This section is from the book "The Farmers Ready Reference Or Hand Book Of Diseases Of Horses And Cattle", by S. C. Orr. . Also available from Amazon: The Farmer's Ready Reference;.
A bog spavin, so called because it is soft, is a puffy tumor situated on the hock joint just above and a little to the front of the seat of bone spavin. It is an over-distention of the capsular ligament with synovial fluid generally caused by a sprain. Many horses of the heavy breeds with large beefy hocks are predisposed to it and often it does not seem to hurt them. It does not always cause lameness but when it does it is generally more serious than in bone spavin, as it is more likely to involve the upper part of joint.
Treatment. - If there is heat and swelling about the hock joint reduce it with cold applications, then follow with the treatment prescribed for bone spavin. If there is no lameness, but only a puffy tumor without heat or swelling, and the tumor is of long standing it will do no good to treat it.
This generally accompanies bog spavin, but not always. It is a soft puffy tumor on each side of the hock near the back part. It seldom produces lameness. Treatment is the same as for bog spavin. A bog spavin and thorough pin truss may be applied.
Sometimes there will be lameness, with all the symptoms which characterize bone spavin, but there will be neither the bony enlargement of bone spavin nor the soft, puffy tumor of bog spavin. This is called an occult or blind spavin." The treatment to be employed is that prescribed in bone spavin.