Synovial infections in horses refer to infections in the synovial structures, which include joints, bursae, and tendon sheaths. These infections can be severe and potentially career-ending for equine athletes.

Most horses develop synovial infections from wounds over top of a joint or tendon sheath. If there is damage to the synovium, bacteria can proliferate within the synovial structure, causing pain and tissue damage.

Symptoms of synovial infections in horses may include lameness, swelling of the joint, heat or oozing wounds. In severe cases, the horse may develop signs of systemic illness, such as fever, lethargy, and loss of appetite.

Any wound near a joint should be thoroughly examined by a veterinarian. Foals are also prone to developing synovial infections from septicemia (blood infection).

Synovial Infections

Synovial infections are infections of the synovium, the specialized tissue that lines joint capsules and tendon sheaths.

These structures contain synovial fluid, a lubricating fluid that ensures smooth joint and tendon movement. Synovial fluid is rich in nutrients, which predisposes it to bacterial infections.

Synovia are present throughout the body; however, they are most common in the limb joints. Common locations for synovial infections include: [1]

  • Navicular bursas
  • Coffin joints
  • Pastern joints
  • Digital flexor tendon sheaths
  • Fetlock joints
  • Hock joints
  • Calcaneal bursa of the hocks
  • Tendon sheaths of the hocks
  • Carpal joints

Foals are more likely to have multiple synovial structures affected, as the bacteria usually spreads from a blood infection. [2][3] The hocks are most commonly affected in foals. [3]

Causes

Synovial infections can develop in several different ways, all of which involve bacteria or other pathogens infiltrating the synovial structures of a joint or tendon.

Traumatic Injuries

The most common cause of synovial infections are wounds that penetrate the joint capsule or tendon sheath, introducing bacteria. [1] Any wound damaging a synovial structure should be considered to have an infection after 24 hours. [1]

Bacteria commonly found in synovial infections from wounds include: [4]

  • Staphylococcus
  • Streptococcus
  • Rhodococcus
  • Corynebacterium

Septicemia

Foals also commonly develop synovial infections, however these infections do not usually arise from traumatic injuries. In foals, joint infections are a consequence of septicemia (blood infection) that spreads to the joints. [1]

Foals experiencing failure of passive transfer have the highest risk of synovial infections. [1] Studies show up to 78% of foals with failure of passive transfer may develop synovial infections. [5]

Common bacteria causing synovial infections in foals include: [2][3][4]

  • Escherichia coli
  • Streptococcus
  • Staphylococcus
  • Actinobacillus
  • Salmonella
  • Klebsiella
  • Clostridium
  • Rhodococcus

Around 0.5-1% of foals develop septic arthritis due to a joint infection. This is an inflammatory joint disease caused by the presence of pathogenic microorganisms, resulting in pain, swelling and difficulty in moving the affected joint. [2]

Joint Injections

Rarely, synovial infections can occur following the administration of joint injections, where a veterinarian injects therapeutic substances into the joint capsule.

Although the skin surface is thoroughly cleaned before the procedure and sterile needles are used, there is still a risk of introducing bacteria into the joint capsule with injectable medications. [1]

Studies show that around 1 out of 1000 joint injections administered to horses result in a synovial infection. [6] There is no difference in the risk of joint infection for injections performed in a hospital setting versus an ambulatory setting. [6]

Known factors that increase the risk of synovial infections after joint injections include: [6]

  • Clipping the hair over the joint
  • Using non-sterile needles or gloves
  • Limited practitioner experience
  • Using multi-dose vials of medications
  • Larger needle sizes
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