Epiphysitis, or Physitis, in Young Horses
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Growth Plate Inflammation, Developmental Orthopedic Disease, and What Horse Owners Should Know
Epiphysitis, more accurately called physitis, is a growth-related condition in young horses that affects the physis, or growth plate. It usually appears as firm swelling, heat, and sometimes pain near the end of a long bone. The areas most often noticed by horse owners are around the knee, fetlock region, and hock.
Physitis is usually discussed as part of developmental orthopedic disease, often shortened to DOD. DOD is not one single disease. It is a broad category of growth-related bone, cartilage, and joint disorders that can affect foals, weanlings, yearlings, and other growing horses.
There are two clinically important forms of physitis:
Non-septic physitis, which is the more common form. This is not caused by infection. It is usually associated with rapid growth, diet composition, body condition, mineral balance, conformation, and mechanical loading on the developing limbs.
Septic physitis, which is caused by infection. This form is most often seen in very young foals when bacteria spread through the bloodstream. Septic physitis is urgent and requires veterinary treatment.
Although many cases of non-septic physitis can improve with appropriate management, physitis should not be ignored. Growth plates are active, sensitive structures, and problems during growth can influence limb alignment, comfort, and future soundness.
What Is a Growth Plate?
A growth plate, or physis, is a layer of cartilage near the end of a long bone. It is one of the areas where a young horse’s bones lengthen as the foal grows. You can think of the physis as a cartilage production zone. New cartilage is made, organized, and eventually replaced by bone.
This process is called endochondral ossification. The word sounds complicated, but the concept is simple:
Endo means within.
Chondral refers to cartilage.
Ossification means the formation of bone.
So, endochondral ossification means bone formation that happens through a cartilage stage. This process is normal and essential for growth.
The epiphysis is the end portion of a long bone. The metaphysis is the flared region next to the growth plate, closer to the shaft of the bone. Because physitis causes visible enlargement around the growth plate and nearby bone, older terms such as epiphysitis are still commonly used. However, physitis is usually the more precise term because the physis itself is the growth plate.
For an in-depth overview of the anatomy of growth plates, read Growth Plates: Everything You Need to Know
Did You Know? A young horse’s growth plates do not all close at the same time. Some close earlier, while others remain active longer. This is one reason why age, breed, growth rate, and stage of development all matter when assessing limb problems in young horses.
What Does Physitis Look Like?
Physitis usually appears as a firm swelling around a growth plate. The swelling may be warm, and the horse may react when the area is pressed. Some horses are stiff or lame, while others show swelling with little obvious discomfort.
Common sites include:
The lower end of the radius, near the knee
The lower end of the cannon bone, near the fetlock region
The lower end of the tibia, near the hock
Less commonly, other active growth plate regions
The swelling may be present on both limbs or may be more obvious on one side. Bilateral swelling means both limbs are affected. Asymmetric swelling means one limb or one side is more affected than the other.
The appearance can vary. Some cases are mild and mostly cosmetic. Others are painful and associated with altered movement, limb deviation, or more serious underlying disease.
Why Physitis Happens
The current scientific understanding is that non-septic physitis is multifactorial. This means it usually does not have one single cause. Instead, several factors may come together during a vulnerable growth period.
The most important contributors include:
Rapid growth
High dietary energy intake
Diets that are not properly balanced
Mineral imbalance, especially involving trace minerals
Excessive body weight for the stage of skeletal development
Conformation that increases uneven loading
Inappropriate exercise
Mechanical stress on active growth plates
In simple terms, the growth plate may become irritated when growth demand, nutritional support, and mechanical loading are not well matched.
Diet, Energy Intake, and Growth Rate
Young horses need enough nutrition to grow, but “more” is not always better. Diets that are too energy dense, especially when paired with rapid weight gain, may increase the risk of developmental orthopedic problems.
Read The Basics of Equine Nutrition to learn more about how to ensure your horses are on an appropriate diet.
High-energy diets can influence hormones and growth signals involved in cartilage and bone development. These include endocrine signals such as insulin and thyroid-related pathways. These hormones interact with cartilage cells, called chondrocytes, and may influence how developing cartilage matures and is replaced by bone.
This does not mean that grain is automatically bad, or that a growing horse should be underfed. The goal is steady, moderate growth, not maximum growth. Young horses need adequate protein, vitamins, minerals, and calories, but those nutrients must be supplied in a balanced way.
