Up to one-third of a horse's red blood cells are stored in the spleen. At the start of exercise, the spleen contracts, releasing the stored cells into the general circulation and providing a tremendous boost in the horse's capacity to transport oxygen.
Medical problems involving the equine spleen are fairly uncommon, writes Dr. Ray Geor in the "AAEP Answer Line" column in the August edition of The Horse. However, splenic rupture and hemorrhage can occur when the horse receives a kick from another horse to the left abdominal wall. Although the spleen is partially protected by the rib cage in this region, a heavy blow can cause severe injury and hemorrhage into the abdominal cavity. In the worst case scenario, the horse will die from acute hemorrhage. More commonly, the bleeding is self-limiting and goes undetected. In other cases, the horse might show signs of mild to moderate colic. Occasionally, hematomas, abscesses, and tumors can develop in the spleen. In these situations, bleeding into the abdomen can occur. Ultrasound examination of the spleen, together with examination of all fluid collected from the abdomen, is helpful in diagnosing these problems. A specific type of colic that involves the spleen is the so-called nephrosplenic entrapment, in which the large colon becomes hooked over the nephrosplenic ligament. Your veterinarian will diagnose this type of colic by palpation through the horse's rectum and by ultrasound examination of the left abdomen. Fortunately, this problem can often be corrected without surgery.