What is all the fuss about, anyway? 

DSLD (degenerative suspensory ligament desmitis) for decades has been a word cloaked in fear and denial. Without a clear understanding of this condition, and no solid data to prove or disprove that any one breed of horse is more affected with DSLD than others, the topic has been the source of much mystery and heated debate. The purpose of this article is to educate the reader about recent advances in the understanding of DSLD, to counter the widespread ignorance concerning what DSLD is and is not, and to discuss how it relates to the Peruvian Paso horse. 

Owners and breeders of Peruvian Pasos are rightfully proud of the renowned qualities of their horses. Celebrated and treasured for their unique essence of nobility, charisma and their characteristic smooth gait, Peruvians evoke fierce dedication and love in their owners. They are a joy to own and to ride. There truly is no other horse quite like a Peruvian in the equine world. Given this admirable devotion, many owners and breeders believe the incidence of DSLD within the Peruvian breed is not the issue. What is important is that even one horse with DSLD is one horse too many. 

DSLD must be understood to be no more - or less - than an equine health issue to be addressed, as have a number of other health concerns in other equine breeds. Combating HYPP in Quarter Horses, SCID in Arabians, eye diseases in Rocky Mountain Horses and lethal white syndrome in Overo Paint Horses are only a few examples of recent scientific advances in equine health. Since humans first started the selective concentrating and breeding of animals, removing the process of natural selection, there came with it the responsibility of custodianship of those animals. In order to ensure a breedís continued health, strength, longevity and viability ? health issues as they arise must be dealt with appropriately to guarantee the survival of that breed. 

The Peruvian Paso is a remarkable example of selective breeding. From termino and brio, to champagne gait - the Peruvian horse was designed to stylishly carry its rider with flair and to delight the eye of the beholder. However, the Peruvian Paso horse and its breeders are not exempt from the process of careful, responsible propagation. Fortunately, we in the Peruvian Paso breed are luckier than some. We have only one affliction to deal with. Many breeds of animals have such a multitude of ailments that breeding them often seems destined to yield only frustrating results. As owners and breeders of Peruvian horses, we have the tools already in place to breed stronger, healthier horses, and the knowledge needed to sell a better product to the equine world. In this light, DSLD is in no way a reason not to use and enjoy each and every horse to the very best of its ability, whether in the show ring, on the trail or in the breeding shed. Rather, it presents the opportunity to each of us to make the commitment to learn about, look for and breed away from DSLD - starting today. 

What have we learned about DSLD? 

DSLD is a crippling, debilitating syndrome involving the suspensory ligaments of the equine lower limb. Degenerative refers to progressive worsening, desmitis (injury/inflammation) of the suspensory ligaments. DSLD as a term was coined by Dr. Jan Young of Arizona in a paper describing the syndrome in several breeds (1).  The disease was first recognized in Peruvian Pasos by Drs. Pool, Pryor and Wheat of the veterinary school at the University of California at Davis in the late 1970's and early 1980's in an unpublished paper (2).  DSLD occurs in many breeds of horse, and in breeds other than the Peruvian Paso is usually associated with aged individuals, broodmares, animals subjected to intense work loads (race horses) and animals that have sustained a prior suspensory injury (3,4). 

Anatomically, the suspensory ligament is a broad band of connective tissue fibers containing variable amounts of muscle that attaches to the backside of the upper cannon bone on all four limbs. The body of the suspensory travels roughly two-thirds down the leg, behind the flexor tendons and tucks in tightly between the two splint bones against the back of the cannon bone. It splits above the ankle into two branches (medial or inside and lateral or outside) with the branches then attaching to the sesamoid bones at the back of the ankle joint. The suspensory ligament formed as an integral part of the "passive-stay apparatus" - the sling that holds the equine ankle joint up off the ground. 

