Introduction | Causes & Prevention | Symptoms & Signs | Diagnosis & Test | Treatment Options


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Polycythemia refers to an increase in either the number or concentration of red blood cells in circulation. Polycythemia is also called erythrocytosis, because red blood cells are medically referred to as "erythrocytes". They also are commonly called "RBCs." The symptoms and treatment of polycythemia can vary greatly, depending upon the cause of the condition in the affected animal.

Causes & Prevention

Causes of Canine Polycythemia
There are several distinct forms of polycythemia in domestic dogs: relative, transient and absolute. Absolute polycythemia can further be broken down into primary and secondary forms. It is important to determine which type of polycythemia is involved in a given dog, so that treatment can be tailored appropriately.

Relative polycythemia, sometimes called spurious polycythemia, refers to an apparent, but not actual, increase in the number of circulating red blood cells. Relative polycythemia can be caused by dehydration, blood loss, shock, decreased water intake, vomiting, diarrhea, kidney disease, hyperventilation and a number of other conditions – essentially, by anything that reduces plasma volume. Plasma is the liquid component of blood. No extra red blood cells are being made by a dog's body in cases of relative polycythemia; rather, the condition is caused by changes in the relative levels of the liquid and solid components of blood. For example, vomiting and diarrhea cause dehydration by depleting the overall amount of circulating fluid in the body, without reducing the number of circulating red blood cells, resulting in relative polycythemia. Fluid loss can happen for many other reasons as well, both externally from wounds and internally from trauma, parasites, hemorrhage, clotting disorders or otherwise. In other words, while the overall red blood cell mass remains normal, the fluid component has lessened, making the blood thicker, or more viscous.

Transient polycythemia is a type of relative polycythemia that is caused by splenic contraction. The spleen is a storage site for mature red blood cells. When a dog becomes extremely excited or experiences fear, anger, restraint, vigorous exercise or surprise, its spleen contracts in response to the release of epinephrine by its adrenal glands. Epinephrine is commonly called adrenaline. When the spleen contracts, it injects large numbers of red blood cells into circulation, thereby increasing the relative ratio of red cells to fluid in the blood. This is beneficial for dogs that are experiencing sudden and strong feelings, because the increased concentration of red blood cells facilitates the transport of oxygen to key organs and body tissues, preparing the dog for unusually energetic activity.

Absolute polycythemia is an actual increase in the number of circulating red blood cells. It is caused by increased proliferation of red blood cell precursors in the dog's bone marrow. Absolute polycythemia can be either primary or secondary.

The cause of primary absolute polycythemia is not known. It is uncommon in domestic dogs and cats, but does occur as an inherited defect in cattle. Also called polycythemia vera or polycythemia rubra vera, primary absolute polycythemia is characterized by increased formation and growth (hyperplasia) of the bone marrow tissue that is responsible for producing red blood cells. This causes an abnormal increase in circulating red cells, while the overall fluid component of blood remains unchanged.

Secondary absolute polycythemia is caused by an increase in the production and release of erythropoietin (EPO) by the kidneys. Erythropoietin is a hormone that stimulates red blood cell production by bone marrow through a process called erythropoiesis. When there is not enough circulating oxygen available for distribution to body tissues (a condition called hypoxia or hypoxemia), the kidneys release EPO. This causes a complex cascade of events that ultimately increase the amount of circulating mature red blood cells. The availability of oxygen can be deficient for many reasons, such as adaptation to high altitudes, cancer, pancreatitis, renal cysts or other forms of renal disease, hyperthyroidism, hyperadrenocorticism (Cushing's disease), heart, disease, lung disease and other causes of circulatory insufficiency. Regardless of the cause, when hypoxia develops, secondary absolute polycythemia usually also occurs.

Relative polycythemia can be prevented by ensuring that a dog is well-hydrated. Transient polycythemia is not preventable and lasts only a short period of time; it usually is clinically insignificant. Absolute polycythemia cannot really be prevented, because the causes are either unknown or highly variable.

Symptoms & Signs

Polycythemia, a condition in which the number or concentration of circulating red blood cells is actually or apparently elevated, can contribute to a number of different clinical signs, depending upon why the condition occurred in the first place.

