Bit & Bridle

Equine Epiphysitis (Physitis)

Other Names

  • Physitis
  • Physeal Dysplasia
  • Growth Plate Disease
  • Physeal Osteochondrosis

What is Equine Epiphysitis (Physitis)?

Equine Epiphysitis — more accurately called Physitis — is a developmental condition affecting the growth plates (physes) of young horses. The growth plate is a zone of cartilage near the ends of long bones where longitudinal bone growth occurs. In physitis, this region becomes inflamed, swollen, and painful, disrupting normal bone development and causing varying degrees of lameness and limb deformity if left unmanaged.

Despite the historical name “epiphysitis” — which implies inflammation of the epiphysis (the end of the bone) — the primary site of pathology is actually the physis (growth plate) itself, hence the preferred modern term “physitis.” The condition most commonly affects the distal radius (just above the knee), distal cannon bone (fetlock level), and distal tibia (just above the hock), though other growth plates can be involved.

Physitis is a condition of growing horses — it is most common in foals and weanlings aged 4–8 months, and in yearlings undergoing rapid growth. It is one of several conditions grouped under the umbrella term Developmental Orthopaedic Disease (DOD).

Causes of Equine Epiphysitis

Physitis is multifactorial — several factors interact to produce disease:

  • Rapid growth: Fast-growing foals and yearlings are at highest risk. When bone growth outpaces the maturation of periarticular soft tissues and the growth plate’s ability to handle load, inflammation develops.
  • Nutritional imbalances: Overfeeding energy (particularly high-starch diets) stimulates rapid growth and increases physitis risk. Imbalances in calcium, phosphorus, zinc, and copper disrupt normal bone and cartilage mineralisation. A high Ca:P ratio (above 6:1) or low copper levels are specifically implicated.
  • Excessive body weight: Overweight foals place greater mechanical stress on developing growth plates.
  • Limb conformational abnormalities: Angular limb deformities create uneven loading across growth plates, accelerating localised physeal damage.
  • Genetic predisposition: Certain breeds — particularly Thoroughbreds and Warmbloods selectively bred for large size and rapid growth — are predisposed.
  • Excessive exercise on hard ground: Concussive forces on immature growth plates contribute to physical trauma and inflammation.

Symptoms and Signs

Signs are typically observed in foals and young horses and include:

  • Firm, warm, and painful swelling immediately above a joint — most visibly above the knee (distal radius), at the fetlock (distal cannon bone), or above the hock (distal tibia)
  • Symmetrical or bilateral swellings (both limbs affected) are common, though asymmetry is also seen
  • Varying degrees of lameness — from subtle reluctance to move freely to overt three-legged lameness in severe cases
  • The foal or yearling may spend more time lying down
  • Reduced growth rate or unexplained stiffness
  • In chronic cases: angular limb deformity as asymmetric physeal damage causes uneven growth on either side of the plate

Diagnosis

Diagnosis is primarily clinical and radiographic:

  • Physical examination: Palpation of the affected physes for warmth, swelling, and pain. The swelling in physitis typically forms a “button” or ring of firm tissue around the physeal region, distinct from joint effusion.
  • Radiography: X-rays of the affected limbs are essential to confirm physitis (showing widening, irregularity, or flaring of the physis) and to rule out other diagnoses, particularly septic physitis and angular limb deformities requiring surgical correction.
  • Nutritional assessment: Review of the mare’s and foal’s diet, body condition scores, and growth rates should be part of every physitis workup.
  • Distinguishing from septic physitis: Septic physitis (growth plate infection) presents similarly but with more acute onset, more severe lameness, systemic signs (fever), and requires urgent antibiotic and sometimes surgical treatment. Synovial fluid analysis may be needed to differentiate.

