Hyperkalemic periodic paralysis (HYPP), glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), malignant hyperthermia (MH), polysaccharide storage myopathy type 1 (PSSM1) … These are some intimidating words that horse owners are likely to come across at some point or another, especially if breeding is on the yearly agenda. So, what exactly do these equine diseases entail? What do they mean for breeding? How can you know for sure that you are breeding to avoid these diseases?
When it comes to genetics, a recessive trait can be carried without showing any signs of disease because the horse would need two copies of the gene for it to cause the disease. So, the horse may have one copy and be a carrier but have a perfect bill of health (these horses may pass that carried gene onto offspring). However, for dominant traits, just one copy means that the disease will likely show through. PSSM1, MH, and HYPP are all autosomal dominant traits, meaning only one copy of the gene is needed to cause problems. GBED and HERDA are both recessive traits.
These five diseases are common in the quarter horse world, which why the American Quarter Horse Association (AQHA) offers what they call their 5-panel genetic test, which will check for these five diseases. Currently, in order for a horse to be marketed as an AQHA stallion for breeding, the stallion must have the 5-panel genetic test on file and it will be open for viewing for potential breeding opportunities. This way, those choosing to breed can make informed breeding decisions for their potential offspring. The test can be ordered through the AQHA website (you can also test your mares), and requires a hair sample to be sent in. This way, if a stallion is a carrier of one of these diseases, you will know and know your risks for breeding. It will also show if there are any dominant traits. So, breeding decisions can be made to avoid these diseases all together.
Hyperkalemic periodic paralysis (HYPP)
Hyperkalemic periodic paralysis (HYPP) is a disease where you may see your horse have random muscle spasm attacks and the muscles will tremble and shake. Sometimes, the horses will end up getting weak and falling. It is caused by a mutation in the sodium channels within the body, which means too much potassium ends up in the blood. This disease is traced back to the stallion Impressive, so any horse from this bloodline is required to be tested for HYPP, and any horse that is H/H (homozygous dominant) for the trait, cannot be registered through the AQHA. These results will show N/N (normal), N/H (has a copy of dominant trait, but symptoms may not be as severe), and H/H (homozygous dominant for the mutation and will show severe symptoms).
Glycogen branching enzyme deficiency (GBED)
Glycogen branching enzyme deficiency (GBED) is fatal. You will not see a mature horse with GBED. These horses do not have the enzyme that they need to get the right kind of glucose (sugars) to their muscles that they need to survive and operate organs, meaning organ failure is inevitable. Often times you will see abortions, stillborn foals, or if the foals are carried to term, they typically die soon after birth. With GBED being recessive, the results would look like this: N/N (normal), N/G (carrier, not affected but can pass to offspring), or G/G (affected, and will not live past a few months of age). The above are examples of both a dominant trait, like HYPP, and a recessive trait, like GBED.
Polysaccharide storage myopathy (PSSM)
PSSM1, which is a mutation that causes excess glycogen (sugar) to be stored in the muscle that can cause severe muscle cramping, especially after exercise, will follow a similar genetic pattern as HYPP when it comes to breeding. The severe muscle cramping is called tying up. This disease can effect draft horses and stock type horses. Type 2 PSSM occurs without any genetic mutation and is often seen in warmbloods.
Malignant hyperthermia (MH) and HERDA
So will MH, which is a mutation that can cause death if the horse receives certain anesthesia (can also happen from extreme stress or excitement), but the horse’s body will release an excess of calcium which causes cramping, extremely high body temperatures, sweating excessively, and breathing problems. HERDA, which is a skin condition where collagen formation is impacted between layers of skin, so the horse is more prone to injuries as well as skin that is oddly stretchy, is recessive so it will follow the same genetic patterns as GBED. This disease is often not evident until the horse is started under saddle and the saddle starts to cause tearing and scarring of the skin.
These diseases are still out there and impact more horses than you would think. Luckily, there are resources available so that responsible and informed breeding decisions are possible, and these diseases can be monitored.
Breed associations like AQHA and APHA offer genetic testing to help breeders and buyers make an informed decision about a horse and it’s career. Many times these genetic diseases will influence performance and often longevity and quality of life for the animal. Many stallion owners will offer the results for the 5 panel genetic testing when advertising the stud service. Be sure to check those results with your mare to ensure there is no chance of a genetic disease in the foal.
In addition to genetic disease testing, horses can also be color tested. In the next issue, we will dive deeper into color genetics and discuss what makes a pretty palomino so golden.
Equine Genetic Disease Resources
By McKenzie Wheeler, Rockdale County ANR Agent, Guest Writer