Equine OCD: Harmless Bone Lesion or Permanent Problem?
Leading researchers from around the world share their findings on this common joint disease of young horses
“Growth itself contains the germ of happiness,” writes Pulitzer Prize novelist Pearl Buck.
Perhaps it does.
But in young horses, it might also include something a bit less appealing—chips of bone. Osteochondritis dissecans (OCD) is a common disease of domesticated horses characterized by loose cartilage and/or bone fragments in the joints. Fortunately, there’s a “germ of happiness” in the prognosis: Many OCD lesions heal spontaneously within a few months. Those that don’t, however, can cause permanent problems when not treated correctly.
How the condition manifests varies considerably from horse to horse, joint to joint, and lesion to lesion. And researchers say its severity and occurrence rates might be linked to breed, genetics, and environment. To give you a better understanding of this complex growth issue, we’ve put together the most recent work and viewpoints from the world’s leading researchers on this disease.
Getting the Terminology Right
First things first, let’s get a grasp on what’s what in the world of OCD and other growth-related conditions.
Integral to our understanding of OCDs is familiarity with osteochondrosis, a disease that causes lesions in the cartilage and bone of growing horses’ joints. When a lesion actually gets to a point where it separates from the underlying bone, it’s termed OCD.
These are two of the most common issues within a wider classification called juvenile osteochondral conditions (JOCC). Coined by Jean-Marie Denoix, DVM, PhD, professor and director of the Centre d’Imagerie et de Recherche sur les Affections Locomotrices Equines, in Normandy, JOCC groups together all developmental disorders related to immature joints or growth plates in young horses.
These disorders fit within an even broader classification of conditions known as developmental orthopedic disease (DOD). This 1986 term comes from the work of Wayne McIlwraith, BVSc, FRCVS, PhD, Dipl. ACVS, Dipl. ECVS, professor of surgery and director of Colorado State University’s Orthopaedic Research Center. The DODs encompass not only JOCC but also other orthopedic disorders related to development, such as Wobbler syndrome and limb deformities.
Just the Joints
Osteochondrosis is a disease of developing bone, but the only place we find it is in the joints. This has to do with the way bones grow and solidify through a process called endochondral ossification. As bones lengthen over time, they build epiphyseal cartilage at each extremity—the ends that meet the ends of other bones at joints. Little by little, the epiphyseal cartilage turns into true bone—the ossification process. In mature animals, only a thin layer of cartilage remains, covering the articulating ends of the bone within the joint. This is called articular cartilage. Problems during the ossification process can cause osteochondrosis and similar conditions.
Horses, of course, have dozens of joints, with each one uniting the ends of several bones. And every one of these joints contains one or more possible sites for osteochondral lesions, each with its own characteristics.
“Depending on the site at which lesions occur, the consequences, progression, and outcome may be different,” says Cathy Carlson, DVM, PhD, Dipl. ACVP, professor in the University of Minnesota’s department of veterinary population medicine, in St. Paul.
René van Weeren, DVM, PhD, Dipl. ECVS, professor in the department of equine sciences at Utrecht University, in the Netherlands, also stresses the great disparity in types of lesions. “Joints react differently, and even sites within joints are not always comparable,” he says.
That’s why Denoix came up with a new scoring system for osteochondrosis with weighted severity indexes (0, 1, 2, 4, and 8) for individual lesions. In doing this he revamped previous incongruous scoring systems. These new scores help veterinarians follow the evolution of lesions more closely and make better decisions about prognosis and treatment, he says.
So where in the horse is osteochondrosis most common? For the most part it’s pretty variable, depending on age and breed. Denoix’s recent study on hundreds of Warmbloods and Thoroughbred and Standardbred racehorses in France showed that osteochondrosis occurs in weanlings most often in the hind fetlock, affecting 28% of the foals. The second-most-common site in Thoroughbreds and Warmbloods was the front fetlock; in Standardbred trotters, it was the hock and the stifle.
The Big Problem: Failed Vascularization
Osteochondrosis develops in response to blood flow problems. Epiphyseal cartilage and young, new bone are heavily vascularized, meaning they are chock full of blood vessels, pumping in nutrients and taking out waste so as to allow growth.
When epiphyseal cartilage doesn’t get the blood flow it needs, osteochondrosis can develop, says Carlson.
The lack of blood flow “creates an area of cell death and matrix (internal bone structure) degeneration,” she explains. “That can make it unable to support the overlying articular cartilage, and it may collapse under pressure, leading to the formation of a cartilaginous flap, accompanied by pain.”
Why would such vascular failure happen? Kristin Olstad, DVM, PhD, of the Equine Section of the Norwegian University of Life Sciences’ veterinary school, in Oslo, has been working with Carlson to answer that question. So far it looks like a problem of missed connections. As the cartilage at the growing end of the bone develops, the blood vessels in that cartilage must link to the vessels in the underlying bone. Sometimes those hook-ups just don’t happen—for reasons yet to be determined, she says.
But that’s not the only way vascularization fails. Sepsis (bacterial infection), physical damage (such as frostbite), or trauma can also cause blood vessel dysfunction leading to degeneration.
A Selected For Problem
The latest research on osteochondrosis is revealing some staggering figures: As many as 50% of Standardbreds and Dutch Warmblood foals, for example, have osteochondral lesions, says Carlson.
“It is a serious problem,” she says, and one that we keep reproducing year after year through bloodlines. “The disease has a heritable predisposition, so it’s important, in particular, for breeding animals,” Carlson notes.