Treatment

Management is directed at reducing physeal loading and correcting the underlying nutritional or growth rate imbalance:

  1. Dietary restriction and rebalancing: Reduction of energy intake to slow excessive growth rate is often the most important step. Reduce or remove grain and high-energy concentrate feeds. Ensure the diet meets mineral requirements without excessive calcium or deficient copper and zinc. Your vet or a qualified equine nutritionist can advise on a specific rebalancing programme.
  2. Controlled exercise restriction: Limiting strenuous exercise and turnout on hard or deep surfaces reduces concussive loading on physes. Consistent, controlled movement (e.g., small paddock turnout) is generally preferable to complete box rest.
  3. NSAIDs: Short courses of phenylbutazone or flunixin can manage pain and inflammation during acute phases, enabling the horse to move comfortably and preventing disuse muscle atrophy.
  4. Corrective farriery: If angular limb deformity is developing due to asymmetric physeal loading, a farrier experienced in developmental orthopaedics should be involved early. Appropriate trimming and potentially extensions can help redistribute load.
  5. Surgical periosteal stripping: In cases where angular deformity is progressing and conservative management is insufficient, surgical intervention (periosteal transection and stripping) to stimulate growth on the shorter side may be required — this must be performed before the growth plate closes.

Prevention

  • Feed broodmares a balanced diet throughout pregnancy — good maternal nutrition means foals are born with better bone quality and mineral stores
  • Avoid overfeeding foals and weanlings — the temptation to “push” a young horse’s growth for show or sale purposes significantly increases DOD risk
  • Ensure diets are appropriately mineralised, particularly for copper and zinc, and maintain correct Ca:P ratios
  • Monitor growth rates and body condition — a consistent, moderate growth curve is healthier than rapid catch-up growth
  • Avoid hard or extremely deep footing for young horses
  • Early farriery assessment of foals with any angular limb deformities enables prompt correction before growth plate closure

Prognosis

Prognosis for uncomplicated physitis managed conservatively is generally good. Most affected foals and yearlings that receive prompt dietary correction and appropriate management resolve without long-term consequences. Cases with significant angular limb deformity developing from asymmetric physeal damage require more intensive management and carry a more guarded prognosis for future athletic performance if deformity is allowed to progress before intervention.

When to Call the Vet

Call your vet if:

  • A foal or weanling develops swelling around any joint or growth plate region
  • There is associated lameness
  • You observe progressive angular limb deviation in a young horse
  • The foal appears systemically unwell alongside joint or limb swelling (to rule out septic physitis or joint infection)
  • Dietary changes are not producing improvement within 2–4 weeks

Frequently Asked Questions

Is physitis the same as osteochondrosis (OCD)?

They are related but distinct conditions, both falling under the Developmental Orthopaedic Disease umbrella. Osteochondrosis (OCD) involves failure of normal cartilage ossification (bone formation), primarily at joint surfaces, and can result in osteochondral flaps that may require surgical removal. Physitis specifically affects the growth plate region and is primarily driven by nutritional and mechanical factors. Both conditions may co-exist in the same horse.

At what age does physitis typically develop?

Most commonly between 4–8 months of age, corresponding to the period of most rapid post-natal growth. A second peak is seen in yearlings (12–18 months), particularly those in intensive preparation for sales. Growth plates typically close by 18–30 months of age depending on the specific physis — once closed, physitis at that site is no longer possible, though any damage already done may have lasting consequences.

Can physitis lead to permanent lameness?

In the majority of well-managed cases, no. Physitis that is caught early and managed appropriately rarely causes permanent lameness. However, if physitis is severe, prolonged, or complicated by angular limb deformity that progresses without correction, permanent joint and limb abnormalities can result that affect future soundness and performance. Early intervention is key.

Should I change my foal’s feed immediately if physitis is suspected?

Yes — prompt dietary review and adjustment is appropriate once physitis is suspected. Reducing grain and high-energy feeds to slow growth rate is a safe first step while awaiting veterinary assessment. Do not dramatically restrict forage (hay/grass) as roughage is important for gut health. Your vet can advise on an appropriate rebalancing plan once they have assessed the foal and reviewed your current feeding programme in detail.