Introduction
Polycythemia is an abnormal increase in the number or concentration of red blood cells (“RBCs”) circulating in the blood. Polycythemia is also known as erythrocytosis (another name for red blood cells are “erythrocytes”). The signs and treatment of polycythemia vary greatly depending on the cause of the condition. There are two types of polycythemia in dogs, relative and absolute. These can further be broken down into primary and secondary polycythemia. It is important to determine which type is involved in a given animal, so treatment can be tailored appropriately. The condition can cause many clinical signs, and they may come on slowly or rapidly.
Frequently, signs show up primarily as central nervous system abnormalities (behavior changes, abnormal motor or sensory skills, etc.). A common sign in dogs is paroxysmal sneezing. The animal may also have bright red mucous membranes, an enlarged spleen and increased thirst. Oxygen toxicity can cause tissue damage due to exposure to unusually high concentrations of oxygen, carried by the abnormally large number of circulating RBCs, over long periods of time. If untreated, polycythemia can lead to lethargy, confusion, trouble walking and seizures.
Treating Polycythemia Depends on the Type
Treating Relative Polycythemia
Relative polycythemia caused by dehydration (for instance, if the dog has been experiencing severe vomiting or diarrhea), is normally diagnosed following routine blood work, including a complete blood count to measure blood protein concentration levels. In cases of relative polycythemia, blood protein levels will be high. A sudden incident of extreme fright or excitement also can cause relative polycythemia, because the spleen can contract and release a sudden burst of red blood cells into circulation. Rehydration resolves relative polycythemia. A veterinarian can easily increase a dog’s overall body fluid levels by well-monitored intravenous or subcutaneous fluid therapy. Keeping the animal as calm as possible can facilitate the rehydration process. Once the dog’s fluids levels return to normal, the condition should be resolved.
Treating Absolute Polycythemia - Primary
Primary absolute polycythemia is the result of an uncommon bone marrow disease in dogs. The cause is not well understood. This is a chronic condition that causes overgrowth of red blood cell precursors in the bone marrow, which in turn causes overproduction of RBCs. Basically, the blood becomes too thick, because the cellular/more solid components are elevated while the fluid component remains relatively normal.
Initial treatment of absolute polycythemia is to reduce the viscosity of the blood by reducing the number of circulating RBCs and thereby reducing or resolving clinical signs. This is accomplished through therapeutic phlebotomy – a procedure where a set amount of blood is removed from a central vein. To prevent blood pressure from dropping dramatically because of rapid blood loss, veterinarians usually simultaneously administer a roughly equivalent volume of saline through a different catheter. Phlebotomies sometimes are called “bleeding the animal,” and may need to be repeated periodically. If the underlying condition is severe, a medication called hydroxyurea, and chemotherapy medications such as chlorambucil, can be administered to suppress red blood cell production by the bone marrow. Discuss the consequences of all treatments with your veterinary professional.
Treating Absolute Polycythemia – Secondary
Secondary absolute polycythemia occurs when the kidneys produce too much erythropoietin (“EPO”), a hormone that regulates red blood cell production through a different mechanism than that causing primary absolute polycythemia via bone marrow disorder. Normally, the kidneys in dogs make and release EPO in response to hypoxia, which is an insufficient availability of oxygen to body tissues.
A number of conditions can cause inadequate oxygen supply and thereby secondary absolute polycythemia, such as adaptation to high altitude, insufficient production of EPO by the kidneys due to cancer, cysts or other forms of renal disease, hyperthyroidism, hyperadrenocorticism, heart or lung disease or other causes of circulatory insufficiency. Regardless of cause, secondary absolute polycythemia is the result. In order to treat secondary absolute polycythemia, the cause of the condition first must be diagnosed. Once the cause is treated or controlled, the polycythemia should be brought under control as well. In some cases, especially if the dog’s signs of polycythemia are severe, periodic therapeutic phlebotomies will be recommended.