A combination of growth, nutrition and inheritance likely come into play.
Osteochondrosis is a manifestation of developmental orthopedic disease widely recognized in young horses across many breeds. This condition is of particular interest because of its potential to cause joint effusion and lameness in horses preparing for yearling sales or entering training.
Osteochondrosis affects many horses, with prevalence estimates greater than 60 percent in some radiographic surveys.1 Although surgical treatment is often curative, severe or untreated lesions can lead to long-term debilitative consequences.
Despite its great impact on the horse industry, the risk factors contributing to osteochondrosis development have been imperfectly understood. But as new findings from researchers emerge, previous theories are being revised, and, we hope, a more definitive picture of its etiology, genetics, metabolic profile and treatment are on the horizon.
Other conditions fall within the developmental orthopedic disease constellation, including angular limb deformities ("crooked-legged foals"), physitis, subchondral bone cysts, cuboidal bone malformation (collapsed or crushed carpal and tarsal bones), cervical vertebral malformation (wobbler syndrome) and flexural limb deformity.2 However, these conditions are quite disparate, and while some risk factors may be shared among them, their pathophysiologies are distinct from that of osteochondrosis.
Osteochondrosis is characterized by a failure of normal endochondral ossification, the process by which a cartilage template becomes bone in the limbs of a growing animal. It is characterized by abnormal cartilage within a joint that may be thickened, soft or collapsed, or it may be separated entirely from the underlying bone. In the latter case, the condition is commonly referred to as osteochondritis dissecans (OCD).3 Osteochondrosis and OCD thus represent the same pathologic process but are at different points along the spectrum of disease. The terms often are used interchangeably.
The disease is most commonly diagnosed in survey radiographs of yearling horses, often before clinical signs have become apparent. It is characterized by irregularities or roughening of the joint surface or by the presence of an osteochondral fragment partially or completely separated from the parent bone (Figures 1 & 2).4
Figure 1: A typical osteochondritis dissecans lesion of the distal intermediate ridge of the tibia in a young horse. The arrow points to the osteochondral fragment.
While osteochondrosis can occur in nearly any joint, certain areas of predilection include the stifle (lateral trochlear ridge of the femur), tarsus (distal intermediate ridge of the tibia, lateral or medial trochlear ridges of the talus, medial malleolus) and fetlock (distal dorsal midsagittal ridge of the third metacarpus/metatarsus).4
Figure 2: An osteochondritis dissecans lesion of the lateral trochlear ridge of the femur. The arrow indicates multiple osteochondral fragments at this location. This lesion is extensive and would be considered severe.
Osteochondrosis is a complex disease involving interactions of inherited genetic and external environmental risk factors. Environmental factors thought to play a role in disease development include nutrition, exercise, conformation and other biomechanical factors, trauma, stress response, the in utero environment, toxins, hormonal interactions and iatrogenic factors.2,5,6 It has been suggested that osteochondrosis could be caused by either abnormal forces on normal cartilage or by normal forces on abnormal cartilage.6 In either case, the pathologic insult to a susceptible bone was likely of short duration, during a window of susceptibility of joint vulnerability.6 While osteochondral abnormalities may be observed early in life (as early as a few days to weeks of age), many lesions go on to heal spontaneously and do not require treatment.7
Nutritional factors have been found to produce cartilage and bone abnormalities, including osteochondrosis, in young horses. These include consumption of high-energy diets or diets with a high glycemic index, an imbalance of calcium and phosphorus or excessive phosphorus with low calcium. Decreased copper and increased concentrations of zinc also are thought to be associated with an increased incidence of various developmental orthopaedic disorders.7
A documented predisposition to osteochondrosis or OCD is noted in several equine breeds, including Warmbloods, Standardbreds, Thoroughbreds, Quarter horses, Arabians and Paints. In fact, in Standardbreds, up to 50 percent of disease risk is thought to be inheritance.8 A specific genetic defect leading to this predisposition has not yet been identified.
Exercise regimen is also suggested as a potential factor in the etiology or progression of various developmental orthopaedic diseases. As opposed to confinement, voluntary exercise may be beneficial to reduced incidence of disease.7 However, the exact role of exercise in osteochondrosis is far from clear-cut. In one large study, forced exercise was found to affect the distribution of osteochondrosis lesions within joints, but not the total number of lesions.9 Another study found that regular but limited exercise seemed to reduce the risk of osteochondrosis development.10
Elucidation of specific risk factors underlying the development of osteochondrosis or OCD is of current interest to researchers in several veterinary laboratories within the United States. Among these are:
Seasonal growth spurts are associated with an increased occurrence of bone abnormalities in young horses. Sustained modest growth rates are preferred to minimize some developmental orthopedic disease conditions, especially before the earliest window of susceptibility for specific joints.
"Foal growth is not even from birth to 1 year of age," says Ralston. "One week they'll gain weight—the next gain in height. And, especially in the first six months, the rates of gain can be very rapid— over 2 pounds a day for foals that will mature at 1,200 pounds.
"A high incidence of osteochondrosis has been correlated with rapid growth, but not all foals that are growing rapidly will develop lesions," Ralston continues. "And in predisposed foals, cartilage defects can appear even during moderate growth. The key is to feed for optimum nutrition to maintain steady growth. If you try to slow growth by depriving the foal of protein or energy, when it is refed, there will be a rapid compensatory growth spurt that can lead to problems, though not necessarily osteochondrosis."
