Etiology and Occurrence.—­Following strains from work in the harness or under the saddle, horses develop an acute synovitis of the hock joint, which often results in chronic synovial distension.  Debilitating diseases favor the production of this affection in some animals.  It is also frequently observed in young horses and in draught colts of twelve to eighteen months of age.  This condition occurs while the subjects are at pasture and often spontaneous recovery results by the time the animals are two years of age.

[Illustration:  Fig. 56—­Bog spavin.  Showing point of view which may be most advantageously taken by the diagnostician in examining for distension of the capsular ligament of the tarsal joint.]

Symptomatology.—­Bog spavin is recognized by the distended condition of the joint capsule which is prominent just below the internal tibial malleolus and this affection is characterized by a fluctuating swelling which varies considerably in size in different subjects.  Except in cases of acute synovitis, lameness is not present and in chronic distension of the capsule of the tarsal joint, no interference with the subject’s usefulness occurs.  In the majority of instances, the disfigurement which attends bog spavin is the principal objectionable feature.  The condition is bilateral in many instances, and in such cases the subjects have a predisposition to this condition or it follows attacks of strangles or other debilitating ailments.  Because of a rapid and unusual growth, bilateral affections are of frequent occurrence in some animals.

Treatment.—­The most practical method of handling bog spavin consists in aspiration of synovia and injection of tincture of iodin.  Discretion should be employed in selecting subjects for treatment, regardless of the manner in which such cases are to be handled.  Where there exists chronic distension of the joint capsule of several years’ standing in old or weak subjects, needless to say, recovery is not likely to result.  When animals are vigorous and two or three months’ time is available, treatment may be begun with reasonable hope for success.

The average subject is handled standing and can be restrained with a twitch, sideline and hood.  Aspirating needles and all necessary equipment must be in readiness (sterile and wrapped in aseptic cotton or gauze) so that no delay will occur from this cause when the operation has been started.  The central or most prominent part of the distended portion of the capsule is chosen for perforation and an area of an inch and a half in diameter is shaved.  The skin is cleansed and then painted with tincture of iodin.  The sterile aspirating needle is pushed through the tissues and into the capsule with a sudden thrust.  With a large and sharp needle (fourteen gauge), synovia can be drawn from the cavity in most instances and the subject usually offers no resistance.  By compressing the distended capsule and surrounding structures with the fingers, considerable synovia