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.