The sheath is a pendulous portion of the body located between the hind limbs of male horses along the lower abdominal wall. The penis is within the sheath for protection from the elements. Returning blood flow at the lowest point of the sheath must travel up and back toward the pelvis, then continue upward to the backbone along the vena cava to the heart, located in the chest between the two front limbs.
The lower parts of the hind limbs have the longest travel distance to the heart, but movement helps to pump blood from the lower limbs helping the heart. However, the sheath offers no help, so blood from this area depends on the heart alone for circulation.
There are two common factors determining whether blood escapes the end of the sheath or accumulates, causing edema. The most common is vessel damage from castration. Sheath edema is more common in geldings for this reason. The other factor is cold temperatures which affect geldings more often than stallions.
The solution is to increase exercise, which helps pump the stagnant blood and lymph out of the area. Another is to house the affected horse in a warm barn, which is usually an unhealthy option due to poor ventilation.
Sheath edema is not usually a problem, even when it looks like it could prevent the penis from coming out to urinate. However, there are other possibilities for sheath edema. If a tumor, disease, trauma, or cancer is located in the sheath or penis or there is an accumulation of debris (called smegma) on the penis, there can be secondary sheath edema. For this reason, the sheath should be thoroughly cleaned, including the shaft and head of the penis (see the topic on how to do this safely). Normally, the sheath has only a normal body odor and no swellings within it. Anything different from this requires a more thorough exam using sedation to relax and exteriorize the penis. Caution must be used as a horse can easily kick your head and kill you while performing this exam, even while the horse is sedated.
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