A chronic, progressive multi-limb (either two or four limb) disorder, DSLD is most often insidious in its onset. Initial signs can be nebulous and easily mistaken for other problems. Back pain, changes in attitude, generalized stiffness and a reluctance to work can be some common early indications. Frequently, well-gaited horses either suddenly or gradually seem to lack reach and extension in the front and appear reluctant to push from behind. They may start shortening their strides, or in other ways attempt to minimize the workload on their limbs. An extreme widening of the rear limbs during walking or gaiting as viewed from the rear is also common, (Peruvian horses normally move fairly close behind). 

As the disease progresses, horses often have a difficult time standing for their feet to be trimmed, especially the rears. Signs mimicking arthritis are not uncommon, as affected horses can appear extremely stiff after a period of inactivity and then seem to work out of it with exercise. Often DSLD animals stab their toes into the ground and appear especially reluctant to load their heel regions. Broken hoof/pastern axes, usually seen as coon-shaped feet, are not unusual. 

By late-stage disease some animals will have visibly enlarged ankles and/or suspensory ligament branches. Some swellings can be diffuse and represent an overall enlargement of the ankle area, while others can have specific enlargements centered over the suspensory branches. Dropped, swollen ankles, or hyperextended fetlocks, are only found in roughly half of Peruvian cases (5). This should not be considered the exclusive diagnostic sign of DSLD. A loss of angulation of the rear limbs is possible, with nearly complete straightening of the joint angles from the hip through to the ankle in some cases. 
 
By end-stage disease many severely affected horses have been noted to be reluctant to move around, and many lie down excessively (some up to 18 hours a day). Other dig holes to stand in to elevate their affected limbs. Some have been described as sitting on feeders, fences or anything else they can use to take the pressure off of their painful limbs. 

Easily apparent lameness is often not present even in advanced cases, and can confuse and easily mislead many owners and their veterinarians. This is likely due to the bilateral (two-limb) or quadrilateral (four-limb) involvement in DSLD. With pain being evenly divided, the horse can not unload the bad limb(s) and preferentially load a sound limb. Perhaps it is also due to the tremendous heart and brio of the Peruvian horse, making their pain tolerance and desire to please legendary. 

What do I need to know? 

Diagnosing DSLD properly is a step-by-step process that usually consists of a medical history, a complete lameness exam and an ultrasound exam. While it is not difficult to make the diagnosis of DSLD in animals in late- to end-stage disease, one must truly look for DSLD in early to mid-onset cases. Consistent findings on clinical exam include pain on palpation of the suspensory ligaments, particularly in the branches, lameness (though often very subtle and described as merely stiffness), and positive fetlock flexion tests.5 The fetlock flexion test is a manipulation where the fetlock is held in an extreme flexed position for a time period of 30-60 seconds, following which the horse is asked to gait away from the examiner. Any resulting lameness is noted on a scale, usually from 0-5. The presence of positive flexion tests in more than one fetlock, especially severe responses in horses with seemingly no visual abnormalities and only mild palpation findings, should raise the examinerís index of suspicion for DSLD (5). 

Once a horse has been examined clinically and suspected of having DSLD, an ultrasound exam will usually confirm the presence of DSLD or not, at the time of the scan. Ultrasound exams are particularly helpful in diagnosing early- to mid-stage cases that might be exhibiting only minimal clinical exam signs. What sets DSLD apart from just an injury, either acute or chronic, is the progressive, continual enlargement of the suspensory ligaments, primarily confined to the branches, over time in more than one limb (5).  Ultrasonograhpically, DSLD usually does not appear as an injury or a tear in the way suspensory lesions are typically thought of. Rather, DSLD appears as diffusely enlarged suspensory tissue, that commonly has lost its normal fiber pattern. The entire suspensory system is affected, not just particular sites. Ultrasound machines are useful as they can accurately measure, record and track the progressive enlargements of the suspensory ligaments. In general, a healthy suspensory ligament branch is no larger than 1.2 sqcm in area, and no more than 1.1cm in the side-to-side, or back-to- front planes (5,6).  Many DSLD horses have been found to have such grossly enlarged suspensory branches that they measure up to 2 to 2 1/2 times normal size. 

