Epiphysitis in horses is an inflammation of the growth plates in the limbs of foals and young horses, producing visible swelling and sometimes lameness at sites where bone is still actively growing. The condition is more accurately called physitis or physeal dysplasia, since the problem sits in the growth plate (physis) rather than the epiphysis itself, but “epiphysitis” remains the term most horse owners encounter first. It is one of the most common developmental orthopedic diseases in young horses, and understanding what drives it can mean the difference between a straightforward dietary correction and a problem that reshapes a horse’s limbs permanently.
What Happens at the Growth Plate
Long bones in a young horse grow in length at their ends, in a zone of cartilage called the physis. Cartilage cells here multiply, stack up in columns, and gradually convert to bone. When the process goes smoothly, the bone lengthens evenly and the growth plate eventually closes, leaving solid bone. In epiphysitis, something disrupts this orderly conversion. The cartilage zone becomes inflamed and swollen, and the region around the growth plate thickens visibly. You can often see it as a distinct bulge just above or below a joint, most commonly around the fetlock, the knee (carpus), or the hock.
Not all growth plates are equally vulnerable. The ones most commonly affected are the distal cannon bone (the bottom end of the third metacarpal or metatarsal), the distal radius (just above the knee), and the distal tibia (above the hock). These sites bear significant load during the rapid growth phases that characterize the first year or two of life. In Icelandic horses, researchers found that certain growth plates, including those in the lower phalanges and the proximal cannon bone, close quite early, while others remain open for much longer and are therefore exposed to mechanical and metabolic stress over a wider window.1PubMed Central. Radiographic closure time of appendicular growth plates in the Icelandic horse The timing of closure varies by breed, but the principle holds across all horses: growth plates that stay open longer have more time for things to go wrong.
Rapid Weight Gain as a Primary Driver
The single most studied risk factor for epiphysitis is how fast a young horse is gaining weight. A study examining the link between body weight gain and bone aberrations found that the major factor affecting the widening of the growth plate was average daily gain in body weight. Foals gaining weight at an above-average rate showed a larger number of bone abnormalities in their growth plate regions, leading researchers to conclude that rapid weight gain is a significant factor in the onset of epiphysitis.2Journal of Equine Veterinary Science. The effect of above average weight gains on the incidence of radiographic bone aberrations and epiphysitis in growing horses
This makes intuitive sense. A foal that is growing rapidly is putting on body mass faster than its skeletal system can remodel to support the load. The growth plates, being the weakest structural link in a developing bone, bear the consequences. The problem is especially common in well-fed Thoroughbred and Warmblood foals on high-energy diets, where owners or breeders are pushing for size and condition in anticipation of sales or early training. The irony is that the diet designed to produce an impressive-looking yearling can create the very skeletal problems that make the horse unsound.
This does not mean all rapid-growing foals develop physitis. Genetics, conformation, the specific nutrient balance in the diet, and exercise all interact. But if you see swelling at the growth plates of a fast-growing foal, the first thing any experienced equine veterinarian will ask about is the feeding program.
Mineral Imbalances and the Copper Connection
Beyond sheer caloric intake, the mineral composition of a young horse’s diet plays a major role. Copper has received the most research attention. A study of foals raised on a low-copper diet found that microscopic examination of their growth plates revealed fractures that disrupted the normal architecture, displacing the orderly columns of cartilage cells and interfering with normal bone growth.3Equine Veterinary Journal. Correlative study of defective cartilage and bone growth in foals fed a low‐copper diet The damage pattern was similar to what had already been documented in copper-deficient cattle, suggesting a shared mechanism across species: copper is essential for the enzymes that cross-link collagen, and without adequate copper, the structural protein in cartilage and bone is weaker than it should be.
