Genetics of degenerative suspensory ligament disease in the horse
Sabrina Brounts DVM, MS, PhD
Diplomate ACVS/ECVS, Diplomate ACVSMR
Clinical Professor of Large Animal Surgery
Peter Muir BVSc, PhD, Diplomate ACVS, ECVS, FRCVS
Melita Grunow Family Professor of Companion Animal Health
Comparative Genetics Research Laboratory, University of Wisconsin-Madison, School of Veterinary Medicine, 2015 Linden Drive, Madison, WI 53706
Ligaments are bands of tough, fibrous tissue connecting bones together in a joint. Ligaments function to stabilize bones within joints so humans and animals can move around normally. The suspensory ligament (SL) in horses primarily functions to support the fetlock joint and to keep it from hyperextending during motion. The SL runs from the carpal bones (knee) in the forelimbs and the tarsal bones (hocks) in the hindlimbs to the fetlock where it splits off into two branches around the sesamoid bones. Suspensory injury is common in athletic horses placing a lot of strain on their lower limbs. Geriatric horses may experience slow dropping of the fetlock as the suspensory ligaments become stretched with age.
Degenerative suspensory ligament disease (DSLD) is a disease in horses that is being recognized more frequently. DSLD is different from other suspensory ligament injuries. The disease onset is subtle in affected horses, typically with no history of injury. DSLD often affects more than one limb, and is usually seen in both fore limbs, both hind limbs or all four limbs. Pain and lameness in multiple limbs then develop over time. DSLD is a disorder that not only affects the SL, but also the cardiovascular system, nuchal ligaments, patellar ligament, deep digital flexor tendon, superficial digital flexor tendon, and the sclera of the eye in affected horses (Halper et al. 2006, Halper et al. 2011). The cause of DSLD is not fully understood. The condition has been diagnosed in multiple breeds such as the Peruvian Horse (Peruvian Paso), the Paso Fino, Warmblood breeds, Thoroughbred, Standardbred, American Quarter Horse, Arabian, American Saddlebreds, and the Akhal-Teke
History of Degenerative Suspensory Ligament Disease
For a long time, DSLD was thought to be a problem that affected older horses, old broodmares, and high-impact sport and work horses. However, cases of DSLD in foals, young horses, and horses that have never participated in high impact activities have also been identified. The condition was first recognized in 1981 by Drs. Pryor, Pool, and Wheat at the University of California, Davis. A study group of 17 Peruvian Paso horses consisting of 8 mares, 7 stallions, and 2 geldings ranging in age from 7 months to 18 years were evaluated. The group had two sets of a mother-daughter relations and one set of father-son. The findings suggested that horses affected with DSLD did not have the typical signs of horses experiencing suspensory apparatus failure due to high impact or high-speed sports. In fact, most of the horses had not participated in such activities and had no previous trauma to the area (Mero 2002). The condition also did not improve with rest as other suspensory injuries do and worsened with time.
Horses with DSLD usually present with a history of lameness, heat or swelling in the fetlocks, enlarged fetlocks, and gradual dropping of the fetlocks towards the ground (Mero and Pool, 2002). The onset of the disease can be really subtle. If the hindlimbs are affected, gradual straightening of the hock angle occurs (Figure 1). Some horses may appear sore in their backs as they adjust their stance to relieve painful limbs. Other horses may dig holes in the pasture to rest affected limbs with their toes down and heels elevated. Affected horses may also lay down more frequently and have trouble getting back up. DSLD usually appears later in the horse’s adult life (greater than 15 years old) but has been diagnosed in foals and horses younger than 15 years old.
In 2005, Drs. Jeannette Mero, and Janet Scarlett developed criteria for diagnosing DSLD in horses. If a horse is suspected to have DSLD, a full lameness exam is done along with an ultrasound image of the suspensory ligament and its branches to confirm DSLD and rule out other possible injuries. DSLD is unique in that it has a bilateral distribution. This means that both forelimbs, both hindlimbs or all four limbs are affected. DSLD horses will be severely lame on affected limbs after a fetlock flexion test and will be noticeably painful on palpation of the suspensory ligament and its branches. The suspensory will also feel harder and thicker than normal, and the area may be hot or swollen. Ultrasound imaging will show visible changes to the suspensory ligament and its branches.
Currently, there is no cure for DSLD. Most treatments focus on making the horse more comfortable. Common treatments include stall rest, controlled exercise, pain relief with non-steroidal anti-inflammatory drugs such as phenylbutazone, flunixin meglumine (Banamine), supportive boots or wraps, and corrective shoeing. Additional palliative treatments such as stem cell injection or platelet-rich plasma injection around the SL, shockwave therapy, laser therapy, magnetic therapy, acupuncture, and chiropractic care have also been tried.
There is no cure for DSLD. In many horses the disease worsens over time. Humane euthanasia may be indicated with severely affected horses.
Genetics of Degenerative Suspensory Ligament Disease
Previous observations and pedigree studies suggest a genetic basis for the disease in some breeds. DSLD may be a simple genetic disease or a complex disease with many genes contributing to risk of disease together with environmental risk. To this date, no estimate of heritability has been made in any horse breed. The prevalence of DSLD in affected breeds has not been reported. Some Peruvian Horse herds have no cases of DSLD, but in other herds the incidence of DSLD may reach ~40% of horses. DSLD clinical signs do not usually appear until the horse is well into or even past prime reproductive years, and the horse may be passing the disease on unknowingly. Relatively few studies have studied the genetics of DSLD. No strong candidate genes have been identified to date. The Comparative Genetics Research Laboratory at the University of Wisconsin-Madison is currently studying the genetics of DSLD in the Peruvian Horse.
Left: A dropped fetlock and a thickened suspensory
is evident in a horse affected with DSLD.
Right: Hyperextension of the hock (straightening of the hock angle)
is evident in a horse affected.
Halper J.; Kim, B.; Khan, A et al. Degenerative suspensory ligament desmitis as a systemic disorder characterized by proteoglycan accumulation. BMC Veterinary Research 2006; 2:12.
Halper J.; Khan A and Mueller P. O. E. Degenerative suspensory ligament desmitis-a new
reality. Pakistan Veterinary Journal, 2011; 31: 1-8.
Mero, J. L.; Pool, R. R. Twenty cases of degenerative suspensory ligament desmitis in Peruvian Paso horses. AAEP Proceedings, 2002; 48
Mero, J. L.; Scarlett, J. M. Diagnostic criteria for degenerative suspensory ligament desmitis in Peruvian Paso horses. J Equine Veterinary Science, 2005; 25: 224-228.
Pryor, P.B.; Pool, R.R.; Wheat, J.D. Failure of the suspensory apparatus in Peruvian Paso horses. 1981 Unpublished paper.
Strong, D. I. The use of a whole genome scan to find a genetic marker for degenerative suspensory ligament desmitis in the Peruvian Paso. University of Kentucky Master’s Thesis. 2005. http://uknowledge.uky.edu/gradschool_theses/419