In post mortem examination, the tissues of DSLD affected horses all look the same, regardless of the breed. Diseased areas typically originate in the center of the suspensory tissue and radiate out. Even in early-onset cases the degenerative areas are easily visible to the naked eye. Late-stage cases represent such tremendous damage and destruction to the suspensory ligaments and surrounding tissues that the normal structure of the ankle area is grossly distorted and enlarged. Under the microscope, DSLD is also extremely consistent, regardless of the breed affected, and is characterized by a primary collagen disorde (5). Collagen is the main tissue comprising the suspensory ligament. Degeneration and swelling within the collagen cells leads to a progressive loss of the architecture within the collagen bundles (collagen bundles are groups of regularly, precisely ordered collagen cells). DSLD becomes a vicious cycle, as repeated attempts to heal the diseased tissue only leads to further swelling and enlargement of the collagen bundles. Some collagen cells die, while others transform into other types of cells that can survive in such an inhospitable environment. These new cells, called chondrocytes, produce aberrant tissues, such as islands of cartilage and bone, within the diseased suspensory tissue. The end result is an entire suspensory ligament that is converted into a mass of disorganized scar tissue that has lost its normal biomechanical properties. 

The challenge and the opportunity ... 

Just over two years ago the privately funded research group named DSLD Research, Inc. was started with the charter to tackle the problem of DSLD. DSLD Research, Inc. is a broad-based collaborative effort involving private practitioners and several universities. Within the last two years concerted research efforts have untangled much of the mystery surrounding DSLD. At the outset of the project, little other than anecdotal information was known about DSLD, and a paucity of that. Research efforts to date have been enormously successful, providing a much greater understanding of what DSLD is and is not - debunking many of the myths surrounding the disease. 

DSLD has now been accurately described in the scientific literature ? clinically, ultrasonographically and pathologically. We have learned that Peruvian Pasos do differ from other breeds affected with DSLD in significant ways. They typically develop the disease at much younger ages (usually well under 12 years), more often develop four-limb DSLD and consistently develop DSLD without meaningful physical exercise (5). Most importantly, we now have an accepted protocol with established parameters to aid in the diagnosis of affected horses. We are extremely fortunate to have come so far in only two years. The information gained will have positive real-life applications for owners and breeders in their efforts to minimize and manage the occurrence of DSLD. 

Looking forward, the view is promising that DSLD will become a disease of the past. Research efforts are underway to further understand the clinical behavior, the biochemical nature and the possible genetic basis behind DSLD. Specific projects are aimed at seeking tests to predict whether a horse will or will not develop DSLD in the future. Until these tests become reality, we must take advantage of the tools we have today to enable us to look for and diagnose DSLD. It is now possible for mares and stallions to be examined and found to be free of DSLD before being bred. Peruvians to be sold can be found to be unaffected with DSLD at the time of a sale. The energies we expend today will help tremendously in our efforts to control DSLD and in demonstrating to the equine world our sincere desire to breed and sell sound, long-lived horses. Consider these efforts to be similar to the annual eye screening process (CERF), or the hip/stifle/elbow screening (OFA or Penn Hip) process that are so prevalent in the dog breeding world ? where precise, predictive tests for these types of conditions are unavailable. 

Buyers and sellers of Peruvian horses should be aware that DSLD is an appropriate consideration in a pre-purchase exam. Purchase exams are usually performed with specific health issues in mind that pertain to the intended animalís breed and type of desired athletic activity. Consider, for example, the parallel in the stock horse world: screening for navicular disease in Quarter Horse types is common practice during the pre-purchase exam on horses of this background (among others), sometimes entailing radiographs (x-rays) of the navicular bones. People do not refrain from buying Quarter Horses just because they seem to have a lot of navicular disease and just because there is no positive, predictive test for that syndrome. Rather, the overwhelming popularity of Quarter-type horses is testament to the power of educated buyers, sellers and veterinarians making informed decisions. Remember the quote "Knowledge is Power." 