A foal or yearling that is growing very quickly, gaining excessive weight, or showing swelling near growth plates should have the feeding program reviewed with a veterinarian or qualified equine nutrition professional.
Trace Minerals and Skeletal Development
Trace minerals are needed in small amounts, but they play large roles in cartilage and bone health. Copper, zinc, and manganese are especially important in discussions of developmental orthopedic disease.
Copper is involved in connective tissue development and collagen cross-linking. Zinc and manganese also contribute to cartilage matrix and bone metabolism. These minerals do not work in isolation. They interact with each other, and excessive levels of one mineral can affect the availability or function of another.
This is why simply adding a supplement without evaluating the full diet can be risky. A young horse may already be receiving adequate levels of a mineral, or the real issue may be the ratio between minerals rather than a simple deficiency.
Peer-reviewed studies in pasture-fed Thoroughbreds in New Zealand examined copper supplementation and developmental orthopedic disease outcomes. These studies showed that copper supplementation could influence copper status and some DOD-related findings in that particular population and management system.
However, those findings should not be interpreted as meaning that copper is a universal cure for physitis. The research was context-specific. Pasture mineral composition, breed, soil type, management practices, and baseline diet all matter.
The practical takeaway is simple: balance the diet before adding more supplements.
Did You Know? Copper, zinc, and manganese are often discussed together because they interact biologically. A diet can contain enough of one mineral but still be poorly balanced if the ratios between minerals are inappropriate.
Mechanical Loading and Limb Conformation
Growth plates respond to mechanical forces. This is normal. Bone and cartilage develop in response to the stresses placed on them. However, too much stress, uneven stress, or stress placed on a rapidly growing limb can contribute to problems.
Examples of mechanical contributors include:
A foal that is too heavy for its stage of development
Excessive exercise on hard ground
Poor hoof balance
Limb conformation that places uneven pressure across the growth plate
Sudden changes in activity level
Prolonged confinement followed by intense activity
Balanced movement and appropriate turnout are important for normal musculoskeletal development. Complete restriction is not usually the answer for every case of non-septic physitis, but neither is uncontrolled exercise on hard or uneven ground.
Management should be tailored to the individual horse. A mildly affected youngster may need feeding adjustments and sensible turnout. A painful or lame foal may need more direct veterinary management.
If a DOD problem is left unaddressed, over time, this may lead to lifelong conformation defects that affect the horse’s soundness, comfort, athletic ability, and suitability for certain types of work. Learn more about conformation defects with the following resources:
Non-Septic Physitis Versus Septic Physitis
It is important to separate non-septic physitis from septic physitis.
Non-Septic Physitis
Non-septic physitis is the more common type discussed in growing horses. It is not caused by infection. It is generally associated with growth rate, nutrition, body condition, and mechanical loading.
Signs may include:
Firm swelling near a growth plate
Mild heat
Mild to moderate discomfort
Stiffness
Variable lameness
Bilateral or asymmetric limb involvement
Many cases improve when the underlying growth, nutrition, and loading issues are addressed.
Septic Physitis
Septic physitis is caused by infection in or near the growth plate. It is most concerning in neonatal foals and very young foals. Bacteria may spread through the bloodstream during or after systemic illness, including septicemia.
Signs that raise concern include:
Fever
Depression
Weakness
Marked lameness
A very hot or painful limb
Joint swelling
A very young foal with systemic illness
Imaging changes suggesting focal bone destruction near the physis
Septic physitis can damage the growth plate and may involve nearby joints or bone. This is an emergency situation and requires urgent veterinary care.
Red Flags Horse Owners Should Not Ignore
Call a veterinarian promptly if a young horse has growth plate swelling with any of the following:
Fever
Depression or dullness
Loss of appetite
Severe lameness
Rapidly worsening swelling
A hot, painful joint
A very young foal, especially a neonate
Recent illness, diarrhea, pneumonia, umbilical infection, or suspected septicemia
One limb much more painful than the other
Swelling associated with obvious limb deviation
A firm swelling in a bright, otherwise comfortable weanling is different from a hot, painful limb in a sick newborn foal. Context matters.
How Veterinarians Diagnose Physitis
Diagnosis begins with a veterinary examination and history. Important questions include:
How old is the horse?
