Hidden Rock Ranch


Labs offering genetic testing:

UC Davis ----- Animal Genetics

Articles covering multiple diseases:

AQHA Offers Five Panel Genetic Test, Results to be Placed on Horse’s Registration Certificate

Genetic Testing Roundup - AQHA

Genetic Diseases in Quarter Horses and Related Breeds
below are excerpts of this article from the equine chronicle:


PSSM (POLYSACCHARIDE STORAGE MYOPATHY) – Symptoms of this condition can include more than just muscle cramping. Some owners report poor performance, lack of energy, trembling after exercise, unwillingness of the horse to move forward, difficulty in backing up, stiff gait, difficulty in performing a balanced canter, unwillingness to lift the feet for hoof care, weakness of the hind limbs, gait abnormalities in the hind limbs due to sore muscles, loss of muscle (atrophy), tense and swollen muscles in the hindquarters, or sensitivity to pressure over certain muscles of the back and hindquarters. Owners say their horses act "cranky" including laying ears back, swishing tail, obvious pain signals.

When research on tying up was first begun in the 1980s, people assumed that all muscle cramping was caused by the same reason, but Dr. Stephanie Valberg at University of Missouri recognized that it wasn’t just one syndrome. She used muscle biopsies to detect the different types, working with donated horses to provide a research herd, and using high speed treadmills in her studies. She found there were basically two categories of horses that tie up—those with PSSM (often found in Quarter Horses, warmbloods and draft horses) and those with recurrent exertional rhabdomyolysis (RER), most common in Thoroughbreds, Standardbreds and Arabians. The horses with PSSM accumulate an abnormal amount of sugar in the muscles, and this can be seen in biopsies. “Many of these horses are heavily muscled and generally tie up when they are not getting regular exercise,” says Valberg. “They develop this problem early in life, are very sensitive to insulin, and don’t seem to be able to regulate energy metabolism properly in the muscles,” she says.

PSSM Article in Equus

PSSM - another case history - Sage Brush Sail Her

diet as recommended by Dr. Valentine

Could My Horse Have PSSM?

EPSM Yahoo Group:

HRR Note: I have been doing more pedigree research. I have come to the conclusion there was at least one source ranch, but there is no way to pin point the source sires. I have done many random pedigree checks now, and started looking for what might be termed a 'clean' pedigree.


Please test ALL your stock type horses with AQHA blood. Many come up positive but "had no symptoms" according to their owners. This is probably not true, they may not have had any classic symptoms, or the horse isn't old enough yet. Researchers say all PSSM N/P1 horses have symptoms by their teens.

There may never be a way to know all lines that have this disease,
this has been around since the middle ages.

Lines indicated by Research: Sonny Dee Bar, Two Eyed Red Buck, Eddie Cinco, Pretty Impressive,
Probable Lines: Midnight, Roan Hancock, Yellow Jacket, 'Waggoner Mare'. 6-10% of all AQHA horses have this. Sometimes suspect horses are 12 or even 15 generations back!

Do not go by pedigree's or bloodlines to feel safe! Testing is the only way to know.
PSSM travels frequently on the mares side.

Pitzer Ranch was proactive and tested horses that passed away - Two Eyed Jack had been highly implicated, but has been cleared thru testing. However, it does appear that the herd of mares he was bred to had positives. John Hancock also appears clear, but was bred to postive mares and has positive offspring. Be Responsible! Test and Disclose!

ANY horse with symptoms above should be tested, including geldings!!!
ANY breeding stock should be tested!
This is a dominate disease, and only one gene can cause symptoms.
Many horse can be managed through diet. Some are unridable. Some have to be put down.
Some very elite AQHA stock is positive for PSSM.

Thoroughbreds have a PSSM type 2 disease, so if your horse has symptoms, but is negative for pssm type 1, then please test for MH and also PSSM type 2 thru muscle biopsy.


HYPP (HYPERKALEMIC PERIODIC PARALYSIS) – First identified in 1985, by 1990 this muscle disease was finally linked to offspring of Impressive. He was very popular during the 1980s, especially in the halter horse industry, and his descendents now include Paints and Appaloosas as well as Quarter Horses. The defective gene is inherited as a dominant trait in both males and females and can be passed to offspring by any horse that has it, if the foal receives that gene. Breeding a heterozygous animal (N/H, with one normal and one HYPP gene) to a normal animal (N/N) will result in half the offspring being normal (50 percent chance) and the other half will have the defective gene. Breeding a homozygous animal (H/H, sometimes called a double positive) results in all the offspring inheriting the defective gene, regardless of the status of the other parent. Breeding a H/H horse to a N/H will produce offspring with a 50 percent chance of being H/H and 50 percent chance of being N/H. Breeding N/H to N/H gives 50 percent chance of being N/H, 25 percent chance of being completely normal (N/N) and 25 percent chance of being H/H.

HYPP affects muscle metabolism and affects the transport of sodium in and out of skeletal muscle cells. David McCarroll, DVM (Interstate Equine Services, Goldsby, OK) has dealt with many HYPP horses and he even treated Impressive, at Pilot Point, Texas, when he was in practice there. “I was called to treat him for colic in the late 1970s. In retrospect, he was having an HYPP attack, but we didn’t know it at the time. He was very, very weak, with his front legs and nose on the ground,” says McCarroll. The stallion had already had several colic surgeries during his lifetime, which may or may not have been necessary, since the pain from HYPP could easily be mistaken for colic.

HRR Note: HYPP started in a horse called Impressive, and look for him in a pedigree, but with mares switching foals and other pedigree errors, any suspect/symptomatic breeding stock should be tested. This is a dominant gene, and only one copy will cause symptoms of the disease.


