Bog Spavin (also known as "serous tarsitis") is a term used to describe distention or swelling of the uppermost hock joint (known as the tibiotarsal joint). Since distention occurs as a consequence of synovial effusion (i.e. the presence of extra synovial fluid) and/ or proliferation (i.e. the presence of extra synovial tissue), the swelling is characteristically soft and fluctuant in nature.
The tibiotarsal joint has four communicating pouches: the dorsomedial pouch (the largest), the dorsolateral pouch, the plantaromedial pouch and the plantarolateral pouch. Swelling is almost always most obvious in the dorsomedial pouch, which is on the inside-front quadrant of the hock. In many cases of bog spavin, however, all 4 pouches are visibly distended.
Joint effusion/ swelling occurs secondary to the presence of intraarticular inflammatory cells which increase the hydrostatic environment within the joint. Increased hydrostatic pressure results in the influx of fluid into the joint which causes distension.
The presence of intraarticular inflammation (synovitis), in turn, is typically an indicator of underlying pathology within the joint. Common causes of tibiotarsal inflammation are osteochondrosis (developmental orthopaedic disease), trauma, degenerative joint disease and infection. In some cases of bilateral bog spavin (which affects both hind limbs), faulty hock conformation with excessive synovial production has been implicated as the cause.
BLOOD SPAVIN is a term used to describe bog spavin that has developed secondary to hemarthrosis (hemorrhage or bleeding into the joint). Intraarticular blood is a potent irritant within joints and usually elicits a considerable inflammatory reaction.
Bog spavin can appear, disappear, and reappear in some weanlings and yearlings, particularly during times of accelerated growth rates.
It is important to realize that although bog spavin itself may not be a cause of lameness, its presence suggests that some form of joint pathology exists.
Bog Spavin is a physical finding, and is not directly associated with lameness in the horse. Many horses with bog spavin are sound.
In our experience bog spavin only results in lameness if the degree of effusion/ joint distention disallows full flexion of the limb during movement. This typically manifests as a nonweightbearing lameness most evident during the trot (during which hock flexion is maximized).
As previously stated, bog spavin is considered to be an indicator of underlying pathology rather than a primary problem.
Unlike bone spavin, osseous abnormalities (changes to the bone) are not always present in association with bog spavin. Consequently, radiographic examination of the tibiotarsal joint may prove inconclusive in some cases of bog spavin. Nevertheless, radiographic evaluation is recommended to rule out underlying pathologic conditions that do affect bone (such as fracture or developmental orthopaedic disease).
Ultrasonographic examination has also proven to be very useful in our hands, especially if bony lesions are very small and/or soft tissue structures (such as collateral ligaments) are involved.
Draining extra synovial fluid from the joint provides temporary relief, but effusion usually recurs within 24-36 hours if the primary source of inflammation (synovitis) is left to persist.
In many cases a combination of corrective shoeing, systemic arthrotherapy and/or intraarticular therapy (i.e. joint injection) can help to alleviate bog spavin, especially after any potential underlying issues have been successfully addressed. This strategy is particularly rewarding in cases of blood spavin, which tends not to recur if properly treated.
Osteochondrosis typically warrants intervention in the form of diagnostic/ surgical arthroscopy.
Bog spavin often recurs if poor conformation is the predisposing trigger.