Hereditary Equine Regional Derma Asthenia (HERDA)

This skin disorder is caused by an abnormality in collagen, a protein described as the “glue” that holds connective tissues together. Any kind of pressure can cause the outer layer of an affected horse’s skin to split away from the layer beneath it—resulting in severe damage and scarring. Most commonly, damage is seen in areas where tack rests, such as on the back or under bridles and breast plates.

How is it inherited? HERDA is a recessive trait, meaning your horse must be homozygous for the condition (have two copies of the gene) to be affected. However, if your horse is heterozygous (has only one copy of the gene), he’s a carrier, meaning he won’t show any symptoms at all but can still pass the gene to his offspring.

How common is it? It’s estimated that 3.5 percent of Quarter Horses carry this gene. It’s most common in cutting horse bloodlines, where its prevalence is as high as 28 percent. A horse with this disease has unregulated synthesis of glycogen, the storage form of sugars, in his body. This causes him to store too much glycogen in his muscles, which then leads to

Glycogen Branching Enzyme Deficiency (GBED)

A horse with this disease lacks the enzymes needed to store glycogen. Without stored glycogen, the brain, heart, and muscles are unable to function. This disease is always fatal. Affected foals are most commonly aborted or stillborn. If born alive, they’re unlikely to survive for longer than a couple of weeks.

How is it inherited? This disease is autosomal recessive, meaning that only homozygous recessive individuals are affected. Just like HERDA, if your horse is heterozygous (has only one copy of the gene) he’ll be a carrier, meaning he won’t show any symptoms at all, but can still pass the gene off to his offspring.

How common is it? Estimates say that eight to 11 percent of Quarter Horses are carriers, and up to 26 percent of horses with Western pleasure bloodlines carry the gene.

Malignant Hyperthermia (MH)

This disease causes a disruption of the channels, or pathways, that control the release of calcium in your horse’s muscles. Affected horses have an excessive release of calcium that leads to a “hypermetabolic state” during periods of stress, such as extreme exercise or when undergoing anesthesia. During an episode, your horse’s heart will race, he’ll sweat, and his temperature can rise to as high as 109 degrees. A severe episode can easily be life-threatening.

How is it inherited? This disease is autosomal