Ralston says that the biggest concern for osteochondrosis is now thought to be associated with excessive caloric intake, or getting foals too fat and pushing their growth with excessively high-energy-type rations. "The concern about excess protein is no longer valid," say Ralston. "We've repeatedly documented that high protein is not a major concern. It's more important to feed adequate protein for growth and good bone deposition."
She notes a strong genetic component to osteochondrosis development in Standardbreds. "There's a large, distinct metabolic profile of horses that get osteochondrosis when others do not, even when they're on the same rations," she says. "Once we discover the details of the metabolic profile, we can hopefully identify the horses that are predisposed and determine the metabolic defect that's the cause."
She says researchers are close to being able to identify affected foals even before they get the lesion. "We're also in the process of developing targeted supplements that potentially will circumvent what the defect is and allow foals to develop normally," says Ralston.
The glucose-insulin responses are not as important of an issue as once thought, Ralston says. But she does recommend avoiding large amounts of high-starch feeds. However, if a young horse is receiving a sweet feed in its ration, it doesn't necessarily mean that the horse is going to develop osteochondrosis. The total ration's mineral content is what is critical, according to Ralston.
"If you're feeding concentrates, they need to be balanced for calcium and phosphorus and have adequate copper and zinc. Oversupplementation is probably just as bad as under-supplementation," says Ralston.
According to the National Research Council, sufficient copper has been shown to be necessary for formation and repair of collagen, the main component of bone matrix.7 It was reported that rations deficient in copper resulted in clinically diagnosed bone disorders in foals. And foals whose dams were given supplemental copper in the latter stages of pregnancy had a lower incidence of osteochondrosis.
McCoy's area of interest is determining the genetic risk factors for osteochondrosis, widely recognized across horse breeds, especially in Warmbloods, Standardbreds and Thoroughbreds. There is also documented evidence from a study done at Kansas State University that it can occur in feral horse populations.11
"The old argument that it's solely a management or nutritional issue has essentially gone by the wayside," McCoy says. "We recognize it as being a complex disease; there are genetic and environmental factors as to whether an individual foal will be affected, ultimately developing disease."
Several groups are attempting to determine the genetic risk factors for osteochondrosis development. One factor might be an underlying shared predisposition to developmental orthopedic disease in general, but researchers must narrow their focus to get at those underlying factors. Researchers have taken a couple of different approaches, says McCoy. One is the candidate gene approach.
"There are certain genes we know are important in skeletal development, in cartilage and joints," says McCoy. "With this approach, it's speculated there are mutations in one or more of these genes that lead to disease development."
In people, a familial form of quite severe developmental orthopedic disease is due to a mutation in a gene that codes a protein important in cartilage. "In horses, we don't think that's the case, though we see a strong predisposition for osteochondrosis development in certain breeds at certain anatomical lesions or within certain family lines," says McCoy. "We don't think there's one gene that causes osteochondrosis or developmental orthopedic disease, but it's much more likely due to a combination of genes that interact with each other."
The newer approach researchers are taking is the genome-wide association study (GWAS), which has become popular not only in the horse genetics community, but in the human genetics community for complex diseases thought to be the result of many genes working together (also known as polygenic traits).
"To investigate these conditions, you look at a large population of horses—hundreds to thousands—and try to figure out what their genetic code is at certain predetermined locations throughout the genome," says McCoy. These are bases known to vary within individuals in a population and are known as single nucleotide polymorphisms.
"Our current technology allows us to evaluate close to 70,000 individual locations throughout the genome," says McCoy. "We then do statistical associations between allele frequencies at the individual locations and disease status. We look to see if there are particular markers more common in horses that have the disease compared to those that don't."
When genomic regions are found to have this statistical association with disease, it allows researchers to focus their efforts. "We don't think those actual markers are going to cause disease, because they're just common markers in the population," McCoy says. "We hope they're tagging the actual variants that are important risk factors for the underlying disease."
That's the approach being taken, not just by the group at the University of Minnesota College of Veterinary Medicine, but by some European groups as well. The EU research is largely in Warmbloods, while the Minnesota research efforts are concentrating on Standardbred racehorses. Both breeds were chosen because of a very high prevalence of osteochondrosis. It's of animal welfare and economic interest in both breeds.
"The current state of genetic knowledge is that several regions of association have been identified that may be contributing to osteochondrosis risk," McCoy says. "The next step, and where our lab group is heading, is harnessing some of the next-generation sequencing technologies that will allow us to focus on the genes in the regions we've identified through our GWAS and actually find gene variants or mutations that combine to produce an increased risk for osteochondrosis."
"The ultimate long-term goal for everyone is that if we come up with a panel of risk factors, then we'll be able to put together a risk assessment to apply to an individual animal," says McCoy. This will help identify an individual animal as having an increased or decreased genetic risk of disease.
"If we can identify those horses with a high genetic predisposition to osteochondrosis, then we might be able to intervene early with changes to some of the management, nutrition or exercise factors that we know are important, and we may be able to help prevent disease development," says McCoy.
For high-risk horses, researchers could then start checking early for development of disease so veterinarians can make a decision about conservative management or surgical intervention at a young age, before pathology develops. The biggest risk for osteochondrosis is that, if it is left untreated, young affected horses can develop arthritis in a joint. Knowing which horses may be affected also can impact breeding decisions. Of note, horses treated early often go on to perform very well, so breeders may be reluctant to remove affected individuals from a breeding program.
"We'll have to deal with some of those decisions down the road, realizing this is a polygenic disease and there are a lot of genes involved, which complicates matters," says McCoy, "Certainly, if you know the stallion and mare both come from a high-risk line, maybe that's not a breeding you would choose to make. It's going to be one more a tool for breeders and veterinarians."
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.
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