When was the swelling first noticed?
Is the swelling on one limb or both?
Is the horse lame?
Has the horse recently grown quickly?
What is the current feeding program?
What is the horse’s body condition?
What type of turnout and exercise does the horse receive?
Has the foal been ill recently?
Is there fever or systemic illness?
The veterinarian will palpate the limb, assess heat and pain, observe movement, and look for signs of limb deviation or joint involvement.
Radiographs
Radiographs, or X-rays, are commonly used to evaluate the growth plate region. In non-septic physitis, radiographs may help assess irregularity, widening, sclerosis, or changes around the metaphysis and physis. They also help rule out other problems.
In septic physitis, imaging can show more aggressive changes. A classic concern is a focal area of bone loss, or osteolysis, centered on the physis and sometimes extending toward the metaphysis or epiphysis. If infection is suspected, the veterinarian may also evaluate nearby joints.
Objective Measurement of Swelling
One peer-reviewed study compared subjective grading of physeal swelling with a quantitative method using digitized images in Thoroughbred foals. The researchers found that objective measurements could help detect and track swelling patterns.
This is useful from a research standpoint because subjective grading can vary between evaluators. However, objective measurement of swelling is not a replacement for veterinary examination, diagnostic imaging, and clinical judgment.
Did You Know? Physitis can be difficult to assess because swelling, heat, and radiographic changes are not always perfectly specific. Similar findings can occur with other developmental or nutritional bone disorders. This is why age, location, clinical signs, diet, and imaging all need to be considered together.
Management of Non-Septic Physitis
The management of non-septic physitis focuses on reducing stress on the growth plate while supporting normal skeletal development.
1. Moderate the Growth Rate
The goal is not to stop growth. The goal is to avoid excessive or uneven growth. If the youngster is gaining weight too quickly, the diet may need adjustment.
This often involves reviewing:
Total calorie intake
Grain or concentrate amount
Pasture quality
Forage intake
Protein balance
Vitamin and mineral intake
Body condition
Growth trends over time
A growing horse should not be placed on an extreme diet. Restricting nutrition too severely can create other problems. Adjustments should be measured and professional.
2. Balance the Diet
A balanced diet is more important than simply adding supplements. The total ration should be evaluated, including pasture, hay, concentrates, ration balancers, and any supplements already being used.
Important nutrients include:
Energy
Protein
Calcium
Phosphorus
Copper
Zinc
Manganese
Other vitamins and minerals involved in growth
Mineral ratios matter. More is not always better.
3. Manage Exercise and Turnout
Young horses need movement for normal development, but the type and intensity of movement matter. Hard ground, excessive forced exercise, abrupt changes in activity, or rough group turnout may worsen stress on growth plates.
Management may include:
Sensible turnout
Avoiding hard or slippery footing
Avoiding forced exercise during painful periods
Monitoring play behavior in groups
Adjusting activity based on lameness and veterinary advice
4. Address Hoof Balance and Limb Loading
Hoof balance can influence how forces travel up the limb. If a young horse has uneven loading, angular limb deviation, or poor hoof balance, veterinary and farrier input may be needed.
Early attention is especially important because growth plates are active for a limited time. In some cases, timely management can help reduce the risk of persistent limb deviation.
5. Pain Control
Anti-inflammatory medications may be appropriate in some cases, but medication decisions should be made by a veterinarian. Young foals and growing horses require careful dosing and monitoring.
Pain control should not be used to mask a problem while the horse continues inappropriate exercise or remains on an unbalanced diet. It should be part of a broader management plan.
Management of Septic Physitis
Septic physitis is treated very differently from non-septic physitis. It requires urgent veterinary care.
Treatment may involve:
Systemic antibiotics
Diagnostic imaging
Bloodwork
Evaluation for septic joints
Synovial fluid analysis if joint involvement is suspected
Surgical or local treatment in some cases
Supportive care for systemic illness
Because septic physitis can occur with septic arthritis or osteomyelitis, the veterinarian must assess more than the visible swelling. The infection can involve the growth plate, adjacent bone, and nearby joint structures.
The prognosis depends on the foal’s age, the organism involved, how quickly treatment begins, whether nearby joints are involved, and how much damage occurs to the growth plate.
