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Understanding PSSM in Brabants

By Stacy Pearsall with Rebecca Courtney, Dehan Courtney, Tim Gunter, Will Beattie and Dr. Hernando Plata, DVM

PSSM has been a hot button topic in the horse community lately, and given the prominence of the condition within draft horse breeds, Brabants included, we thought it was worth investigating the implications of PSSM on European Brabants while also outlining the European Brabant Registry of America's views on the issue. Unlike other draft horse registries, we require PSSM testing for all breeding stallions and encourage all European Brabant owners to test their horses, so they may make informed decisions about health, exercise, diet and breeding. That said, as a Registry we do not prohibit the breeding of European Brabants who carry the gene, and we hope this article clarifies our reasoning and stance on the matter.

It's important to consider that most medical journals and literature available on PSSM skews toward PSSM's impact on light horse breeds, rather than draft horses. Given draft horses respond to PSSM significantly differently from their light horse cousins, we researched and condensed draft horse specific studies and articles into this succinct overview for our European Brabant owners and breeders.

“PSSM is estimated to have emerged as far back as 1,600 years ago, when the great horse was being developed from European draft and light horse breeds to carry knights with heavy armor into battle,” says Stephanie Valberg, D.V.M., Ph.D. "There is a 90% prevalence of PSSM in Trekpaards, with 40% of tested Belgian Trekpaards being homozygous for the trait. Many with PSSM are asymptomatic."

Initial examination of pedigrees from registered Belgians with PSSM suggested that most Belgians, at least in North America, trace back to a small number of foundation lines from mainland Europe. This is not surprising, given the relatively few purebred Draft horses left for breeding following the Great Depression and the Industrial Revolution. If the data regarding incidence of PSSM in Draft-related breeds are even close to accurate (45-86%), it would appear that trying to breed away from this trait in these breeds would be extremely difficult. In fact, there is some suggestion that horses with PSSM, when they are able to deal with the condition, are superior in temperament, conformation, and even more importantly, performance. (citation: Susan A. Mende, DVM, Dipl ACVP)

"This trait is what allowed the draft horse to survive long hours of field work on poor feed," says Hernando Plata-Madrid, DVM and Founding Member of the EBRA.

But I'm getting ahead of myself here. Let's start by defining PSSM. It is the excessive and abnormal storage of sugar (polysaccharide) in muscle cells. There are two types of PSSM, type 1 PSSM (PSSM1) and type 2 PSSM (PSSM2). PSSM1 is the form of PSSM caused by the genetic mutation and PSSM2 represents one or more forms of muscle disease found in biopsied tissue that are characterized by abnormal staining for muscle glycogen under a microscope exam. (citation: Michigan State University).

Polysaccharide storage myopathy (PSSM1) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (amylase-resistant polysaccharide) in muscle tissue. By definition, horses with PSSM1 have a distinctive genetic mutation in the gene.

PSSM affects virtually every draft horse breed, including Belgian, Percheron, Clydesdale, Shire, Haflinger, Norwegian Fjord, Irish Draught, Friesian, Gypsy Vanner, draft cross, and draft mules. As previously stated, approximately two-thirds of all draft related horses show evidence of PSSM and it has likely been around for hundreds of years. (citation: Dr Beth Valentine DVM Ph.D, who studies focused primarily on PSSM/EPSM in draft horses). Since all draft horses are insulin sensitive, draft horse owners have already adapted their feed and exercise regimes. So in many ways, they are ahead of the game when it comes to caring for horses who are diagnosed or undiagnosed for PSSM. (citation: Dr Eleanor Kellon, DVM Ph.D)

In some breeds, horses with the genetic mutation for PSSM1 are asymptomatic - ie: most draft breeds. This may relate to differences in diet, exercise and impact of different genotypes in different breeds. For instance, carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, appear to exacerbate PSSM1. The majority of draft horse owners, including Brabant owners, don't feed their horses starchy foods. Instead, they seek feeds that provide extra calories in the form of fat. Again, for hundreds of years, draft owners have developed food and exercise regimes to best suit the draft horse's slow metabolism functions, says Michael R. Stone, DVM. It just so happens that this regime is the remedy to managing horses with PSSM.

NOTE: Tying up, muscle spasms, and elevated muscle enzymes are most common in Quarter Horses and other light breeds. Draft horses, Warm Bloods and other Draft Crosses may show only muscle tenderness or reluctance to engage the hindquarters. (citation: Dr Eleanor Kellon, DVM Ph.D)

An important part of the management of PSSM1 horses is daily exercise. This enhances glucose utilization, and improves energy metabolism in skeletal muscle. If only the diet is changed, we found that approximately 50% of horses improve. If both diet and exercise are altered, then 90% of horses have had no or few episodes of tying-up.

