Physitis is the most common developmental disease that affects the bones of growing horses.
This condition involves swelling around the growth plates (physes) of long bones in a young horse’s legs. It can lead to pain, stiffness in the joints, and lameness.
Physitis tends to occur in foals less than 7 months old and is predominantly seen in Thoroughbreds and sport horses. In rare cases, it may appear in horses up to two years old.
Rapid growth linked to feeding too much energy is a major contributor to the development of physitis. This condition is also associated with excessive exercise, obesity, and hormone imbalances.
In most cases, physitis does not cause serious problems but it could have long-term effects if not addressed early. Prompt recognition of the clinical signs and appropriate management are key to supporting a horse with physitis.
What is Physitis?
Formerly known as epiphysitis, physitis is one of several developmental orthopedic diseases (DODs) that can affect growing horses.
It is characterized by inflammation in the growth plates, which are the areas in bone where new growth occurs. This condition can lead to abnormal bone development.
Physitis most commonly involves the radius, tibia, cannon bones, and long pastern bone (first phalanx).
In severe cases, both front legs or hindlimbs may be affected. Occasionally, all four limbs are affected. 
The first sign of physitis is often a warm swollen area above the knee, fetlock or hock. Usually, the horse will show signs of pain when the area is touched.
Other signs of physitis include:
- Flared knees
- Enlarged or boxy joints
- Stiffness and altered movement
- Pain or discomfort
- Change in behaviour
How Physitis Develops
When a foal is developing in the uterus (in utero), its skeleton is composed of cartilage – a type of connective tissue.
As the fetus matures, this cartilage is replaced by bone in a process known as endochondral ossification.
Most of the cartilage has ossified to bone by the time the foal is born. However, cartilage remains at the growth plates or physeal regions to allow these bones to continue to grow in length.
Breed and sex of the horse and the specific joint affet when growth plates close. However, most are fully closed by 28 months of age. 
Each part of the bone has a different name:
- Diaphysis: the middle of the bone
- Metaphysis: the slightly flared parts before the growth plate
- Epiphysis: The ends of bones
In young horses, the growth plate is situated between the metaphysis and the epiphysis. 
When any type of disruption occurs in the growth of long bones, inflammation can occur at the growth plates.
This typically occurs as body size increases quickly in conjunction with increased activity levels, resulting in stress on the growth plates. 
Causes of Physitis
Similar to other developmental orthopedic diseases, researchers believe physitis is caused by multiple factors.
Suggested causes include malnutrition, conformational defects, excessive exercise, obesity, endocrine disorders, and infection. 
Studies indicate that overfeeding and rapid growth alone are not sufficient to cause physitis. Instead, several factors act together to bring about this condition. 
Physitis is either generalized (occurring in multiple growth plates) or localized (occurring in one specific growth plate).
Generalized physitis is usually caused by nutrition or exercise that puts stress on the growth plates.  Localized physitis is more commonly the result of trauma or infection.
The nutritional composition of your horse’s diet can contribute to physitis by over-supplying calories and promoting excessive growth.
If a foal grows at a rate that exceeds the ability of the growth plates to ossify and adapt, structural damage in the bones can occur.
For this reason, fast-growing foals consuming a high level of soluble carbohydrates are prone to developing physitis. Examples of high-carbohydrate feeds include grain-based concentrates and rich pasture.
In commercial horse operations, foals that are weaned and switched to a higher concentrate diet often develop physitis in the lower part of the cannon bone of the front or back legs.
This is thought to occur because the growth plates in the cannon bones close around the age that many foals are weaned on these farms. 
In addition to nutritional excesses, imbalances within the diet can lead to physitis. For example, horses on a diet with an imbalanced calcium to phosphorus ratio have a higher rate of this condition. 
Trace mineral imbalances, especially involving zinc, copper, manganese, and iron are also linked with an increased risk of physitis.
Trauma and uneven weight bearing in the horse’s joints can also lead to physitis.
Angular or flexural limb deformities can lead to uneven weight bearing. Localized physitis often occurs in the location where there is a conformational defect that overloads the growth plates.
Some cases of physitis occur after a period of under-stimulation where the foal has had restricted movement due to illness or lameness, followed by an abrupt return to exercise.
Traumatic physitis may also develop following surgery. If the growth plates are disturbed during a surgical procedure, inflammation can occur in the area. 
Foals and growing horses can develop infectious physitis from a penetrating wound near the growth plate or from an infection in the blood (hematogenously). Foals with infectious physitis may have a fever and generally appear unwell.
In rare cases, physitis could also begin in utero in mares with placentitis.  In this condition, the placenta becomes inflamed due to an infectious agent.
Infectious physitis is a potentially life-threatening form of this disease and requires immediate veterinary intervention. 
Of all types of physitis, the infectious forms are associated with the lowest survival rate. The short-term survival rate is between 42% – 84%, but early treatment can improve the prognosis for affected horses. 
Foals and growing horses are usually diagnosed with physitis based on the presence of an enlarged physeal region or inflammation around the growth plates. Other clinical data may also be considered, such as the age of the horse and the site of the lesion. 