Prognosis
The prognosis for non-septic physitis is often fair to good when the condition is recognized early and management changes are made. Many young horses improve with appropriate nutritional adjustment, growth monitoring, hoof care, and sensible exercise management.
However, persistent, severe, or asymmetric physitis may be more concerning. Uneven growth plate stress can contribute to angular limb deviation, ongoing discomfort, or future soundness concerns.
The prognosis for septic physitis is more guarded. Infection can damage the growth plate and adjacent structures. Some foals return to athletic function, while others may remain sound but not reach intended performance goals. Early recognition and aggressive veterinary care are important.
What Horse Owners Can Do
Horse owners play an important role in early detection. Regularly observe growing horses from the front, side, and behind. Look for changes in limb shape, swelling around joints or growth plate regions, altered movement, and differences between left and right limbs.
Keep records of:
Diet changes
Body condition
Growth rate
Exercise changes
Turnout conditions
Lameness episodes
Veterinary findings
Farrier adjustments
For breeding farms, young-horse programs, and owners raising foals, routine monitoring can make a major difference. Small changes are easier to address than advanced limb deviations or severe lameness.
FAQ: Epiphysitis, or Physitis, in Young Horses
1. What is epiphysitis in horses?
Epiphysitis is an older term commonly used for inflammation and enlargement around a growth plate in a young horse. The more precise term is physitis because the physis is the growth plate itself.
2. What causes physitis in young horses?
Non-septic physitis is usually multifactorial. Rapid growth, high energy intake, mineral imbalance, body condition, conformation, hoof balance, and mechanical stress can all contribute.
3. Is physitis the same as developmental orthopedic disease?
Physitis is usually considered part of the broader category of developmental orthopedic disease, or DOD. DOD includes several growth-related bone, cartilage, and joint disorders in young horses.
4. Where does physitis usually occur?
Common sites include the growth plates near the knee, fetlock region, and hock. These areas correspond to active growth regions near the ends of long bones.
5. Is physitis painful?
It can be. Some horses show firm swelling with mild discomfort, while others have heat, pain, stiffness, or lameness.
6. Can physitis affect both legs?
Yes. Physitis may be bilateral, meaning both limbs are affected, or asymmetric, meaning one limb or one side is more affected than the other.
7. Is copper supplementation helpful for physitis?
Copper is important for skeletal development, and peer-reviewed studies in pasture-fed Thoroughbreds showed that supplementation could influence copper status and some DOD-related outcomes in that setting. However, copper is not a universal fix. The entire diet must be balanced.
8. When is physitis an emergency?
Physitis is urgent if the horse is a very young foal, has fever, depression, severe lameness, a very hot painful limb, joint swelling, or signs of systemic illness. These signs raise concern for septic physitis or related infection.
9. How do veterinarians diagnose physitis?
Veterinarians use history, physical examination, lameness evaluation, palpation, and often radiographs. If infection is suspected, additional diagnostics may be needed.
10. Can a horse recover from physitis?
Many horses with non-septic physitis improve with proper management. Septic physitis has a more guarded prognosis because infection can damage the growth plate, bone, or nearby joint structures.
References
Ineson AC, Rogers CW, Firth EC, Gee EK. A comparison of subjective and quantitative methods for detection of physitis in Thoroughbred foals. New Zealand Veterinary Journal. 2004;52(2):65–69.
Dietary Aspects of Developmental Orthopedic Disease in Young Horses. Veterinary Clinics of North America: Equine Practice (ScienceDirect abstract page).
Clinical and Radiographic Findings of Septic Physitis in Foals (case series; published online 2020-09-29).
Septic Arthritis, Physitis, and Osteomyelitis in Foals. Veterinary Clinics of North America: Equine Practice.
Pearce SG, Firth EC, Grace ND, Fennessy PF. Effect of copper supplementation on the evidence of developmental orthopaedic disease in pasture-fed New Zealand Thoroughbreds. Equine Veterinary Journal.1998;30(3):211–218. DOI: 10.1111/j.2042-3306.1998.tb04490.x; PMID: 9622322.
Pearce SG, Grace ND, Wichtel JJ, Firth EC, Fennessy PF. Effect of copper supplementation on copper status of pregnant mares and foals. Equine Veterinary Journal. 1998;30(3):200–203.
This article is for educational purposes only and does not replace professional veterinary or farrier advice.
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