5 Panel Testing and Understanding the Alphabet Soup

Information about the tests within the 5 panel test

This day and age, people abbreviate things to save time and make well known phrases easier to type in a story. However in proper writing, the abbreviation is listed at the first part of the story so everyone understands what the letters stand for. In the last several years, this practice has been left by the wayside leaving many people wondering “what the heck does that stand for”.

One of these areas, that can be very frustrating, involves medical terminology, especially when it comes to equine disorders. Not only are these disorders long named, they are constantly adding new disorders and some even have improper names.

One of the most listed group of abbreviations is found with the five panel test offered by several breed registries. After properly pulling mane hair and submitting it to the lab facility, this panel test checks the horse’s genetic makeup for the genes or markers for the specific disorders they are looking for. Many people can name most of the abbreviations in the panel but they will also admit that don’t know much about the disorders themselves. I hope to help clarify a few things in this blog.

All stallions listed with the AQHA and APHA (requires 6 Panel test) must be five panel tested in order for any offspring to be eligible for registration. All five panel results are listed on the horse’s pedigree on AQHA.com and can be seen by any member of AQHA.

Hyperkalemic Periodic Paralysis (HYPP)

The “all feared” Impressive disorder, HYPP is probably the widest known abbreviation of the five panel tests. HYPP is found in some descendants of the AQHA halter horse, Impressive. It is characterized by a gene mutation that causes muscle tremors, sudden paralysis, collapse, and even death. The disorder is dominantly inherited which simply means if the horse has one copy of the mutated gene the horse has the disease. A horse can inherit one (heterozygous) or two copies (homozygous) of this mutated gene. Having one or two copies does not necessarily effect the severity of the disorder as multiple heterozygous horses have had severe, deadly attacks. It does, however, affect the offspring of these horses as breeding a homozygous (2 copies) will result in foal guaranteed to have the disorder. A heterozygous, as long as they are bred to a negative horse, will result in a positive horse 50% of the time.

Horses with HYPP have “leaky” sodium channels in their muscles caused by elevated levels of potassium in their blood. I know this sounds complicated but all you need know is when the horse eats excess potassium (which most horses do every day) it causes the channels to leak sodium. When the muscles are receiving this excess sodium it causes them to twitch uncontrollably. This is great for the skeletal muscles as it really beefs them up, but remember the heart is a muscle as well as all the muscles we have in other areas of our bodies such as intestines.

Some medications, changing the horse’s diet to reduce the potassium levels, and reducing stress helps reduce the signs of HYPP but it can never be cured.

Not all Impressive bred horses carry this mutated gene, many are completely clear, this panel test also helps prove the horses that are disorder free. These horses are listed as HYPP N/N and with this testing, proves that these horses cannot, I repeat cannot, ever have this disorder nor can they ever pass it on.

AQHA requires all descendants of Impressive to be tested for HYPP prior to registration unless the horse is proven negative through parentage (Sire and Dam are negative). Currently any foal found homozygous (HYPP H/H) is no longer eligible for registration in the AQHA. Heterozygous foals (HYPP H/N) are still eligible for registration, however, every offspring from that horse must be tested.

Hereditary Equine Regional Dermal Asthenia (HERDA)

With a name like that, there is a reason HERDA is abbreviated. HERDA is genetic mutation characterized by a lack of adhesion of the skin to the inner layer. This results in the skin moving around move than it was designed and can actually cause the skin to tear off and scar. Most horses show signs during training as two year olds resulting in them being unable to be ridden and for most euthanasia. This disorder is found in foundation stock horse lines and may be linked to descendants of Poco Bueno.

HERDA is recessively inherited so only the horses that have 2 copies (HERDA hrd/hrd) of the mutation have the disorder, however, horses with one copy are carriers (HERDA Hrd/N). Meaning the horse, does not have the disorder but can pass it on to offspring and if the carrier horse is bred to another carrier horse or worse a horse with the disorder offspring can be affected. How often is the offspring affected? If a carrier is bred to another carrier, 25% of the foals born will be affected, 50% of the foals will be carriers and 25% will be negative. If a carrier is bred to a horse with 2 copies 50% of the foals will have the disorder and 50% will be carriers.

The biggest problem with the carrier state of a disorder is the horses show no signs themselves, so without testing, the breeder never knows his or her chances of a HERDA Hrd/Hrd horse, until they have one.

Malignant Hyperthermia (MH)

MH, one of the lesser known and rarest of disorders, results in horses excessively sweating, having elevated blood pressure and overheating to the point of heat stroke and sudden death. These horses have a mutation that causes excess calcium to release which causes the hyperthermia. Excess calcium is not constant however it is triggered by things such as anesthesia, stress, or extreme exercise. A lot of horses show no signs until their first veterinary procedure requiring anesthesia. This disorder is dominantly inherited so like HYPP a single copy means the horse is affected. Horses with MH can still undergo veterinary procedures, however it is crucial to know if the horse is affected so preventative medications can be given BEFORE the procedure. If the horse begins to overheat, most horses do not recover. Interestingly, greyhounds can also have this disorder.

