6.2 Mitral valve disease
Mitral valve (MV) disease is the most important valvular condition affecting athletic performance. Degenerative myxomatous change is the most common cause; however, the exact aetiology of this disease is poorly understood. Anecdotal reports link the occurrence of the condition with previous viral or bacterial respiratory infections, but there is little clear evidence of this. Recently, there has been interest in the possibility of a valvulitis associated with streptorespiratory infections which may affect the mitral or other valves. However, although the concept is attractive, there is little evidence for such a link at present. It may be that murmurs are more commonly detected in these animals because they are more likely to be auscultated. If the degenerative change is sufficient the valve may become incompetent. This may be due to a change in the collagenous structure of the valve resulting in a billowing of the valve leaflet, rather than due to the nodules on the margin of the valve. Horses of all ages can be affected, but chronic degenerative disease is most frequently seen in middle-aged or older horses. MV disease may also be caused by bacterial endocarditis, but this is relatively rare (endocarditis is discussed in section 6.6).
6.2.1 Clinical signs
6.2.2 Clinical examination
6.2.3 Other clinical findings
6.2.5 Clinical guides to prognosis
6.2.6 Diagnostic and prognostic aids