Link Between Appaloosa Coat Coloring and Night Blindness

New research by Dr. Lynne Sandmeyer, DVM, DVSc, DACVO has established that Appaloosas with coat patterns indicating they are homozygous for LP, the main gene responsible for Appaloosa spotting, are affected by congenital stationary night blindness (CSNB).

CSNB is an inherited disorder that is present from birth and does not progress over time. Affected horses suffer from a complete lack of night vision, yet their day vision is normal. In the late 1970’s W.A. Witzel first published his research on the subject, demonstrating that a form of CSNB occurs in the Appaloosa breed. However, no subsequent research had been performed to determine the prevalence of CSNB in the breed or its mode of inheritance.

Seeking to expand the understanding of this disease, Dr. Lynne Sandmeyer, Dr. Carrie Breaux, Ms. Sheila Archer and Dr. Bruce Grahn joined forces in 2005-06 to study CSNB in the Appaloosa. With financial assistance from the Equine Health Research Fund, the Western College of Veterinary Medicine, University of Saskatchewan team was able to conduct a large scale, comprehensive study. They aimed at investigating a possible association between CSNB and the leopard complex of white spotting patterns. They also sought to further characterize the clinical and electroretinographic aspects of CSNB in the Appaloosa.

The results of this study have just been published in the November 2007 issue of “Veterinary Ophthalmology”, the journal of the American College of Veterinary Ophthalmologists. The researchers confirmed a direct relationship between type of coat pattern and the presence of CSNB in the Appaloosa. “Horses with homozygous coat patterns are affected by CSNB, while those with heterozygous coat patterns have normal night vision,” says Sandmeyer. Coat patterning is classified according to probable genotype with respect to LP, the main Appaloosa gene. Coat patterns associated with homozygosity for LP typically have few or no spots in white areas, and include snowcaps and fewspots. Coat patterns associated with heterozygosity for LP have numerous spots scattered over white areas, and include spotted blankets and leopards.

Sandmeyer and associates also confirmed Witzel’s observations regarding the structure of the inner eyes of affected horses, which were found to be normal in all respects. However, abnormalities found in affected Appaloosas during ERG recordings (recordings of the electrical activity in the retina in response to light stimuli) support the hypothesis that CSNB is caused by a defect in neural transmission through the rod pathway involving the inner nuclear layer.

There is good news associated with this last finding – the defect that causes CSNB in the Appaloosa is an excellent candidate for gene therapy. "We may eventually be able to provide those cells with the material that they need to activate the physiological process and allow them to work at night," explains Dr. Sandmeyer. "That's many years down the road, but someday, we may have the means to help these horses."

Meanwhile, the search for the causative mutation continues. Led by chief molecular biologist Dr. Rebecca Bellone, this effort will ultimately confirm the nature of the relationship between Appaloosa spotting and CSNB. The LP mutation, responsible for Appaloosa spotting, may also be what causes CSNB. When the genetic mechanism that causes CSNB has been determined, a DNA test will become available and the development of a gene therapy treatment can begin.

What does this mean for Appaloosas and their owners? "CSNB-affected horses are healthy, viable and useful horses," states Sandmeyer. "This is a manageable condition, and if owners know their horses are affected by this disease, they can put measures into place that keep their horses - and their families - safe." Co-researcher Sheila Archer adds, “Most owners who participated in the CSNB study were surprised to learn their horse was affected. This tells us that these horses are very well adapted to their state. With a little help, we can make their lives even easier and safer.”