Bog spavin is a common term for effusion in the horse’s tarsocrural joint. This painful condition describes inflammation and fluid accumulation in the joint capsule between the tibia and the tarsal bones in the hock.

Bog spavin is characterized by a ring of swollen lumps around the hock. When one of the swellings is compressed, the others expand as the fluid moves around the swollen joint.

Joint effusion in horses is usually connected to an underlying joint or connective tissue disorder such as osteochondrosis, osteoarthritis, traumatic injuries, chronic strain, poor conformation, or joint infection. Bog spavin develops most often in working horses and equine athletes.

Treatment for mild cases includes rest and anti-inflammatory drugs. In more serious cases, treatments may include corticosteroid joint injections, compression bandaging, and fluid drainage. In extreme cases, hospitalization may be required.

The prognosis ranges from good to guarded depending on the underlying cause and response to treatment. Prevention of bog spavin in horses requires avoiding injury, providing excellent nutrition, and allowing plenty of rest after exertion.

Bog Spavin in Horses

Bog spavin describes fluid accumulation and inflammation of the lining of the joint and the joint capsule (joint effusion) inside the horse’s hock. [1][2] The condition appears as a characteristic ring of swollen lumps around the hock.

This condition is known by multiple names, including: [2][3]

  • Tarsocrural joint effusion
  • Tarsocrural effusion
  • Talocrural effusion
  • Tarsal hydrarthrosis

Bog spavin specifically affects the tarsocrural joint (TC joint or tibialtasal joint), which connects the tibia (the long bone in the horse’s shin) with the tarsal bones in the hock. This joint is fundamental to equine locomotion (walking) and overall mobility. [1]

When a horse has bog spavin, fluid accumulates in the capsule between the bones, then pushes out of the tight space, resulting in visible swelling around the hock. These swellings are referred to as “bog spavins“. [4] Bog spavin can occur in one or both hocks of the hind legs. [3]


Bog spavin is the result of inflammation in the membrane that lines the tarsocrural joint. [3] It occurs when the synovial cavity (space between the two bones) becomes inflamed and causes swelling of the capsule that cushions the joint between the tibia and the hock. [4] In some cases, the cause remains unknown.

In general, bog spavin is a complication of another underlying joint disease. Possible underlying conditions include:


Osteochondrosis is a disorder of bone development during the horse’s growth phase. In severe cases, this can result in osteochondrosis dissecans, where the cartilage separates from the underlying bone and forms a flap that causes further pain, inflammation, and joint dysfunction. [4][5]

Osteochondrosis is suspected as an underlying cause of bog spavin when it develops in horses under 3 years of age or shortly after the horse is broken or begins athletic training. Osteochondrosis typically affects both joints. [4][5]