Degenerative Suspensory Ligament Disease or Desmitis (DSLD) is a chronic and debilitating disorder that was once thought to be limited to the suspensory ligaments of Peruvian Paso horses. These horses experienced a progressive weakening of the suspensory ligament, which caused painful hyperextension of the fetlocks, hocks, and stifles. This unmanageable condition would eventually lead to humane euthanasia.
A SYSTEMIC CONNECTIVE TISSUE DISORDER
Since DSLD was first identified as a disease several decades ago, the research into its cause and genetic inheritance has greatly advanced. Researchers today know that the disease is not limited to the suspensory ligaments and affects tissues and organs made of connective tissue throughout the body. Additionally, many more breeds are affected by DSLD than just Peruvian Pasos. The disease affects Arabians, American Saddlebreds, American Quarter Horses, Thoroughbreds, Warmbloods, Friesians, and several other breeds.
WHAT IS CONNECTIVE TISSUE?
Connective tissue is a specific type of tissue that connects, supports, binds, or separates tissues or organs. Connective tissue contains three kinds of fibers delineating the tissue’s purpose – collagen, elastic, and reticular fibers.
Collagen fibers are found in connective tissue that binds bones and other tissue together, and they can be found in tendons, ligaments, skin, cartilage, blood vessels, and many other places.
Elastic fibers allow the expansion and contraction of organs and arteries and are located in the extracellular matrix – an intricate network of proteins and other molecules that surround and support tissue.
Reticular fibers crosslink and form a fine meshwork that provides structural support in connective tissue. These fibers are found in the bone marrow and the organs in the lymphatic system, like the kidneys, spleen, and lymph nodes.
TOO MANY PROTEOGLYCANS
Researchers learned DSLD is not caused by the fibers in connective tissue as once thought, but rather an abnormal accumulation of proteoglycans between the fibers of connective tissue. Proteoglycans are a type of carbohydrate-modified protein with high molecular weight. Proteoglycans are found in and make up a major part of the extracellular matrix of connective tissue. Why does this matter? The extracellular matrix is what gives connective tissue its physical scaffolding for housing cells. Proteoglycans are meant to fill up the spaces between the cells and provide structural support. However, in horses with DSLD, there are simply too many proteoglycans, and this weakens the affected tissues.
While the tendons and ligaments in the hind legs of horses were found to have the highest accumulation of proteoglycans, tendons from other areas of the body, as well as tissues in the aorta, arteries, sclera (outer layer of the eye), and other organs were also found to be affected by DSLD.
AN ABNORMAL HEALING RESPONSE
Over time, tissues affected by DSLD begin to fail when resisting regular forces of tension. In healthy tissue, a strain is repaired with cells that produce collagen called fibroblasts. Fibroblasts 'bridge' the damage and, in time, repair the damaged tissue. New collagen fibers then orient themselves in line with the stress on the tissue as healing progresses.
Horses affected with DSLD have an abnormal healing response. Their damaged tissue heals with cartilage instead of collagen and is unable to restore itself to normal strength. As the disease progresses over time, the damaged tissue continues to 'breakdown' even further with just the strain of its normal function.
NO CURE FOR DSLD; ONLY MANAGEMENT
Diagnosing DSLD is primarily based on clinical signs such as dropped fetlocks and ultrasonography of the affected tissues. Unfortunately, there is no cure or treatment for DSLD.
Various management techniques have been attempted to make horses in the latter stages of DSLD more comfortable, primarily through support to the affected limbs and treating inflammation and pain. However, none have been effective at halting the progression of the disease. Researchers believe DSLD to be hereditary, though the exact inheritance has yet to be determined.
PARTICIPATE IN DSLD RESEARCH!
Correlations Between Connective Tissue Diseases in Friesian Horses
The University of Georgia (UGA) has developed a diagnostic test for DSLD, a disease caused by an inappropriate accumulation of proteoglycans in connective tissue. UGA would like to determine if their test could also be used to identify other connective tissue-related diseases in Friesian horses. A DNA profile will be extracted from the tissue samples and sequenced, and the tissue's histology will be examined. The FHANA Health Committee is helping to recruit Friesian study candidates.
Friesians with a documented diagnosis of DSLD
Friesians with a documented diagnosis of Megaesophagus
Friesians with a documented diagnosis of Chronic Proliferative Pastern Dermatitis (CPPD)
Friesians with a documented diagnosis of regular pastern dermatitis
Control subjects (Friesians unaffected by DSLD, megaesophagus, CPPD, or pastern dermatitis)
Study Type: Punch biopsy taken from under the mane (2-3 biopsies, approximately 7 millimeters in diameter).
Remote Participation: Yes. Samples will be shipped to UGA.
Recruitment Period: Open. No end date established
Enrollment Information: Email email@example.com
Degenerative Suspensory Ligament Disease (DSLD) The University of Wisconsin-Madison is currently recruiting study candidates for a project which will attempt to discover the gene or genes that cause DSLD. The DNA of the affected horses will be compared with that taken from control horses and checked for differences between the samples. Preliminary data indicates certain genes are worth noting, but more horses need to be analyzed. After looking at the Peruvian Paso, the study will include other breeds and compare those results with that found in that breed. This research is expected to lead to a genetic test to identify horses at risk for DSLD and help guide patient management. Requested Subjects: Peruvian or other breeds affected with DSLD. Study Type: DNA sample (blood or hair) Remote Participation: Yes Recruitment Period: Open. No end date established Enrollment Information: https://www.vetmed.wisc.edu/lab/corl/dlsd-study/
Halper, J., Kim, B., Khan, A. et al. Degenerative suspensory ligament desmitis as a systemic disorder characterized by proteoglycan accumulation. BMC Vet Res 2, 12 (2006). https://doi.org/10.1186/1746-6148-2-12
Brounts, S., Muir P. Genetics of degenerative suspensory ligament disease in the horse. (2019). University of Wisconsin Madison School of Veterinary Medicine. Online.
Ramey, D. Degenerative Suspensory Ligament Disease (DSLD, ESPA). (2021). Dr. Ramey. Online.
Koontz, P. Degenerative Suspensory Ligament Disease in Horses. (2018). The Horse. Online.
Young, J. Degenerative Suspensory Ligament Desmitis. Virginia Therapeutic Farriery. Online