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| Background Information on CSNB Research |
History of Research on CSNB in the Appaloosa
CSNB in the Appaloosa was first investigated formally in the late 1970's by D.A. Witzel and associates. Witzel performed a series of experiments involving ERG's (electroretinograms) on small groups of related Appaloosas. He also looked at the structure of the retinas of several CSNB-affected Appaloosas.
Witzel found that Appaloosas with CSNB had normal day vision, but were totally blind at night. He also discovered that the inner eye structure of affected horses was normal in all respects. The CSNB-affected Appaloosas Witzel studied proved to have normal retinas with rods (night-time photoreceptor cells) that did function (as evidenced by the ERG recordings). However, the electrical message their rods attempted to send to the brain was not being received.
At the time, relatively little was known about about the variety of types of CSNB that occur in mammals, including humans. Based upon what he saw, Witzel proposed that the type of CSNB observed in the Appaloosa was inherited recessively. He did not base this on a large amount of inheritance data - he could not, for his sample was very small. Rather, the ERG results of affected Appaloosas were most similar to those found in humans with a type of CSNB called Schubert-Bornschein. Since Schubert-Bornschein CSNB is recessive, and the ERG's were a reasonable match, he assumed he was looking at an analogous disorder in the Appaloosa. To read Witzel's 1978 paper, click on the title:
Congenital Stationary Night Blindness: An Animal Model
The limitations of Witzel's research are important to consider. He looked at a very small number of horses, and most of them were related to each other. He also did not record or consider coat pattern as part of his study data. These facts likely contributed to the long period that followed, in which Appaloosa-specific CSNB attracted little attention from researchers. Perhaps those who read Witzel also assumed that it was caused by a recessive mutation. Also, since it seemed that there were not many affected animals (lack of reporting to veterinary ophthalmologists), there seemed to be little impetus to pursue the cause or find some form of treatment.
Anecdotal Evidence From Breeders
However, though few cases of Appaloosa-specific CSNB were being reported to veterinary ophthalmologists, breeders were making observations. Though they were not entirely sure just what their horses could or could not see, they noted that some horses were having trouble in dark conditions.
When the Appaloosa Project began to operate its first "Electronic Classroom" through a Yahoo-based site, it became possible for breeders to contact our research team directly through Sheila Archer, researcher and head moderator for the discussion group. Each time the topic of night blindness arose, a few more breeders would write privately to Archer and share their observations.
What they reported was that the horses most likely to display behaviour indicating they did not have normal night vision had coat patterns that indicated they were homozygous for LP, the main Appaloosa gene.
By 2004, Archer had gathered enough observations from breeders to see a significant correlation between coat pattern and poor night vision. This sort of information, though based on anecdotal evidence, had the potential to be very useful in the search for the identity of LP. Any strong relationship between traits that could help to narrow down the field of candidate genes was worth looking at. In addition, Archer had several Appaloosas that behaved in ways that suggested they too could not see at night, and all of them had homozygous coat patterns...
The 2004 Pilot Study
In the fall of 2004, Archer contacted Dr. Bruce Grahn of the Western College of Veterinary Medicine, University of Saskatchewan. Grahn is a senior figure in the field of veterinary ophthalmic research, with expertise in the identification of congenital abnormalities of the retina in the dog and horse.
Archer came to Grahn with a proposal - Would he be interested in examining some of her Appaloosas to see if they were night blind? Grahn agreed, and that October, the examinations were conducted at the University of Saskatchewan by Dr. Grahn and Dr. Carrie Breaux. Sheila brought two mares, a snowcap and a fewspot, that had no common ancestors in their extended pedigrees. Both horses underwent ERG's (electroretinograms) as well other forms of ocular examination. Their results were identical. Though each mare had normal day vision, they were both completely night blind.
Based on these results, Dr. Grahn, the newly-hired Dr. Lynne Sandmeyer (who became lead researcher for the study), Dr. Carrie Breaux and Sheila Archer decided to proceed with the design and implementation of a major study of night blindness in the Appaloosa. During 2005 and 2006, horses selected for the study were examined using ERG's and various other procedures. To read a summary of the results of the study, click here.
Abstract of 2005-06 Study
Where Does the Future Lead?
The form of CSNB found in the Appaloosa appears to be a good candidate for gene therapy. Unlike many genetic abnormalities, this is one that may not have to be permanent. This is because the necessary cells for night vision are present, and no other inner eye structural abnormalities exist.
Rather than missing parts, the eye is missing a required ingredient necessary to send the electrical message generated by the rods when they respond to light onward to the brain. If this situation can be changed, CSNB-affected horses might be made able to see in the dark.
The discovery of the causative mutation for CSNB in the Appaloosa is therefore the key to future research and the development of a potential treatment. We may also find that a treatment for CSNB in the Appaloosa will lead to better understanding and treatment of some types of CSNB in humans.
Looking to the future, there are many reasons to be optimistic! Though CSNB is a serious defect, it is not by any means lethal. Consider that horses affected by Appaloosa-specific CSNB have survived for thousands of years, as long as this mutation has existed, for certainly there have been homozygotes occurring in nature during that time. Consider also that most owners of CSNB-affected horses that took part in the CSNB study did not realize their Appaloosa was night blind!
That is how well-adapted affected these Appaloosas are to their condition. Even should there prove to be no treatment possible, they can continue to live productive lives with simple management strategies in place to maximize their well-being. What is truly amazing to all of us involved in this research is just how well they have carried on, right under our noses!
Created on 09/09/2007 10:02 AM by SheilaArcher
Updated on 11/01/2007 09:26 AM by SheilaArcher
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