Years ago, I read a report [I think it was from New Zealand] about deafness being associated with "Splash" overo Paints/Pintos. All the subjects in the study were related so it wasn't given too much consideration.
"Splash" is a color pattern that's thrown with "Frame" & "Sabino" into the overo category. Splash horses typically look like they've dunked their heads into a paint bucket.
The Tobianos that article mentioned must really be "Toveros". I'm guessing they have the Splash overo gene, too, since blue eyes aren't associated with Tobianos. [In theory, one horse could have ALL the Pinto pattern genes.]
I think it's interesting when you consider the other species that seem to have this problem, too. Typically, the pure white cat who also has both eyes blue is deaf. In dogs, a pup [usually from the Australian Shepherd, Collie, etc breeds] that has a solid white head & two blue eyes is deaf. It seems to be only associated with merles & the reason breeders warn to never mate two merles. That's telling me it's a recessive.
Hmm... In all these animals, I'm thinking that we're looking at some kind of "rider" associated with a recessive gene. What do you think?
Here's a couple of pics I found of Splash White [the gene that seems to be associated with deafness] horses:
See how it looks like that they're heads were dipped in white paint & the body white seems to start at the bottom, going up? Of course sometimes it can be hard to tell if a minimal color horse is really a Splash White or if he just has a very exaggerated bald face. [I'm not sure if they've isolated the Splash gene, yet, or not.]
Here's some sites with lots more Splash White pics:
Hmm... I was just reading up on Splash Whites. It was said that there are no homozygous Splash Whites so the homozygous foals are probably aborted in utero. That's somewhat like Frame Overos. In that case, the homozygous foals are "lethal whites" & don't survive.
Hmm... that means Splash is dominant - like all equine color modifiers & unlike the merle color pattern in dogs. So why are some deaf?
What bothers me is the cavalier attitude shown by many breeders, as well as the APHA [read the below article], about breeding deaf horses. Why create a problem??? [Of course APHA is one of the registries that won't even note HYPP status...]
Splashed white is the least common of the spotting patterns in horses, although it is increasing in frequency as breeders use more and more splashed white horses in their breeding programs. It occurs sporadically in a number of widely divergent breeds, such as Welsh Ponies, Finnish Draft Horses, Icelandic Horses and Paints.
The pattern usually makes the horse look as though it has been dipped in white paint. On a dark horse, the effect can be that of an ice cream cone dipped in chocolate. The legs are usually white, as are the bottom portions of the body. The head is also usually white, and the eyes are frequently blue.
The edges of the white are consistently crisp and clean, with no roaning. Some of these splashed whites have dark toplines, but on some the white crosses the topline.
The splashed white pattern was originally studied in Finnish Horses, and was reputed to be a true recessive pattern. If this were the case, the pattern would be unlikely to occur unless two splashed white horses mated, which is not the case.
Recent evidence is consistent with this pattern being caused by a dominant gene. The main problem in the past appears to have been that minimally marked horses were classed as nonspotted, which resulted in erroneous conclusions.
Some people have observed that many splashed white horses are deaf. This is not much of a problem if the trainer realizes the limitations of the horse in question, and many of these horses go on to lead full, normal and productive lives. If trainers rely on many verbal cues, though, these horses will be labeled as difficult, stubborn, and can be psychologically ruined by techniques inappropriate for a deaf animal.
No homozygous splashed white horses have ever been documented, leading to the suspicion that this is yet another gene that cannot exist in homozygous form. If this is true, the loss of hearing probably occurs early in gestation rather than at term, so this is distinct from the lethal white foal problem where defective foals are born.
The best strategy for splashed white horses, then, is to mate them to horses without the splashed white pattern.
Thanks for the correction, Wendy! Some of my dog info is woefully outdated.
Of course, it wasn't all that long ago that we thought the overo pattern in horses was recessive, too. Now, we know it's dominant but just so minimally expressed at times that we don't recognize it. The fact that merle is an incomplete dominant is pretty cool!
I wonder if a loose comparison could be drawn between deafness in merle dogs & the lethal white gene with homozygous frame overo horses?