The facts and data about what DSLD really is, what it looks like and how to diagnose it ante mortem (before the horse is put to sleep) are still very new ? giving us the first concrete opportunity to use scientific information to help us select away from this disease. We must begin now. With this new knowledge not another DSLD horse need knowingly be bred or sold. We urge you not to wait until the "perfect" test comes along that will give us all the answers we hope for. It is humankindís duty to preserve and protect the well-being, longevity and viability of our domesticated species. Given the current state of scientific knowledge, why would anyone choose otherwise? 
 

References 

1. Young J.H. Degenerative Suspensory Ligament Desmitis. Hoofcare and Lameness. 1993;61:6-19. 

2. Pryor P.B., Pool R.R., Wheat J.D. Clinical and pathological characterization of suspensory apparatus failure in Peruvian Paso horses. Unpublished paper: 1984. 

3. Dyson S. Diagnosis and prognosis of suspensory desmitis, in Proceedings. 1st Dubai Int Symp  1996;207-225. 

4. Gibson K.T., Steel C.M. Conditions of the suspensory ligament causing lameness in horses. Equine Vet Edu 2002;4:50-64. 

5. Mero J.L., Pool R.R. 20 cases of Suspensory Ligament Desmitis in Peruvian Paso Horses, in Proceedings. AAEP  2002;48:329-334. 

6. Cuesta I.C., Riber C., Pinedo M. et al. Ultrasonographic measurement of palmar metacarpal tendon and ligament structures in the horse. Vet Radiol Ultrasound 1995;36:131-136.

  
Above:  Suspensory ligaments of the forelimb. 
 
 
Above:  Mare affected in all four limbs, mid to advanced stage disease.  Note the loss of clean outline of the fetlock joints and protruding bumps on the outer and inner edges of both fetlocks.  These bumps represent the progressive, degenerative scar tissue that DSLD horses produce in abundance to try to heal their diseased suspensory ligaments. 

 
Above:  Mare in beginning to mid stages of rear limb DSLD with mildly enlarged rear ankles. 

 
Above:  Gelding in advanced stages of rear limb DSLD. Note greatly enlarged rear ankles and straight limb angles from top of limb to bottom (post legged). 

 
Above:  Shows tissue specimens from leg necropsy exam of gelding in figure above. The left front leg (LF) is not affected. The left rear leg (LR) shows grossly distorted and enlarged shape and size of tissues compared to unaffected left front leg. The view is from the back of the leg, showing all soft tissues behind the cannon bone. 

 
Above:  This figure illustrates the tissue parts seen in specimens shown in photo above.  
 
 
Above:  Photo of front limbs from four limb affected mare. Note bump just below both knees that represent bony deposits in her suspensory tissues. This mareís limbs from knee to fetlock were grossly enlarged and solid. 

 
Above:  Ultrasound image of the affected enlarged tissue with the bony deposits seen in above mare's limb. 
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Above:  Ultrasound image shows cross section view from back of ankle (top) towards front (bottom) with greatly enlarged suspensory ligament branches. 

 
 Above:  Ultrasound image shows cross section view from back of ankle (top) towards front (bottom) with greatly enlarged suspensory ligament branches. 
 

Dr. Jeanette Mero graduated from the College of Veterinary Medicine at Cornell University in 1994. She then completed an internship in large animal medicine and surgery at the Ontario Veterinary College at the University of Guelph in Canada. She has been in private equine practice since her internship. She is married to Peruvian breeder/trainer and judge Rick Mero. She and Rick make their home in upstate New York on a working horse ranch, with their three young children. Dr. Mero also competes in endurance racing with her Arab gelding. 

More Information 

Need more information? Go to www.dsld.org to learn more about the researchers, for current information on the research and to find photos and ultrasound scans of affected horses. The website http://www.dsld.org/  also allows the downloading of a diagnostic protocol for DSLD and its accompanying exam form.