Supplementation research supports this link. When pregnant mares in New Zealand were given copper supplements, their foals showed significantly lower radiographic signs of physitis in the distal cannon bone at 150 days of age, along with fewer cartilage lesions.4PubMed. Effect of copper supplementation on the evidence of developmental orthopaedic disease in pasture-fed New Zealand Thoroughbreds An important detail from that same study: supplementing the foal directly did not produce the same benefit. It was the mare’s copper status during pregnancy that mattered. This suggests the critical window for copper’s protective effect may be during fetal development, when the skeletal template is being laid down. Even with copper supplementation, though, the researchers noted that developmental orthopedic disease was not eliminated, underscoring that physitis is a multifactorial condition rather than a simple deficiency disease.
Calcium-to-phosphorus ratios also matter. An inverted ratio, where phosphorus exceeds calcium, can impair bone mineralization and stress growth plates. High-grain diets tend to be phosphorus-heavy, which is one reason nutritionists recommend balancing concentrates with adequate forage and a mineral supplement designed for growing horses. Zinc is another trace mineral implicated in cartilage integrity, though the evidence base is thinner than for copper.
The Influence of Diet Composition Beyond Minerals
It is not just how much a foal eats or which minerals are in the feed. The type of energy in the diet appears to make a difference on its own. Research comparing foals raised on diets that differed in their carbohydrate and fat content found that the source of dietary energy influenced bone mineral content and that clinical leg evaluations showed differences in physitis and joint problems related to age and possibly season.5Oxford Academic (Journal of Animal Science). Dietary carbohydrates and fat influence radiographic bone mineral content of growing foals High-starch diets can trigger insulin spikes, and there is growing interest in whether the hormonal cascade that follows a starchy meal may directly affect cartilage metabolism in the growth plate. Fat-supplemented diets offer the same caloric density with a lower glycemic response, which is why some nutritionists now recommend replacing a portion of grain calories with oil or high-fat supplements for at-risk foals.
Season also plays a role, though it is hard to separate from pasture quality. Spring grass is energy-dense and high in sugars, and many cases of physitis appear in late spring or early summer when foals are hitting growth spurts while grazing lush pasture. Restricting pasture access or using a grazing muzzle on the mare (which slows the foal’s access indirectly by limiting the mare’s milk production somewhat) is a strategy some breeding farms employ, though the evidence for this is more anecdotal than controlled.
Recognizing the Signs
The hallmark of epiphysitis is a firm, warm swelling at one or more growth plate sites. The most common spot is just above the fetlock joint on the front legs, where the bottom of the cannon bone meets the growth plate. The swelling is usually symmetrical, affecting both front legs, though one side can be worse than the other. In more advanced cases, the area is painful to pressure and the foal may shift its weight, stand with its legs camped out in front of it, or show obvious lameness at the walk or trot.
When the distal radius is involved (the growth plate above the knee), the swelling appears as a widened, boxy look to the front of the knee. Foals with distal tibial physitis will show enlargement above the hock. In severe cases, asymmetric inflammation can start pulling a limb out of alignment, creating an angular limb deformity where the leg angles inward (varus) or outward (valgus) below the affected growth plate.
A large Irish study tracking Thoroughbreds from birth to 18 months found that angular limb deformities and physeal dysplasia together accounted for nearly three-quarters of all developmental limb problems treated in the cohort.6PubMed. The incidence of abnormal limb development in the Irish thoroughbred from birth to 18 months That figure gives a sense of how dominant physitis and its related deformities are in the landscape of young-horse orthopedic issues.
How Veterinarians Diagnose It
Mild physitis is often diagnosed on clinical examination alone. A veterinarian palpates the growth plate region, assesses pain, and watches the foal move. Radiographs are the standard next step when the swelling is significant, when lameness is present, or when an angular deformity is developing. On X-ray, an affected growth plate appears widened, with irregular margins and sometimes a flared metaphysis (the flared zone just below the growth plate). These findings help distinguish routine physitis from more serious problems like fractures through the growth plate or osteochondrosis.