Also, says van Weeren, it’s a disease that’s almost exclusive to domesticated horses. Wild horses have occurrence rates of around 2% in certain joints, down to zero in others. Ponies (even domesticated ones) are mostly exempt from the disease, he adds.
What does that mean? The statistics strongly suggest that we’ve—accidentally, of course—specifically selected for the genes that cause osteochondrosis, perhaps by breeding for larger, faster-growing animals.
Unfortunately, selective breeding probably won’t help at this point, our sources say. “There’s little hope of eradicating the problem by breeding strategies alone, as environmental factors are known to play a large role and because the disease is definitely polygenic (involving multiple genes),” van Weeren says.
In fact, genomic advances and molecular genetics “have only given further evidence of the complexity of the disease,” says McIlwraith, adding that osteochondrosis genes are present in at least 22 of the horse’s 33 chromosomes.
“Because of that complexity, it’s going to be difficult to ever breed it out,” he says. “And that effort might lead to inadvertently selecting out other desirable traits.”
Links to Feeding and Metabolism
Indeed, there’s such a thing as growing too fast, and it can open the door for osteochondrosis.
“A high plane of nutrition has been associated with thicker growth cartilage, which is more dependent than thin growth cartilage on blood supply for nutrition,” says Carlson.
What’s more, those same foals had a greater risk of developing osteochondrosis in the first six months of life as they nursed from draft mares producing more and possibly richer milk, Peugnet adds.
Environment Plays a Role
It’s case of damned if you do and damned if you don’t: A young foal living in a big field is probably more likely to injure himself than one confined to a stall. But confining a youngster only encourages osteochondrosis development. In a recent study INRA researcher Johanna Lepeule, MS, PhD, showed that foals (particularly those younger than two months) exercising freely in a moderately sized pasture every day developed significantly fewer osteochondral lesions by six months than foals kept in stalls.
To prevent these problems, she says, “they have to get outside and moving.”
One of the most common ways veterinarians discover osteochondral lesions in mature horses is during prepurchase exams, says McIlwraith. That’s because most of these lesions don’t actually cause any clinical signs, so owners are probably unaware their horses even have them.
Young horses, however, often do show clinical signs, such as joint effusion (swelling) and lameness, that merit a work-up, including imaging, he says. These are often caught when young horses first start training, on radiographs of the effused joints. In-clinic radiographs are common, but some owners opt for a more thorough (and more expensive) MRI exam. A less expensive option is ultrasound, which van Weeren says is showing some good results in initial diagnostic studies.
A (Mostly) Self-Healing Disease
Most osteochondral lesions are nothing to worry about because they’ll heal on their own. Van Weeren says lesions diagnosed before a horse is a year old stand a good chance of improving. Carlson places the “age of no return” at 18 months for some fetlock lesions and for the stifle joint, areas where osteochondral lesions continue to evolve a bit longer, she says.
The researchers warn that location does make an important difference in outcome. Ostechondrosis in the femur (the long upper bone of the hind leg) seems to resolve well in nearly all cases. As for hock lesions, Denoix and co-authors on a study reported there was no clinical progression in 60% of cases. And fetlock lesions appeared just as likely to resolve as they would progress.
When Surgery Becomes Necessary
There are situations where OCD becomes problematic enough to warrant arthroscopic surgery to remove the lesion. McIlwraith says that’s usually because the horse will have permanent clinical problems (e.g., arthritis or lameness despite treatment) without surgery or because the high-value horse can’t be sold for a good price without correcting the lesion.
Shoulder lesions, however, depend on the location within the joint. Select cases limited to the glenoid (socket) typically do not require surgery. If they involve the humeral head (the end of the humerus bone that articulates with the scapula), surgery is necessary; still, these operations only have a 50/50 success rate.
Sometimes, McIlwraith says, it’s a “question of your pocketbook,” because, again, it’s possible to just wait things out to see how they progress. Surgery can help improve performance, increase sale values (through clean X rays), and limit the onset of secondary (caused by another condition) osteoarthritis.
But if you choose not to operate, rarely is it a welfare concern. “The only time OCD would be a welfare issue is with certain shoulder lesions, or a severe OCD lesion in the stifle, which we see extremely rarely nowadays, now that people know how to feed their foals better,” McIlwraith says.
Keeping a Reasonable Outlook
Yes, OCD is serious in that it affects a significant number of horses, particularly Warmbloods and Thoroughbred and Standardbred racehorses. But all in all, we might actually be overdoing it when we worry too much about osteochondrosis in our young horses.
There’s no reason, for example, to routinely radiograph our youngsters to check on their osteochondrosis status, say our sources. “Due to the multiple predilection sites in horses and the fact that different sites are affected at times during development, it would not be reasonable or cost-effective to X ray horses at multiple time points,” Carlson says. “Additionally, we have no current therapies for the subclinical lesions (those that do not produce clinical signs) other than resting the affected joint, and this is not usually possible with horses,” because they’re usually standing and frequently moving.
That being said, there are situations when OCD causes enough pain that it becomes a real concern—again, depending on the lesion’s location. “Some OCD lesions in the stifle and shoulder in horses carry a guarded prognosis, and some of these horses have to be euthanized,” Carlson says.
Osteochondrosis is a common but often temporary disease of young horses’ joints. Some lesions come and go without any clinical signs, while others are accompanied by swelling and/or lameness. Surgery is usually pursued for those that don’t heal on their own. While there are some exceptions, horses with naturally or surgically resolved osteochondrosis frequently go on to lead healthy lives to their full athletic potential.
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