While diagnosing this condition is usually straightforward, it may be more challenging in cases with only subtle clinical signs.
Radiography (x-ray) is typically used by veterinarians to make a diagnosis as it is readily available, affordable, and can be performed in the field if needed.
The most common radiographic finding is an irregular and widened physis.  Radiographs can also show moderate synovial fluid accumulation in joints.
Ultrasonography (ultrasound) is another useful tool for identifying joint swelling. Ultrasounds also show early sites of erosion or joint cartilage loss.
Another diagnostic tool is magnetic resonance imaging (MRI), which allows for visualization of soft tissues.  MRI is a more expensive technology and is not as commonly used by veterinarians.
Diagnosing Infectious Physitis
In addition to imaging techniques, an arthrocentesis can be performed to diagnose infectious physitis. In this procedure, synovial fluid is extracted from the affected joint and analyzed.
Normal synovial fluid will be clear, yellow and viscous. Synovial fluid from infected joints will be turbid (cloudy), darker yellow or orange, and have reduced viscosity (thickness). 
In foals with infectious physitis, nearby joints should always be evaluated through arthrocentesis to determine whether the infection has spread. 
Blood work is also important to diagnose and monitor possible infections in other parts of the body, including organs. Blood tests reveal nutritional, immune, and electrolyte status. 
Treating Physitis in Horses
In mild cases, horses with physitis often spontaneously heal on their own without much intervention. However, some cases require therapeutic treatment to heal completely.
If your horse is diagnosed with physitis, it is a good opportunity to review their nutrition and exercise program and make any required changes to better support bone health.
Reducing excess energy in the diet is often the first step in addressing physitis. It is also important to ensure the calcium to phosphorus ratio is correct.
Attention should be paid to the inclusion rates of trace minerals and protein. In some cases, vitamin D supplements may be beneficial. 
Your horse’s exercise levels may need to be modified as well to prevent excess stress on the joints. Foals may need to be temporarily confined to a stall with soft bedding or a small paddock to avoid further trauma to growth plates. 
Most growth in the long bones occurs while a foal is at rest. To support healthy bone growth, foals should be allowed sufficient periods of recumbent rest (lying down).
The younger the horse, the more rapid the growth rate, and the greater the importance of these rest periods. 
Keep in mind that the overall goal of rehabilitation for foals with physitis is to support mobility and to avoid limbs becoming too loose or too tight. Foals should be returned to pasture as soon as possible to maintain healthy joints.
If physitis is caused by a bacterial infection, antimicrobial treatments (antibiotics) are used to combat the infection.
In some cases, surgical removal of the infected tissue may be required. Removal of infected tissue may shorten the amount of time that a horse needs to be treated with antimicrobials. 
Systemic non-steroidal anti-inflammatories (NSAIDs) are also commonly prescribed for foals with physitis, especially those with lameness or severe swelling. These medications can help provide pain relief, control the inflammatory response, and prevent tendon contraction.
Only use NSAIDs to treat physitis under the guidance of your veterinarian. Some of these drugs can cause toxicity in very young foals.
COX-selective NSAIDs, such as meloxicam, are considered safer to use. Phenylbutazone (bute) is not recommended due to an increased risk of side effects in young horses. 
Anabolic steroids are another treatment option – especially in rapidly developing horses. These medications can help to advance the maturation of the growth plates and reduce further harm to the area. 
Foals that do not respond to initial treatment should be monitored regularly with x-rays. In some severe cases, surgery may be needed to stabilize bones and to prevent displacement or fractures. 
Prognosis for Foals with Physitis
In most cases, the prognosis for horses with physitis is favorable. Most symptoms resolve on their own as horses reach skeletal maturity and growth plates close. 
However, there are rare cases in which physitis causes premature closing of affected growth plates and leads to angular limb deformities. 
In foals with infectious physitis, lameness may persist because of damage to joints, growth plates, or soft tissue around the joints. 
This form of physitis more commonly leads to deterioration of articular cartilage, thickening of the joint capsule, reduced range of motion, and the development of osteoarthritis. Unfortunately, these symptoms can impact a foal’s future athletic career.
This condition cannot be prevented in every instance, but there are measures owners can take to reduce the incidence of physitis and other developmental orthopedic diseases.
You can help prevent physitis by feeding your horse a balanced diet with all the essential nutrients required to support bone growth. Avoid over-feeding concentrates and do not aim for quick growth in foals.
Instead, your feeding plan should focus on supporting slow and steady growth that won’t overload developing joints. Learn more in our article on 8 key principles for supporting joint health in horses.
Looking for assistance in designing a balanced diet for your growing horse? Our nutritionists can help for free! Submit your horse’s information online for a complementary consultation with one of our nutritionists.
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- Whitton, C. Physitis in Horses. Merck Manual. 2015.
- Lindegaard, G., et al. Haematogenous septic arthritis, physitis and osteomyelitis in foals: A tutorial review on pathogenesis, diagnosis, treatment and prognosis. Part 1. Equine Veterinary Education. 2020.
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