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. 
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.
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 importanc