Polysaccharide Storage Myopathy Type 1

PSSM-1 is disorder occurring in stock breeds and other breeds such as warmbloods and drafts. Glycogen (normal form of sugar found in muscle) and Polysaccharide (abnormal sugar in muscle) begin to accumulate in the muscles at a faster rate than normal. The levels become excessive, up to 4x higher than normal, causing the muscles to become extremely tight. Signs very similar to tying up such as reluctance to move, muscle stiffness, and sweating develop. These horses can also show signs of severe colic or even an unexplainable lameness during an episode. PSSM-1 horses can begin showing signs with very little (10 min walking) to no exercise and usually occurs after a horse in consistent training has a layoff and is brought back to work. This gave PSSM the common name of Monday Morning Disease in draft horses.

PSSM-1 is dominantly inherited so a single copy is all that is needed for the disorder to raise its ugly head. As with any genetic disorder once affected, always affected, but the occurrence of symptoms can be reduced by a consistent exercise program, 24/7 turnout and a special feeding program. However, a one size fits all solution does not exist, the feed program and exercise may have to be continually tweaked as the weather and hay values change. In general, PSSM-1 horses need a low starch, high fat nutritional program, so sweet feed, oats, corn and molasses are a big no-no. There is hope, studies have shown 75% of PSSM-1 horses no longer had symptoms of PSSM after correction of their nutrition and exercise program.

Glycogen Branching Enzyme Deficiency (GBED)

GBED could almost be the opposite of PSSM-1, as it is characterized with the inability to store glycogen. The foal is born unable to branch glycogen, because it cannot produce the enzyme required. Without glycogen, the foal does not have enough energy to fuel important structures such as muscles and brain.

Most foals are aborted while in development and do not make it full term. Even if born full term, the disorder is 100% fatal, with most dying before 8 weeks of age. It is estimated that 3% of all equine abortions are caused by this disorder and 10% of all quarter horses are carriers. GBED is recessively inherited so the disease can be prevented by testing and never breeding carriers together. As all affected horses die before breeding age the disorder is only spread by carriers (GBED G/N).

Summary of 5 Panel Disorders (Offspring Assumes Horse was bred to Negative Horse)

HYPP (Hyperkalemic Periodic Paralysis) –dominantly inherited

HYPP H/H – Affected, 100% offspring affected

HYPP H/N- Affected, 50% offspring affected

HYPP N/N- Unaffected, no offspring affected or carriers

HERDA (Hereditary Equine Regional Dermal Asthenia)-recessively inherited

HERDA Hrd/Hrd-Affected, no offspring affected, 100% carriers

HERDA Hrd/N- Unaffected, No offspring affected, 50% carriers

HERHA N/N- Unaffected, no offspring affected, No carriers

MH (Malignant Hyperthermia)- dominantly inherited

MH Mh/Mh- Affected, 100% offspring affected

MH Mh/N- Affected, 50% offspring affected

MH N/N- Unaffected, no offspring affected or carriers

PSSM-1 (Polysaccharide Storage Myopathy Type 1)- Dominantly inherited

PSSM-1- PSSM1/PSSM1- Affected, 100% offspring affected

PSSM-1 – PSSM1/N- Affected, 50% offspring affected

PSSM-1 – N/N – unaffected, no offspring affected or carriers

GBED (Glycogen Branching Enzyme Deficiency)- recessively inherited

GBED – Gb/Gb – Affected, 100% fatal, No offspring produced

GBED- Gb/N – Unaffected, Offspring unaffected, 50% offspring carriers

GBED- N/N – Unaffected, no offspring affected or carriers

Owners will ask, “My horse is positive for X, Y, and Z should I breed them?” or worse “My horse has X and I can’t ride her, so I am going to breed her.” My advice to them is consistent and easy for multiple disorder and double positive (homozygous) horses. They should not be bred. No ifs ands or buts, horses with multiple disorders or horses that are homozygous for a single disorder should NOT be bred. Now it gets trickier with the horse that is single positive (heterozygous), I ask them a series of questions. First question, is the single positive horse affected or is it just a carrier state, if it is a carrier state only breed to negative horses. Next question, is the horse genetically valuable? I am not talking about the backyard baby sitter that you have had for years and now want your children to experience the miracle of birth. I am talking about a show horse that has proven himself/herself in their chosen discipline and the offspring from that horse would be considered valuable by potential buyers. Maybe that should have been the first question….. Final question applies to affected heterozygous horses, are you willing to care for the affected offspring for the rest of their life? Most buyers will not purchase a known affected horse, and if so they will not pay the value you were expecting for the foal. Remember half of the offspring born from a single affected horse will be affected. There may come a day when a heterozygous horse may not be eligible for registration.

I hope this helps clarify some of the “letters” of the 5 panel test. Look for my future blogs including PSSM-2 In depth, Wound Care, Vaccines and Health Paper 411.