Genetic Disease in the Horse

Confusion often surrounds the terminology used to describe birth defects in horses and other animals. The terms “congenital” and “genetic” are used interchangeably by many. Actually, “congenital” merely refers to any defect present at birth. It may indeed be due to a genetic or inherited problem, but it also describes defects due to non-genetic causes, such as developmental problems caused by toxins, infection, poor uterine environment, or malposition. Genetic defects include anything that is due to a change in the DNA that affects development so that the foal is not normal when born. Sometimes a new mutation occurs in the embryo’s DNA that causes a problem but is not inherited from either parent. Other genetic defects are indeed inherited from one or both parents.

Compared to other species such as the dog, the horse has a comparatively low incidence of inherited disorders. New technologies have resulted in identification of the actual mutations, which permits breeders to screen their animals and identify carriers through DNA analysis.

One of the first recognized inherited disorders in the horse was Severe Combined Immunodeficiency (SCID) in the Arabian horse. Affected foals die within the first few months of age due to the inability to fight infection. It is estimated that about 3% of Arabian foals were born with this condition. Inherited as an autosomal recessive, both parents must be carriers. Since carriers appear healthy and normal, the development of a DNA test that detects the mutation allows the breeder to avoid carrier-to-carrier matings.

Another inherited disorder, which affects extensive numbers of Quarter horses descended from the stallion Impressive, is hyperkalemic periodic paralysis (HYPP). Unlike SCID, this is a dominant disorder. Only one affected parent is needed to pass the gene on to the foal. The mutation causes a defect in a sodium channel gene that regulates the movement of sodium into and out of muscle tissue. Horses with HYPP experience muscle tremors and can become temporarily paralyzed. Severe attacks can lead to heart and respiratory failure, resulting in death. Horses with two copies of the defective gene (homozygous) usually do not survive very long. Horses with one copy of the mutation can lead functional lives if fed a controlled diet. In fact, the disease causes enlargement of the muscles in these horses, which gives them an advantage in halter classes at shows. Again, a DNA test is available to determine which horses carry this mutation, and matings that would produce a foal homozygous for the mutation can be avoided.

A third heritable disease for which a DNA test has been developed also occurs in Quarter horses and Paint horses. Glycogen Branching Enzyme Deficiency (GBED) is a fatal disease affecting foals. The mutation causes a lack of the functional form of the enzyme needed to properly store sugar molecules as glycogen. Tissues that require glycogen as an energy source, such as heart muscle, skeletal muscle, and brain, cannot function properly. Foals are stillborn or die at a very young age.

The Overo Lethal White Syndrome was the bane of Paint horse breeders. When two carriers are mated, a white foal, or nearly all-white foal, might be produced that dies or needs to be euthanized within days of birth. These foals have abnormalities of the intestine that prevent them from passing feces. A DNA test is also available for this disorder so breeders can determine which horses are carriers.

A DNA test is not yet available for Equine Hyperelastosis Cutis, which occurs in Quarter horses descended from the Poco Bueno/King bloodline. It does not occur in all horses descended from these sires, and its expression is variable, with some horses being more severely affected than others. Skin layers in affected horses are not attached normally, creating loose areas of skin that are easily stretched away from the body. This makes the skin very susceptible to trauma, and the condition is often initially diagnosed when the horse goes into training and is subjected to saddle pressure. There is no treatment, and currently there is no genetic test.

Fortunately, the technology exists to eventually identify the mutation for this disorder as well as other genetic diseases. While it may never be possible to prevent congenital defects that are due to novel mutations that occur during development or due to environmentally induced defects, we do have the tools to assist breeders in preventing the predominant inherited diseases that are recognized in horses.

By Dr. Kathryn T. Graves, Equine Parentage Verification and Research Laboratory, University of Kentucky.
Equine Disease Quarterly newsletter, Gluck Equine Research Center, University of Kentucky College of Agriculture