Peruvian Paso, Arabian, American Saddlebred, and American Quarter Horse owners have been patiently waiting for an update on the diagnosis and treatment of equine systemic proteoglycan accumulation (ESPA). While no new data has been published recently, researchers have been working diligently to make progress in this field.
ESPA is a "new" name for an old disease: degenerative suspensory ligament desmitis (DSLD), a heritable, debilitating syndrome characterized by "an insidious onset of either bilateral or quadrilateral lameness with no history of trauma or injury," according to P.O. Eric Mueller, DVM, PhD, Dipl. ACVS, professor of surgery and Director of Equine Programs at the University of Georgia Large Animal Veterinary Teaching Hospital.
The condition was (and often continues to be) referred to as DSLD because it was widely thought to affect only the suspensory ligament. Either way, the condition is often progressive, leading to recurrent, incurable lameness and, oftentimes, euthanasia. The underlying cause is an overabundance of molecules called proteoglycans (part protein, part carbohydrate) in the suspensory ligament.
In 2006, researchers found that the excessive accumulation of proteoglycans occurred in a variety of anatomic structures with high amounts of connective tissues. Along with the suspensory ligament, these tissues include the:
- Deep and superficial digital flexor tendons;
- Patellar ligaments;
- Coronary arteries;
- Ocular sclera, and
- Nuchal ligament.
Subsequent to this published study, which was previously reported on TheHorse.com, Mueller, Jaroslava Halper, MD, PhD, from the University of Georgia's College of Veterinary Medicine, and colleagues suggested ESPA as it better describes the condition as a systemic disease in which the proteoglycan decorin accumulates in a number of tissues throughout the body.
An affected horse.
Decorin is a small proteoglycan intimately involved in collagen fibrillogenesis. That is, decorin plays an important role in the formation of the strong collagen fibers that are found in connective tissues such as tendons and ligaments.
"We hypothesize that the biochemically and biologically altered decorin found in ESPA leads to impaired regulation of collagen fibrillogenesis, secondary decreases in biomechanical strength of the affected tissues, with eventual tendon and ligament failure as the clinical consequence," explained Halper.
The condition continues to be a diagnostic challenge.
"On ultrasound, the affected structures have a loss of normal appearance, an irregular fiber pattern, and occasionally calcification in the ligamentous tissues," explained Mueller. He did add, however, that it is not possible to differentiate between injury and ESPA based on ultrasound alone. "ESPA-affected tissues postmortem do not have the characteristic fibrosis (scaring) and inflammation observed microscopically that is seen with chronic injury."
The research team started evaluating nuchal biopsies after their paper on ESPA was published.
"The test involves biopsying the nuchal ligament, staining the proteoglycans, and examining the biopsies microscopically to determine if excessive amounts of the proteoglycan called decorin are present in the tissues," Muller said. "It is a subjective test and at the present time it is not possible to make any recommendations on breeding or removing affected horses based solely on the test results."
If the test is positive in a horse demonstrating signs consistent with ESPA, it helps support the diagnosis of ESPA.
"If, however, the horse is asymptomatic and has abnormal amounts of proteoglycan in the nuchal ligament tissues, we still do not have enough information and the test is not specific enough to make any conclusions on whether or not the horse will ever develop clinical symptoms," Mueller said.
Other aspects of the disease that continue to frustrate researchers and horse owners alike is the fact that the underlying mechanism remains undefined.
Halper is hoping that more detailed information regarding the molecular characterization of ESPA will be published in the veterinary literature within the next year.
In the interim, additional research is ongoing.
"We have submitted a grant for funding in collaboration with scientists at Texas A&M in an attempt to design a more specific test that will identify affected horses earlier in the course of disease with the hope that one day we can identify affected horses before the onset of clinical symptoms," Mueller said.
Accurate and early identification of affected horses before clinical signs are apparent will allow owners to make informed decisions on breeding and management of these horses in the hopes of minimizing the morbidity, emotional impact, and economic losses associated with this debilitating disease.
Veterinarians who would like more information regarding nuchal biopsies and tissue handling can contact Halper at UGA.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.