In unusual or severe presentations, advanced imaging can reveal more. A case report of a Thoroughbred foal with septic apophysitis of the proximal humerus, a relatively rare location, demonstrated the value of computed tomography: standard radiographs showed only mild irregularity and sclerosis at the growth plate, but CT revealed a distinct area of bone destruction that radiographs missed.7Equine Veterinary Education. Multimodal Diagnostic Imaging to Identify Septic Apophysitis of the Proximal Humerus in a Thoroughbred Foal Septic physitis, where a bacterial infection settles in the growth plate via the bloodstream, is a distinct and more urgent condition than the nutritional or mechanical physitis discussed so far. It requires aggressive antibiotic treatment and sometimes surgery, and the prognosis depends heavily on how quickly it is caught.
Managing Mild to Moderate Cases
The good news about garden-variety physitis is that most cases respond to management changes without surgery. The core strategy involves several elements working together:
- Dietary adjustment: Reduce total caloric intake to slow the growth rate. Cut or eliminate grain concentrates and switch to a ration balancer that provides vitamins and minerals without excess energy. Ensure the calcium-to-phosphorus ratio is at least 1.5:1 and that copper, zinc, and other trace minerals meet requirements for growing horses.
- Controlled exercise: Confine the foal to a stall or small paddock to limit concussive stress on inflamed growth plates. Complete stall rest is rarely necessary and can be counterproductive, since some controlled movement stimulates healthy bone remodeling. Avoid turnout on hard ground.
- Anti-inflammatory support: Short courses of non-steroidal anti-inflammatory drugs can reduce pain and swelling. Your veterinarian will select the appropriate drug and dose based on the foal’s age and weight, since very young foals metabolize some medications differently than adults.
- Monitoring: Repeat radiographs every few weeks allow tracking of growth plate width and any developing angular deviation. Catching a worsening deformity early broadens the surgical options available.
Most mild cases resolve within weeks once the growth rate is brought under control. The swelling subsides, the growth plate returns to a more normal appearance on radiographs, and the foal grows normally. Owners sometimes panic when they first see the enlarged joints, but physitis caught early and managed promptly rarely has lasting consequences for the horse’s athletic career.
When Surgery Is Needed
Surgery enters the picture when physitis has led to, or is accompanied by, an angular limb deformity that is not self-correcting. The concept behind most surgical approaches is transphyseal bridging: placing hardware across the growth plate on the side of the limb where growth needs to slow, so that the opposite side catches up and straightens the leg. It works only while the growth plate is still open, so timing is critical.
A retrospective study described placing a single transphyseal screw across the distal radial growth plate in Thoroughbred yearlings with carpal varus deformities. The procedure was performed standing under sedation rather than under general anesthesia. A single 4.5-millimeter cortical screw was used, and screws were removed once the leg straightened, at an average of 46 days after placement. No short-term or long-term complications were identified in any of the eight horses treated.8PubMed Central. Standing placement of transphyseal screw in the distal radius in 8 Thoroughbred yearlings
Even younger foals can benefit from the same principle. A five-week-old Thoroughbred foal with unilateral carpal valgus was treated with a single transphyseal screw and showed improvement within three weeks without the need for additional periosteal stripping. A follow-up seven months after screw removal confirmed no over-correction.9Journal of Veterinary Clinics. Single Screw Transphyseal Bridging for Correction of Unilateral Carpal Valgus in a 5-Week Old Thoroughbred Foal Comparing techniques, another study found that a single screw produced more rapid correction of fetlock varus deformities in foals up to five months old than a traditional two-screw-and-wire tension band, with minimal complications and a better cosmetic result.10Equine Veterinary Education. A single screw technique compared to a two screw and wire technique as a temporary transphyseal bridge for correction of fetlock varus deformities
The trend in equine surgery has been toward simpler, less invasive techniques and away from general anesthesia when possible. Standing surgery reduces anesthetic risk and recovery complications, and single-screw methods require smaller incisions and are easier to remove. For breeders and owners, the practical takeaway is that surgical correction of physitis-related angular deformities is now a routine, low-risk procedure when performed within the window of remaining growth plate activity.
