D. Phillip Sponenberg, DVM, PhD
Virginia-Maryland Regional College of Veterinary Medicine
Virginia Tech, Blacksburg, VA

An increasing number of defects in horses are known to have genetic origins, and managing these can be very important in order to produce sound, healthy horses. Specific strategies vary with the defect, the breed, and the mode of inheritance. A few examples can help.

D. Phillip Sponenberg

D. Phillip Sponenberg is Professor of Pathology and Genetics at the Virginia-Maryland Regional College of Veterinary Medicine. He teaches pathology, reproduction, genetic resource issues, and small ruminant medicine. His interest in coat color genetics includes horses, donkeys, sheep, goats, dogs, and other species. Dr. Sponenberg is the author of the definitive book on coat color genetics in horses, Equine Color Genetics.

HERDA, GBED in Quarter Horses
These are both recessive, and both lethal or nearly so. At any rate, a serviceable horse is impossible with these traits. One strategy would be to disallow registration of any carriers. Equally effective, and market-driven, is to insist on the testing of stallions that produce over five foals a year. These stallions are the most likely to spread any recessive disease, and by knowing their genotype it is possible to avoid mating carrier to carrier. Remember – when an animal is culled for a single trait, all of the other traits go as well!

HYPP in Quarter Horses
This is a dominant trait, and is related to the heavy muscling that is sought in halter horses. The frequency of this trait has not gone down, even though a test for it has been available for many years. This is because the showring still selects this type as superior, and it then becomes in the economic best interest of a breeder to continue to use carriers of this trait. The only way this can change is for the trait to become negative instead of positive – and that will likely take a redirection of show ring excellence.

Ocular Dysplasia, Rocky Mountain Horses
Ocular dysplasia appears to be a consequence of a specific color gene, silver dapple. This is a dominant gene, and changes color in heterozygotes. In homozygotes it also changes the eyes, and in severe cases it limits the visual ability of the horses to the point that they make poor choices for saddle horses.

This gene can be managed in populations so that it only occurs in heterozygotes. In order to do this, specific breeding restrictions must be placed on silver dapple horses. These horses should only be mated to horses not carrying the silver dapple gene. In order to be certain of this, those choices should generally be limited to bay, brown, and black horses. The gene does not betray itself in chestnut horses, or in colors derived from it. It also can be trickier to spot in duns, buckskins, and champagnes than it is in darker colors. The safest route, therefore, is to mate silver dapple to dark, black pointed colors. This is only important in those breeds in which this color is common, such as Shetland Pony, Miniature Horse, Rocky Mountain, and Mountain Pleasure horses.

JEB in Belgian and American Cream Horses
Junctional Epidermolysis Bullosa is a recessive disorder in Belgian and American Cream Draft horses. This is a lethal condition in which the skin of foals becomes hopelessly damaged and they need to be put down soon after birth. A DNA-based test is now available, so it is possible to determine if a given horse is a carrier or not.

With the existence of a DNA test it becomes possible to identify and potentially eliminate carriers of this disease. In a relatively common breed, like the Belgian, the removal of carrier horses from reproduction has minimal affect on the breed and its overall population health. In a rare breed, such as the American Cream Draft Horse, the result of removing all carriers is to remove several bloodlines in a breed that is already very restricted. In this case it makes more sense to insist on testing, but then to not insist on removal of carriers. Instead, they can be mated to noncarriers in order to eliminate the chance of an affected foal. Selection against the gene itself can then be safely postponed until breed numbers can absorb the loss of breeding animals.

Chestnut in Cleveland Bay and Friesian Horses
Cleveland Bay and Friesian Horses have very stringent rules for color, and neither breed registers chestnut foals. In both breeds, though, there are carriers of chestnut. With DNA tests now available it is possible to detect carriers. That information could lead to elimination of carriers, which is detrimental to the overall genetic health of these rare breeds. Or it could be used to direct matings so that carriers are not paired, and therefore the off-standard foals will never occur.

As more and more DNA-based tests become available for diseases as well as colors it will be increasingly important for breed societies to decide if the standard should be “freedom from disease” or “freedom from the gene.” Those two situations are very different!

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