Symptoms of Canine Polycythemia
There are several different forms of polycythemia, each of which can cause symptoms that may appear slowly or suddenly. Owners of affected dogs may notice one or more of the following signs:

Behavior changes
Abnormal motor skills
Abnormal sensory skills
Paroxysmal sneezing (sudden; intense; recurrent)
Nose bleeds (epistaxis)
Injected mucous membranes (brick red gums)
Cyanotic mucous membranes (pale gums)
Enlarged spleen (splenomegaly; palpable)
Enlarged liver (hepatomegaly)
Increased thirst and/or water intake
Increased volume and/or frequency of urination
Unusually high concentrations of oxygen, carried by unusually high numbers of circulating red blood cells, can cause mild to severe tissue damage over time. Left untreated, polycythemia – especially the absolute form of the disease can lead to additional symptoms, including:

Difficulty walking/ambulating
Exercise intolerance (fatigue)
Difficulty breathing (dyspnea)
Lack of coordination (ataxia)
Lack of interest in play and/or normal social interactions
Seizures (rare)
Uncontrolled shaking
Vision impairment
Dogs experiencing these symptoms should be taken to their veterinarian.

Dogs at Increased Risk
There is no particular breed, age or gender predisposition to the development of polycythemia in domestic dogs.

Diagnosis & Tests

Polycythemia, which is the actual or apparent increase in the number of red blood cells in circulation, is not particularly difficult to diagnose. However, it can be tricky to differentiate between the various forms of polycythemia in a given dog.

How Polycythemia is Diagnosed
The initial database for a dog presenting with the nonspecific signs associated with polycythemia usually includes a baseline complete blood count, total plasma protein measurement, serum chemistry panel and urinalysis, in addition of course to a thorough history and physical examination. These tests may help to distinguish between the relative and absolute forms of this disorder.

Other diagnostic tools include assessment of blood erythropoietin levels, blood gas oxygen values and pulse oximetry oxygen saturation values. Radiographs (X-rays) may be useful to identify pulmonary (lung) and/or cardiac (heart) disorders or other diseases that may be contributing to polycythemia. Electrocardiographs (ECGs) and ultrasonography can be used to assess the heart, kidneys, liver and adrenal glands.

Special Notes
Dehydration and certain drugs can affect the results of some of these diagnostic techniques. A veterinarian is the best person to advise owners appropriately about the diagnosis and treatment of polycythemia in companion dogs.

Treatment Options

Polycythemia, also known as erythrocytosis, is an abnormal increase in the number or concentration of circulating red blood cells (RBCs). The symptoms and treatment of polycythemia vary greatly depending upon the underlying cause of the condition. It is important to determine which form of polycythemia is involved, so that an appropriate treatment protocol can be implemented. Most forms of polycythemia are treatable, and the sooner the treatments are started, the better. Secondary absolute polycythemia tends to be the most difficult (although not impossible) to treat. A veterinarian is in the best position to assess a dog's condition and recommend the best treatment approach. The therapeutic goals are to restore the appropriate solid-to-fluid ratio in the affected dog's blood and to relieve the symptoms of the disorder.

Treatment Options
Relative polycythemia can usually be resolved by rehydration. The veterinarian can easily increase a dog's blood fluid level by well-monitored intravenous or subcutaneous fluid therapy. Keeping the animal as calm as possible facilitates the rehydration process. The dog's kidney function, electrolyte balance and other factors are important to the selection of the appropriate fluid or fluid combinations. Once the dog's plasma level returns to normal, the relative polycythemia condition should be resolved. Transient polycythemia caused by splenic contraction normally is not clinically significant and does not require treatment.

Primary absolute polycythemia is an uncommon, chronic disease of domestic dogs that involves increased production and circulation of mature red blood cells due to abnormal proliferation of certain red blood cell precursors in bone marrow. The goal of treating primary absolute polycythemia is to reduce the viscosity (thickness) of the blood by reducing the circulating red blood cell mass and increasing the fluid component of blood (plasma). This is accomplished through a process called phlebotomy - a procedure where a defined amount of blood is permanently removed through a catheter inserted into a central vein. To prevent a dramatic drop in blood pressure, most veterinarians simultaneously administer isotonic fluids through a different intravenous catheter, in order to restore normal blood volume. A phlebotomy is sometimes referred to as "bleeding the animal." It may take several of these procedures to correct polycythemia, and they may need to be repeated from time to time. If the underlying condition is quite severe, a medication called hydroxyurea, and/or chemotherapy medications such as chlorambucil or busulfan, can be administered to suppress red blood cell production by bone marrow. Owners should always discuss all treatment options with their veterinary professionals.

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 the one involved with primary absolute polycythemia. In order to treat secondary absolute polycythemia, the underlying cause of the condition must first be diagnosed. Once the cause is identified and treated (or at least controlled), polycythemia should be well-managed, if not eliminated entirely. In some cases, especially if the dog's symptoms are severe, periodic therapeutic phlebotomies may be required.

The prognosis for most dogs with polycythemia is usually quite good, as long as appropriate therapy is implemented.

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