Over the last decade, several hereditary diseases have been found to be prevalent in horses descending from certain American Quarter Horse Association bloodlines. Perhaps the most well-known of these is hyperkalemic periodic paralysis (HYPP) which has had a genetic test available for some time.  The other four which have just come to light in recent years are glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), malignant hyperthermia (MH) and polysaccharide storage myopathy (PSSM1).

AQHA is now having breeders test their horses for these five genetic diseases and have the results included on the registration papers. The five disease panel test is a simple mane or tail hair kit available through AQHA at a discount to members. The diseases range from those which the effects are manageable, to those that are deadly. Even if your horse doesn’t appear to come from the bloodlines associated with one of the diseases, there is always a possibility of a different stallion breeding a mare unknowingly, or mares switching foals on pastures many years ago that can cause inaccuracies in all pedigrees, so testing for all five genetic diseases is important in every breeding horse.

While HYPP is widely recognized and understood in stock breed circles, the other four diseases may be new to some of our readers. In this month’s article, I will touch on two of the diseases briefly, and explain why testing your breeding stock for them is just as important as it has been to test for HYPP.

Glycogen branching enzyme deficiency (GBED) is perhaps one of the most important in terms of late term abortions and the birth of sickly foals which will die by just a few months of age. GBED is the genetic inability to produce glycogen, an important sugar used for energy by the muscle cells in the body.

Many GBED foals will be aborted late term, but those that are born alive will have weakness and low body temperature at birth. They may have a high respiratory rate due to weakness of the muscles used to breathe, have contracted tendons in all four legs, and just show an overall weakness and inability to get up. These foals may also die from heart failure or from seizures due to low blood sugar. GBED is always fatal – there are no known surviving foals with GBED past the age of 18 weeks. For large-scale breeders with a big foal crop, testing for GBED is important to test for in terms of controlling foal losses. For the individual broodmare owner breeding for that one special foal, it is equally as important.

GBED is an autosomal recessive disease. Autosomal recessive is a genetic term meaning that each parent must carry the abnormal gene, and the foal must receive two copies of the gene (one from the sire, one from the dam) in order to have the disease. The stallion bloodline that is commonly associated with GBED is one of the most famous foundation AQHA sires, King-P234 and likely from his sire, Zantanon. Their genetic material is the foundation of many popular bloodlines today.

If a sire and a dam that are each carriers of GBED are bred together, there is a 25 percent chance of the foal having GBED and dying before or at birth. If the horse just receives one copy of the gene, then he is a carrier of the gene, but will not actually have the disease.  If you are breeding a mare who is a GBED carrier, it is imperative that you breed her to a stallion that has been tested GBED negative in order to avoid the disease completely. Some breeders are taking the step in gelding all of their male foals that are GBED positive in order to help stop the perpetuation of the disease. Technically, GBED could be eliminated within one generation of horses by simply not breeding any GBED carriers – mares or stallions – together.

Malignant hyperthermia (MH) is probably the least known genetic disease in Quarter Horses. While it is extremely rare, it also has life-threatening complications. Malignant hyperthermia is uncontrollable skeletal muscle metabolism which overwhelms the body’s ability to regulate oxygen, carbon dioxide, and body temperature. The body overheats quickly, causing circulatory collapse, severe brain damage, and death. MH is also a disease in humans as well as in Landrace pigs. MH is linked a specific Quarter Horse bloodline that has not been publically named, and it often coexists with another genetic muscle disease, PSSM1 (which we will discuss next month), and they may exacerbate each other. Less than 1 percent of Quarter Horses carry the MH gene.

MH can be brought on by exercise, stress, and the inability of the horse to process certain gas anesthetics or succinylcholine, drugs which are commonly used during anesthesia and surgery.

MH is an autosomal dominant disease in horses, like HYPP. This means that a horse only needs to carry one copy of the faulty gene – ryanodine receptor 1 gene (RyR1) – from either the sire or the dam, in order to show signs of the disease. If you breed a carrier horse to a noncarrier horse, 50 percent of the resulting foals will be affected with MH. If you breed a carrier horse to a carrier horse, 75 percent will be affected. It is very important to test for MH in horses that are high performance athletes, or for any horse that may undergo general anesthesia or surgery.

Next month we will discuss polysaccharide storage myopathy (PSSM1) and hereditary equine regional dermal asthenia (HERDA).

Tanis MacDonald Walker, DVM graduated from the Atlantic Veterinary College on Prince Edward Island and currently practices both small animal emergency and equine medicine in Delaware. You can email her at tanis_macdonald@hotmail.com or visit her at www.drtanis.com. You can also write to her in care of InStride

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