As it circulates through the body, blood provides oxygen and essential nutrients to every cell and tissue, and at the same time it removes waste and toxic products. In order for the blood to perform its life-sustaining functions, the lining of the vessels (arteries and veins) through which the blood travels must be preserved.

The vessel walls that delineate this "blood highway" are subject to many different insults that can cause injury. These insults include not only obvious damage from cuts or external trauma, but also minor defects that occur in the everyday wear and tear of life as the cells that form the vessels age and are replaced. As platelets circulate, they plug any small hole or imperfection in the vessels almost immediately.

Without platelets, the quantity of blood lost through these small defects would be greater than that which could be replaced in a reasonable time frame, leading to collapse of the system and death. For this reason, platelets are vital to life.

Platelets originate in the bone marrow from cells called megakaryocytes (from the words mega = large, karyo = nucleus, and cyte = cell). As the name implies, megakaryocytes are big cells. Throughout their lifespan, they grow very large, and once they complete their maturation, they are fragmented into thousands of platelets. In the circulation system, platelets are active, seeking places where they can clump and form clots.

Disturbances in the lining of vessel walls can activate platelets. So can disturbances in the patterns of flow or the presence of other activated platelets nearby. Low platelet numbers in the circulation (thrombocytopenia) can result from improper platelet production by megakaryocytes in the bone marrow or by excess utilization of platelets in the circulation. Decreased platelet production can occur from various diseases that impair bone marrow function, or even as a side effect of potent cytotoxic drugs used to treat cancer or severe infections.

Increased utilization can be caused by conditions that alter the delicate balance that prevents excessive or insufficient clotting in the blood. Alternatively, low platelet counts can result from processes where the dog destroys its own platelets. When a dog is found to have thrombocytopenia, it's critically important for the veterinarian to identify the cause, so that the proper treatment can be instituted.

The diagnosis of thrombocytopenia begins with a complete blood count from a small blood sample. Various procedures are commonly used to obtain platelet counts. These include automated methods that rely on sophisticated medical instruments and manual methods where platelets are counted under a microscope using specialized grids. If a low platelet count is identified and confirmed on an additional sample, the veterinarian must then attempt to identify if the thrombocytopenia is due to decreased production in the bone marrow, or to increased utilization or consumption.

Often, the history and any potential accompanying disease or condition can provide important clues to determine the cause of thrombocytopenia. In other instances, the veterinarian must rely on examination of a bone marrow sample. These samples are obtained by placing the dog under anesthesia or heavy sedation, inserting a large needle into the bone marrow space (usually in a bone in the shoulder or the hip), and withdrawing the sample. The sample is then preserved and sent for analysis by a pathologist.

The procedure is well tolerated by dogs, but it can be uncomfortable. It also carries potential risks from the anesthetic procedures. Moreover, the test results, while usually reliable, can be subjective. For these reasons, development of a low cost, reliable alternative to determine the cause of thrombocytopenia in dogs is desirable.

Morris Animal Foundation is supporting a study by Dr. Carol Grindem and Dr. Heather DeHeer at North Carolina State University's College of Veterinary Medicine entitled "Reticulated Platelet Assay -- An Alternative to Bone Marrow Biopsy in Thrombocytopenic Dogs." In this study, Drs. Grindem and DeHeer propose to evaluate the presence of "reticulated" platelets in dog blood.

These "reticulated" platelets appear to represent immature forms released from the bone marrow during periods of high demand. They can be used as an indicator of bone marrow function. Thus, their presence could indicate a process of platelet overutilization or overconsumption, while their absence may indicate improper platelet production in the bone marrow.

The validation of this assay may provide veterinarians with a suitable, inexpensive alternative to bone marrow evaluation to diagnose the root cause of thrombocytopenia in dogs. Once your veterinarian can determine the cause of thrombocytopenia, proper treatment can be started.

Rob Hilsenroth, DVM, is Executive Director of Morris Animal Foundation. This nonprofit organization sponsors animal health studies at veterinary schools and institutions throughout the U.S. and in other parts of the world. All annual unrestricted contributions to the foundation support health programs -- not the cost of administration or fund-raising.

If you'd like to make a contribution, write Morris Animal Foundation, 45 Inverness Drive East, Englewood CO 80112. For more information, call (800) 243-2345.