NOTE: Tying up is a symptom rather than a specific disorder. There are two categories with tying up, horses that have sporadic/isolated episodes and those that have repeated episodes. Isolated episodes are more likely to be due to management/dietary factors. Common causes include electrolyte imbalances to plain overworking of a horse to heat stroke. (citation: Dr Beth Valentine DVM Ph.D).
Not all cases of tying up are caused by the PSSM1 mutation. If a horse is N/N but is showing signs of tying-up or muscle pain, it is possible that the horse has another muscle disorder which must be diagnosed by muscle biopsy. (citation: Stephanie Valberg, DVM, PhD, DACVIM)

By now you're probably asking yourself, "Does my horse have PSSM?" Great question! The European Brabant Registry of America offers a PSSM test through UC Davis and encourages all owners and breeders to test their Brabants. There is a caveat when it comes to DNA testing. While the PSSM test exists, a negative result can occur in horses with a somewhat milder form and therefore, a negative test does not entirely rule it out in a horse. Also, a n/P1 (heterozygous) or even a P1/P1 (homozygous) result does not mean your horse will be sympathetic. However, knowing the genetic makeup of your horses can be useful when planning breeding and management though, advises Dr. Beth Valentine, DVM.

So the ultimate question is to breed, or not to breed. Dr. Susan Menda says that owners of horses with PSSM should seriously consider whether homozygous horses should be used as breeding animals. But she goes on to say that, "Given the high incidence of PSSM within certain breeds and the apparent performance capabilities of PSSM horses, it may not be feasible or even appropriate to try to breed away from this condition."

"'The mission of the European Brabant Registry of America is to preserve and maintain purebred European Brabants,' and given the critically low numbers of such horses globally at this time we are presented with a unique set of challenges," says Stacy Pearsall, President of the EBRA. "But I believe the Registry has established thoughtful programs to both grow the purebred numbers while maintaining and improving the overall health of the breed."

The ERBA understands just how difficult it can be to find viable mates within a breed that has such critical numbers - That's why we also established the "Breed Up Program" to help increase our purebred herds in America, while also widening our gene pool. We offer Stallion Listings online, where EBRA Members and mare owners can view genetic testing results and leg health reports of each candidate. Then they can use the Grassroots "Test a Mating" system to ensure optimal genetic pairings for stallions and mares.

We know the challenges are real, but with the aforementioned programs, we hope to make breeding choices easier. No matter what, the EBRA encourages you to test your horses for PSSM and to avoid breeding homozygous horses to other PSSM carriers when possible. For example, a mare who is P1/P1 (homozygous) should ideally be bred to a stallion who is n/n. The resulting offspring from such a pairing will then be n/P1 (heterozygous). Ideally, that offspring should then be bred to another n/n. Alternatively, the n/P1 may be bred to another n/P1 which will result in a n/P1 foal 50% of the time, 25% n/n and 25% P1/P1. (citation: Stephanie Valberg, D.V.M., Ph.D.)

Dr. Beth Valentine does not advise trying to breed away from this trait. She believes that working horses may actually benefit from it and that may be why it is so common in so many breeds. The key to managing these horses is to feed the right diet and provide enough exercise.

To that end, let's talk about how best to maintain our Brabants. Successful PSSM diets can include either grass or alfalfa hay. Many higher fat and fiber feeds are being developed for horses, but to date all still require some additional vegetable oil or additional 100% fat source to achieve the proper ratio of calories. For many horses, addition of about 480 ml of vegetable-based oil to a forage-based feed has proven to be the most economical, feasible, and effective type of diet. Regular exercise, even if it is just turnout, is also important. It takes about four months for full fat adaptation in these horses, according to Susan A. Mende, DVM, Dipl ACVP.

"The ability to control signs of muscle dysfunction with diet change and exercise suggests that PSSM may best be considered a problem with a genetic component that can often be controlled effectively with proper management," says Dr. Susan Menda.

To summarize, the EBRA believes that it's important to understand PSSM and how it impacts the European Brabant specifically. We require PSSM tests on all breeding stallions and require stallion owners to release the results to mare owners upon request. We believe that honesty and openness about PSSM is the best policy. After all, it's here. It's been here. We provide a number of great medical journals, which guide owners toward best feeding and exercise practices for European Brabants. These can be found on the Members Portal. We also encourage owners to engage each other on the Forum about this topic! In the end, this breed has thrived for hundreds of years and we believe it will continue to do so. To that end, we will leave it up to the horse owner whether to the breed or not to breed.

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