Description

Genetic conditions often affect both the health and the physical condition of the foal or horse. Most genetic conditions appear to be inherited from one or both parents.

Genetic defects include any abnormality that is due to a change in the DNA that affects development, such as a new mutation occurring in the embryo's DNA that causes a problem, but is not inherited from either parent.

With scientists working to reveal equine genomic information and how to use molecular tools to prevent and/or heal inherited or DNA-based diseases, a number of breakthroughs can be expected during the next few years.

Inheritance of genetic disorders depends on the transmission of dominant genes and recessive genes. With dominant genetic disorders, only one copy of the defective gene is required for the foal to have the disorder.

With recessive genes, horses that have only one copy of the defective gene and do not exhibit symptoms of the disease can still be carriers. For the disease to be manifest, a foal must get a copy of the defective gene from each parent.

Disorders with dominant transmission are rare, but genes for recessive disorders are relatively common and are more difficult to track in a population. Keeping these facts in mind, selective breeding is considered to be the key to minimizing or preventing genetic conditions at the present time.

This article will cover a few of the presently recognized equine genetic diseases.

Symptoms

• Death of a foal within the first few months of age due to Severe Combined Immunodeficiency that affects Arabian horses. Both parents may appear healthy and normal, but are carriers of the condition.

• Muscle tremors or paralysis known as hyperkalemic periodic paralysis, which is a dominant disorder that is characterized by a defect in the gene that regulates the movement of sodium into and out of muscle tissue. Severe attacks can lead to heart and respiratory failure. Horses with two copies of the defective gene usually do not survive very long, but horses with one copy can lead functional lives with a controlled diet.

• Still-born foals that die from Glycogen Branching Enzyme Deficiency. Tissues that require glycogen for energy, such as the heart muscle, skeletal muscle, and brain cannot function properly and foals are stillborn or die at a very young age.

• Abnormalities of the intestine that prevent the passing of feces is known as Overo Lethal White Syndrome, which affects Paint horses. When two carriers mate, a white or nearly all-white foal may be produced that dies or needs to be euthanized within a few days of birth.

• Loose skin that is easily stretched away from the body occurs in Quarter horses descended from the Poco Bueno/King bloodline. This disease does not affect all horses descended from these sires and some are more severely affected than others. Usually the disease becomes apparent when the horse goes into training and saddle and/or tack pressure makes the skin susceptible to trauma.

• Hereditary equine regional dermal asthenia, or HERDA, is a skin condition that develops into seromas and hematomas, especially when the skin receives pressure from saddles and tack. The skin in the neck area is especially susceptible, and blisters develop that often leave unsightly scars when they don't heal properly. Poorly organized collagen fibers are thought to be the cause.

Causes

In all cases of genetic disease in horses, the DNA background or changes within the DNA cause the disease, which is often apparent as soon as a foal is born.

Prevention

Selective breeding and DNA testing are used to determine the chances of a foal being affected by a genetic condition. Careful attention to background of breeding stock, plus DNA testing when any questions arise as to whether or not a horse may be a carrier, can reduce the number of foals born with genetic conditions.

Treatment

For most genetic conditions, little can be done in the way of treatment. In some cases, special diets and exercise programs may help minimize the condition. As scientists gain knowledge about genetic factors involved in conditions and diseases, it is hoped that both preventive and treatment capabilities will be enhanced.