Polycythemia is a condition involving an increase in the relative or absolute number or concentration of circulating red blood cells (RBCs). Polycythemia is also called 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 general types of polycythemia in cats: relative and absolute. It is important to determine which type is involved in a given animal, so treatment can be tailored accurately.
How Polycythemia Affects Cats
Polycythemia can cause a number of vague clinical signs, which can appear abruptly or over time. These include lethargy, difficulty breathing, anorexia, fatigue, exercise intolerance, nose bleeds (epistaxis), stunted growth, development of small red spots on the skin, shaking, seizures, vision difficulties and either brick red or sometimes pale mucous membranes. Owners often notice increased thirst, increased water intake and increased urination. Owners also may notice that their sneeze excessively. Changes in behavior can also be seen, including neurological signs of altered motor skills, confusion and incoordination.
Causes of Polycythemia in Cats
Relative polycythemia, sometimes called spurious polycythemia, is an apparent or relative elevation in circulating RBC numbers normally due to dehydration, blood loss, shock or splenic contraction. In cases of relative polycythemia, no extra red blood cells are actually made by the cat’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 cat’s body, resulting in relative polycythemia. Blood loss can occur for many reasons as well, either externally from a wound or internally from trauma, parasites, surgical hemorrhage or otherwise. Transient polycythemia is a type of relative polycythemia that can occur when a cat experiences extreme excitement, fear or shock. This can cause the spleen to contract, releasing a large number of red blood cells from storage into circulation and elevating the relative ratio of cells to fluid in the blood.
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 uncommon in cats and in dogs. This is a chronic condition that leads to overgrowth of red blood cell precursors in the bone marrow, which in turn causes overproduction of new, normal RBCs. Basically, the blood becomes too thick, because the cellular/more solid components are elevated while the fluid component remains relatively normal.
Secondary absolute polycythemia is caused by an abnormal increase in the kidneys’ production of erythropoietin (EPO), a renal hormone that stimulates red blood cell production when there is an inadequate oxygen supply to bodily tissues (called “hypoxia”). Availability of oxygen supply to the body can be insufficient for many reasons, such as adaptation to high altitude, various forms of cancer, renal 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.
There is no way to reliably prevent polycythemia. Obviously, preventing dehydration and blood loss will help prevent relative polycythemia. Primary absolute polycythemia cannot be prevented as veterinary medical experts do not yet understand its cause. Secondary absolute polycythemia is only preventable if the underlying cause of hypoxia can be prevented or managed.
Most cats with polycythemia can be successfully managed for many years.