Introduction
Polycythemia is an increase in the number or concentration of red blood cells 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. 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.
It is important to determine which type is involved in a given animal, so treatment can be tailored accurately.
Types of Polycythemia
Relative Polycythemia
Relative polycythemia, sometimes called spurious polycythemia, is an apparent elevation in circulating red blood cell (“RBC”) numbers due to dehydration (high levels of plasma protein), a large loss of blood (low levels of plasma protein), shock, or contraction of the spleen, which is a storage site for RBCs. In cases of relative polycythemia, no extra red blood cells are actually made by the dog’s body. The condition results from changes in the relative levels of the liquid and solid components of blood. For example, vomiting and diarrhea can cause dehydration by depleting the overall amount of fluid in the body, resulting in relative polycythemia. Transient polycythemia is a type of relative polycythemia that can occur when a dog experiences extreme excitement, fear or shock. This can cause the spleen to contract, releasing a large number of red blood cells into circulation in this case. In either case, there is an increase in the relative ratio of cells to fluid in the blood.
Absolute Polycythemia
Absolute polycythemia results from increased bone marrow production of RBCs and may be either primary or secondary. The cause of primary absolute polycythemia is unknown, but it occurs as an inherited defect in cattle and is not commonly seen in dogs and cats. Also called polycythemia vera, this type of polycythemia results from hyperplasia of the bone marrow tissue responsible for making red blood cells and leads to an abnormal increase in formation of new, normal RBCs. The overall fluid component of blood remains normal.
Secondary absolute polycythemia occurs from an increase in production of erythropoietin (“EPO”), a renal hormone that stimulates red blood cell production (“erythropoiesis”) in the presence of hypoxia – or insufficient availability of oxygen to bodily tissues. EPO is produced and released by the kidneys in dogs, causing a complex cascade of events that ends up with more red blood cells in circulation. Availability of oxygen supply to the body can be insufficient for many reasons, such as adaptation to high altitude, insufficient production of EPO by the kidneys due to cancer, cysts or other forms of renal disease, hyperthyroidism, heart or lung disease or other causes of circulatory insufficiency. Regardless of cause, secondary absolute polycythemia is the result.