Causes
Bog spavin is a physical finding, and does not directly create lameness. Causes include synovitis (inflammation of the lining of the joint capsule), degenerative joint disease, or excessive strain of the joint capsule. In horses younger than the age of three, most cases of bog spavin are caused by a defect in the tibiotarsal joint, while in older, fully mature horses, it is most likely because of chronic strain of the joint capsule. Infection of the joint causes a severe synovitis, and should be treated as an emergency.
Many horses with bog spavin will not be lame. However, bog spavin can be a sign that the horse has joint disease, which is a very significant finding. Usually lameness will occur if the workload of the horse is increased. Bog spavin should not be treated lightly, and it is best if a veterinarian examine the horse to find the cause, even if the horse does not appear lame.
Unlike bone spavin, bog spavin does not show any changes to the bone itself. For this reason it is considered to be of no interest to those studying animal paleopathology (Baker and Brothwell, 1980).
Read more about this topic: Bog Spavin