Why the Name “Epiphysitis” Persists
The terminology around this condition is a source of quiet frustration in veterinary circles. Strictly speaking, the epiphysis is the cap of bone beyond the growth plate, while the physis is the growth plate itself. The inflammation in this condition is centered on the physis, which makes “physitis” or “physeal dysplasia” the more accurate terms. Yet “epiphysitis” remains stubbornly entrenched in the vocabulary of horse owners, farriers, and even some older veterinary texts.
Part of the confusion is historical. The condition was described before the precise anatomy was well understood radiographically, and the swelling visible on the outside of the leg does not cleanly distinguish physis from epiphysis. By the time researchers sorted out the anatomy, the old name had stuck. You will see both terms used interchangeably in modern practice, and for practical purposes it does not matter which word you use when talking to your vet. But if you are reading research papers or product labels, knowing that “physitis,” “physeal dysplasia,” and “epiphysitis” all describe the same condition will save you from thinking you are dealing with three different problems.
Breed and Conformational Susceptibility
Thoroughbreds and other rapidly maturing breeds are overrepresented in physitis cases, partly because they are selected for early skeletal maturity and partly because the sales cycle incentivizes high-plane nutrition. A Thoroughbred yearling heading to auction in January needs to look like a two-year-old, and the feeding programs that produce that appearance push growth rates into the danger zone. Warmbloods, which mature more slowly, tend to develop physitis later, sometimes in the second year, particularly at the distal radius and distal tibia as their larger frames load those growth plates.
Draft breeds and ponies are not immune, but their lower metabolic intensity and slower growth trajectories make clinical physitis less common. Interestingly, some breeds with naturally later growth plate closure, like the Icelandic horse, may have a wider window of vulnerability but also a wider window for self-correction, since the growth plate remains active longer and has more time to remodel.11PubMed Central. Radiographic closure time of appendicular growth plates in the Icelandic horse
Conformation also plays in. Foals born with a mild angular deviation, even one within the normal range that typically self-corrects, place uneven loads on their growth plates. If physitis develops on top of an existing conformational tendency, the deformity can accelerate. This is why breeders who are serious about limb quality evaluate foal conformation within the first weeks of life, before the growth plates have narrowed the window for conservative management.
Long-Term Outlook and Common Misconceptions
One of the most persistent misconceptions about epiphysitis is that it permanently damages a horse’s soundness. In reality, most cases that are caught early and managed with diet and activity changes resolve completely. The growth plate remodels, the swelling disappears, and the horse goes on to a normal athletic career. Problems arise when the condition is ignored, when the diet is not adjusted, or when an angular deformity develops and is left untreated past the point where the growth plate closes.
Another common misunderstanding is that physitis is caused by too much protein. This belief has been remarkably durable among horse owners despite decades of evidence pointing toward energy excess, mineral imbalance, and rapid weight gain as the true drivers. Protein levels in most commercial horse feeds are not high enough to cause skeletal problems. The confusion likely stems from the fact that high-protein feeds are often also high in calories, and it is the caloric load, not the protein itself, that pushes growth rates into risky territory.
A subtler misconception involves the idea that simply adding a mineral supplement will fix or prevent the problem. As the copper supplementation research demonstrated, even meaningful improvements in mineral status did not abolish developmental orthopedic disease in the foals studied.12PubMed. Effect of copper supplementation on the evidence of developmental orthopaedic disease in pasture-fed New Zealand Thoroughbreds Physitis is the product of multiple converging factors: genetics, growth rate, diet composition, mineral status, exercise, and conformation. Addressing only one variable while ignoring the others is unlikely to solve the problem. The most effective prevention programs take a whole-picture approach, starting with the mare’s nutrition during pregnancy and continuing through the foal’s first two years with careful attention to growth curves and regular veterinary evaluation.