Degenerative suspensory ligament desmitis (DSLD) is a complex, progressive condition that often causes pain and lameness in horses. Typically, veterinarians recognize horses with DSLD by the classic dropped-fetlock appearance, meaning the fetlocks lack their normal upright position when standing and consistently appear to be hyperextended.

Though researchers currently consider DSLD to be uncurable, they are working to better understand the degenerative changes that occur in the suspensory ligament and identify genes potentially involved in the development of DSLD.

Understanding the Suspensory Ligament in Horses

The suspensory ligament is also known as the interosseous muscle because it contains between 2% and 11% muscle tissue, but some consider the suspensory ligament to be more tendonlike in its characteristics.

Regardless of its exact classification, the suspensory ligament is found in all four limbs. In forelimbs the suspensory ligament starts just distal to (below) the accessory carpal bone in the knee. In hind limbs the suspensory ligament begins immediately distal to the tarsometatarsal (hock) joint. In all limbs the suspensory ligament runs down the back (palmar/plantar aspect) of the cannon bone between the second and fourth metacarpal/tarsal bones, also called the splint bones. About halfway along the length of the cannon bone, the ligament divides into two branches, and the branches continue down the back of the cannon bone where they ultimately insert on the proximal sesamoid bones in the fetlock.

The suspensory ligament is a key structure in the equine suspensory apparatus because it supports the fetlock joint and prevents overextension during limb loading.

Degeneration of the Suspensory Ligament

The suspensory ligament contains both collagen and ligament fibers, says Jaroslava Halper, MD, PhD, professor of pathology in the University of Georgia College of Veterinary Medicine and Augusta University’s Department of Basic Science Medical Partnership, in Athens. In DSLD the proteoglycans (large molecules composed of both proteins and carbohydrates) normally found between the fibers that make up the ligament begin to accumulate, disrupting the normal fiber alignment. Halper explains that similar lesions are found in other tendons and ligaments, especially in the superficial and deep digital flexor tendons.

“In addition, accumulated proteoglycans begin looking like cartilage, a connective tissue that naturally consists of large amounts of proteoglycans,” says Halper. “(Increased numbers of) chondroid cells, those that produce cartilage, are also appreciated.”

This abnormal accumulation of proteoglycans in connective tissue can affect the tissue’s tensile properties, adds Sabrina H. Brounts, DVM, MS, PhD, Dipl. ACVS, ECVS, ACVSMR, professor of large animal surgery at the University of Wisconsin-Madison, School of Veterinary Medicine.

Not Limited to Peruvian Horses

Researchers have reported DSLD occurs more frequently in Peruvian Horses (Pasos), in which the incidence of disease can reach as high as 40% in some families. Other breeds, however, are also affected, including Warmbloods, Morgans, Akhal-Tekes, Arabians, and Quarter Horses. Typically, affected horses have no known history of trauma. The condition often affects more than one limb and is usually seen bilaterally, but sometimes all four limbs are involved.

“Initial clinical signs include heat or swelling in the fetlocks, enlarged fetlocks, and gradual dropping of the fetlocks towards the ground,” says Brounts. “Disease onset is typically subtle as the suspensory ligament tissue ruptures gradually over time.” 

In general, affected horses are mid-career, or about 5 to 10 years of age when the first clinical signs are noticed. It is not common for DSLD to first appear beyond 15 years of age.

Diagnosing DSLD in Horses

The specific characteristic that helps veterinarians identify DSLD in a horse is dropped fetlocks. Injury or trauma to other tendons and ligaments around the fetlock might appear similar initially. “Once you see changes to the suspensory ligament in more than one limb and it is really generalized and not just one branch or one limb, especially without a history of trauma, DSLD should start coming to mind,” says Brounts.

“As noted above, DSLD is a systemic disease affecting not only the suspensory ligament but also other tendons and ligaments,” says Halper. “Additionally, the aorta and coronary arteries, among other tissues, are sometimes affected as well.”

Ultrasound can help veterinarians rule out other injuries and might reveal a generalized change to the suspensory ligament in multiple limbs if they are affected; however, ultrasound cannot provide a definitive diagnosis. The only true way to diagnose DSLD is by taking biopsies of the ligament, but this is considered unethical in live animals and the suspensory ligament does not heal well from this procedure.

Post-mortem biopsies of horses with DSLD show proteoglycans accumulating between the individual collagen fibers, disturbing the normal, lengthwise fiber alignment and replacing collagen fibers in more advanced stages of the disease.

Halper says she has also observed a lack of inflammation and fibrosis (scarring) in DSLD cases.

Genetic Aspects of DSLD

The high incidence in some lines of horses suggests a strong genetic contribution to DSLD, most likely polygenic (i.e., involving more than one gene). “No single gene or gene mutation has been identified which would be responsible for DSLD,” says Halper.

To help diagnose DSLD in vivo (in the live animal), Halper and her colleagues have identified at least two genes that could potentially be used as markers of DSLD—FOS and the growth factor bone morphogenic protein 2 (BMP2). These genes can be identified in skin biopsies, making the procedure more ethical than biopsies in live animals. Her group is expanding the number of horses undergoing testing and she hopes the procedure will be available soon to test horses of different breeds, not just Peruvian Pasos.   

Taking a different approach, Brounts and her colleagues were able to confirm these findings. In one of their studies they identified 183 Peruvian Horses with and without DSLD and conducted a genome-wide association study. This is an approach that involves analyzing healthy and affected horses’ genetic markers for differences. In total, they found 151 single nucleotide polymorphisms (SNPs), which are point mutations in DNA that identify genes that might be associated with the disease. Looking further at those SNPs, the team identified 66 different genes that could be involved in the disease process, including genes responsible for proteoglycan metabolism and extracellular matrix homeostasis.

“Recently, the University of Wisconsin-Madison has developed a genetic risk test for DSLD in Peruvian Horses based on a panel of SNP markers using polygenic risk score prediction that is approximately 90% accurate at predicting horses as a DSLD case or control,” says Brounts.  

She adds that genetic testing can be performed on any Peruvian Horse of any age, including foals.

Forward Directions for DSLD

“We will be looking at the markers that we have found in our research so far, how they interact with each other, and how important they are for tendon/ligament disease,” says Brounts, with the hopes of answering these questions:

• Are there markers in DSLD that have been identified in human tendinopathies (disease or damage to tendon tissue) that are similar?
• Is there a way we can influence the disease (i.e., slow it down, maybe reverse, or cure)?
• Are other horses with DSLD like the Peruvian Horse and, if not, what is the difference?
• Can we develop a prediction test for other breeds?

“There is still a lot that we do not know about DSLD, and further research is still needed,” Brounts says.