GBED (GLYCOGEN BRANCHING ENZYME DEFICIENCY) – This is a mutation that causes inability to store sugar properly as big-branched glycogen molecules. “The glycogen in skeletal muscles, cardiac muscle, or liver can’t be mobilized very well so the foals are weak—and often die in the uterus or in the first days or weeks of life. They don’t have reserves to call on, so this becomes very serious,” says Winand.

This inherited defect was first noticed in 1997 when muscle biopsies from a foal at Kansas State University were submitted to the Neuromuscular Diagnostic Laboratory at the University of Minnesota. It took several more years to pinpoint the problem. Dr. Valerg (Director of the UM Equine Center) in 2001 found that the muscles of these foals had a lower than normal amount of glycogen and an overabundance of abnormal polysaccharide within the cells. Researchers realized that some of the mysterious late term abortions, stillbirths, and weak foals that die soon after birth are due to this condition. Valberg’s research suggests that at least three percent of abortions in Quarter Horses are caused by GBED, and that this defect may now be present in about 10 percent of all Quarter Horses and related bloodlines—such as Paints and Appaloosas–that have incorporated certain Quarter Horse bloodlines.

HRR Note:Traces to King, Not thru Poco Bueno are the what to look for in a pedigree, but with mares switching foals and other pedigree errors, all breeding stock should be tested. This disease is recessive, and two copies are needed to show the disease.

“Breeders need to become more aware of GBED because it’s not rare,” says Winand. “This disease is so misunderstood that people often don’t have a clue when they lose foals. Many people I talk to have lost several foals and don’t even think to test their stallions or mares for GBED. One of my HERDA clients lost several foals and decided to test her horses, discovering she had carriers of GBED. She prompted me to test my mare. Breeders should test for this if they experience foal loss, particularly from repeated matings. It’s not a very expensive test ($40), considering the price of producing a foal.”


HERDA (HEREDITARY EQUINE REGIONAL DERMAL ASTHENIA) – This term simply means dermal weakness in certain areas of the body. It is an inherited connective tissue disorder characterized by abnormal skin that separates readily from the underlying tissues, and tears easily. Any trauma or pressure can literally pull the skin apart. The problem is often not discovered until a young horse goes into training, when the simple act of wearing a saddle creates massive injury to the skin.

In humans there are about 12 different types of this condition—a disorder known as Ehlers-Danlos syndrome—which is similar to the disease in horses. Research at Cornell led by Dr. Winand, and at UC-Davis, and Mississippi State University (where Dr. Ann Rashmir kept a group of affected horses in a study program) showed that the defective gene is a recessive (hidden) trait that must be inherited from both parents in order to show up in their offspring.

This trait has been associated with some of the most popular bloodlines in cutting and reining horses and found in many Quarter Horse pedigrees—and some Paints and Appaloosas, sine Quarter Horse genetics have been used in those breeds. The researchers traced the defect back to Poco Bueno and his immediate ancestors. Skin biopsies were used to diagnose this problem until a DNA test was created. Now breeders can readily determine whether any horse in their herd is a carrier.

HRR Note:Traces to Poco Bueno are the what to look for in a pedigree, but with mares switching foals and other pedigree errors, all breeding stock should be tested. This disease is recessive, and two copies are needed to show the disease.

Malignant Hyperthermia

There are several other defective genes that sometimes cause problems. One condition that occurs in certain families of Quarter Horses is MH (malignant hyperthermia). According to Beth Valentine, DVM, PhD, Oregon State University, “The genetic basis for malignant hyperthermia is thought to be the mutation detected by Dr. Monica Aleman at UC-Davis. This is the same mutation that Dr. Valberg at University of Minnesota is looking for as a modifier gene that makes things worse for horses that tie up with polysaccharide storage myopathy (PSSM). The MH mutation is similar to the situation in people. In most cases, however, we are not aware of it, because it generally does not cause problems until the person, or the horse, is exposed to gas anesthesia.”

Dr. Valberg says the problem had already been identified in humans, dogs (such as Greyhounds) and pigs. “It’s due to a mutation in a gene that regulates a protein that moves calcium around within the muscle cell. If this movement is not regulated properly and excessive amounts of calcium are exposed to the contractile filaments in the muscle (that enable the muscle to contract), then the muscles stay in contraction—and metabolism greatly increases. This causes the body to become very hot, due to all that energy. The muscles get very rigid and there is often a lactic acidosis that develops in the whole body. The episodes of muscle damage are very severe, involving the whole body metabolism,” she explains.

When Dr. Aleman had 2 Quarter Horses develop severe anesthetic reactions during an experimental situation, she looked at the gene that causes these same problems in humans, and was able to identify a genetic mutation in these 2 horses. “After her discovery we screened a large number of horses that had had muscle biopsies at our lab, plus a large number of normal horses, to see how common this is. We found that the MH mutation is only in Quarter Horses (one bloodline), and in less than 1 percent of Quarter horses,” says Valberg.

“One thing we discovered, looking at muscle biopsies, is that there’s a group of Quarter Horses that have both type 1 PSSM mutation and the MH mutation. This makes their signs much worse when they tie up. Owners have told us their horses not only tied up following very light exercise, but some of these horses died acutely—which is not common with PSSM. Some horses had very high temperatures (up to 106 degrees F) when tying up. When we looked at some of these horses we found they had both mutations. So now we are recommending that if a horse ties up severely it should be tested for both.” PSSM is more common—affecting 10 percent of Quarter Horses—while MH is less common, but you might want to test for both if you have any reason to be suspicious. It might save your horse if you know ahead of time, in case he ever needs to be anesthetized. A horse with MH can be pre-treated (60 minutes ahead of giving the anesthesia) with a drug called dantrolene that may minimize the chances for severe reaction.