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. [1]

Symptoms

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
  • Lameness
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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. [2]

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. [3]

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. [4]

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. [3]

Studies indicate that overfeeding and rapid growth alone are not sufficient to cause physitis. Instead, several factors act together to bring about this condition. [1]

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. [4] Localized physitis is more commonly the result of trau