Spleen Problems

Q. Can you help me settle a bet? I remember reading that a horse's spleen is unique, but my friend said that horses don't even have a spleen. What's the truth?

A.  Horses do in fact have a spleen, so you win that part of the bet. However, although some aspects of the anatomy and function of the horse's spleen are different from the human spleen, these physical and functional characteristics are shared by a number of other mammals.

First of all, what is the spleen, and what function does it serve in the body? The spleen is a dense, red-colored structure that is situated high on the left side of the abdomen, lying against the rib cage. A ligament (called the nephrosplenic ligament) connects the spleen to the left kidney. In all species, one of the primary functions of the spleen is in the formation and filtration of blood. However, the bone marrow is the primary site for blood formation and normally the spleen does not produce new red blood cells. On the other hand, the spleen throughout life is one of the body's organs that produces lymphocytes, a type of white blood cell that is extremely important in immune function.

The filtration of blood by the spleen is important in the removal of foreign materials (including bacteria) and old blood cells. Blood cells, particularly the red blood cells, have a fairly short lifespan, and it is the job of the spleen to recognize and remove old and degenerating cells from the circulation.

There are two basic types of spleens, defensive and storage types. The human spleen is the defensive type, so its primary function is blood filtration, removal of foreign material and bacteria, and production of lymphocytes. The storage type spleen is larger and, in addition to these defensive functions, it serves as a reservoir for red blood cells. The spleen of horses, dogs, and cats is classified as the storage type. The primary difference between the horse and human spleen relates to this storage function. However, the horse is not unique in this regard because the spleen of the dog and cat serve a similar storage function.

Although the spleen performs several important functions, it is not essential to life. That is, animals and humans can lead a healthy life after removal of the spleen. However, the reservoir of red cells contained within the spleen of the horse plays an important role during exercise. At the start of exercise, the spleen contracts and thereby releases the stored red cells into general circulation. In fact, up to one-third of the horse's red cells are stored within the spleen. One of the main functions of red cells is to transport oxygen from the lung to other parts of the body. Therefore, the increase in blood volume associated with splenic contraction provides a tremendous boost in the horse's capacity to transport oxygen. Indeed, this high capacity for oxygen transport contributes to the high athleticism of the horse. We know this to be true because following removal of the spleen, horses suffer a sharp decline in athletic ability.

Splenic contraction and release of stored red cells can occur under other circumstances, for example when the horse is excited. As a result, measurements of red cell numbers (hematocrit) in samples taken from excited horses do not provide a true indication of a horse's resting state. Conversely, because the horse stores up to one-third of its red cells in the spleen, "blood counts" measured in samples obtained from calm, resting horses cannot be used to assess oxygen-carrying capacity. Despite this, many racehorse trainers routinely use such resting measurements as a guide to the fitness of their horses.

Medical problems involving the spleen are fairly uncommon. 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 the 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 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 non-surgically.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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