Increased number of platelets call for review of thrombocytosis

Q: Several dogs seen recently in our clinic have increased numbers of platelets. What does this mean?

Causes of canine thrombocytosis

A: Many veterinarians in their interpretation of the routine canine CBC often pay little to no attention to the number of platelets, especially if the total platelet count is increased above the normal expected range. This is not good! Therefore, let's proceed with a brief review of canine thrombocytosis.

Platelets are an integral part of the blood's clotting mechanism, enabling the dog's body to control bleeding at the site of injury. When a wound occurs, several changes occur to minimize blood loss. First, the blood vessel slows the flow of blood past the wound site. Next, platelets collect at the wound site to form a plug, which is replaced by a cellular clot until the wound heals.

Platelets are produced in the bone marrow and are removed from the blood by the spleen. Therefore, the bone marrow and spleen work together to regulate platelet numbers in the blood. Platelet numbers can increase because of increased bone marrow production or decreased spleen removal.

Thrombocytosis is defined as increased number of circulating platelets.

Fundamental causes

The fundamental causes responsible for thrombocytosis in dogs are:

• It may be a temporary response to epinephrine release.

• It may be a reaction to a disorder or disease in another part of the body.

• It may be a sign of an underlying disease in the bone marrow.

Temporary thrombocytosis may occur in response to exercise, pregnancy and delivery, or other factors causing increased epinephrine levels in the blood. The spleen releases stored platelets in response to temporary increases in blood epinephrine.

Thrombocytosis as a reaction to another disorder in the dog's body results from increased production of platelets in the bone marrow and increased release of stored platelets from the spleen. Clinical problems that may cause a secondary or reactive thrombocytosis are infections, inflammatory disorders, acute or chronic blood loss, tissue damage from trauma or surgery, drugs (steroids, vincristine), hyperadrenocorticism, splenectomy, cancer and rebound from chemotherapy.

Infections (acute and chronic) may cause either reactive thrombocytosis or a thrombocytopenia. Infections may be accompanied by a reactive thrombocytosis, which is thought to be due to cytokines that are produced as part of the body's normal defense against infection.

The reactive thrombocytosis usually resolves as the infection resolves, although it may take longer time to settle into the normal expected range. Inflammatory disorders may also cause reactive thrombocytosis by similar mechanisms to infections.

The response of the bone marrow to acute or chronic blood loss is to produce not only more red blood cells but also more platelets - this occurs in response to cytokines.

The total platelet count will also increase when a relatively large amount of body tissue is damaged either intentionally following surgery or with accidental damage. This is part of the natural defense mechanism to ensure adequate clot formation and prevent fatal bleeding.

Platelet count

Treatment with some drugs (steroids, vincristine) causes a transient increase in total platelet count. Underactive or absence of spleen is a major site for destroying platelets, and if the spleen is removed, the total platelet count usually increases. The increase may remain for a long time but usually it will settle back into the normal expected range. In some conditions, the spleen is present but either it does not function properly or it is shrunken and the total platelet count is sometimes increased in these conditions.

Some cancers can cause an increased total platelet count either by causing damage to tissues, causing blood loss or by erroneously producing a cytokine that stimulates the bone marrow to produce platelets. Some chemotherapy drugs exert their effects directly on dividing cells in the body - including the bone marrow where platelets are produced. When the body is recovering from the effects of such chemotherapy, a transient overproduction of some cells can occur, too.

Also, remember that a reactive thrombocytosis is a common observation in canine hyperadrenocorticism.

Potential problems

The potential problems arising in a reactive thrombocytosis include excessive clot (or thrombus) formation due to the increased number of active platelets and, much less commonly, bleeding as platelet function may also be defective in reactive thrombocytosis. Although both thrombosis and hemorrhage are potential problems, their occurrence in reactive thrombocytosis is very rare. Generally, they will only occur in dogs with a combination of a total platelet count more than 1,000 million per ml plus other risk factors such as dehydration. The major problems that usually arise in this situation actually relate to the underlying cause of the reactive thrombocytosis.

If reactive thrombocytosis is a response to another disorder, the dog will note signs of that disorder. The primary treatment for a reactive thrombocytosis should be directed against the underlying condition causing the reactive thrombocytosis. In rare circumstances, some treatment to protect against inappropriate clot formation might be advised. Such treatment includes ensuring adequate hydration and optimal mobility. Less often drugs such as aspirin and anticoagulants such as heparin injections may be used.

Essential thrombocytosis, polycythemia vera, chronic myeloid leukemia and myelofibrosis are uncommon conditions that form a group of diseases known as the myeloproliferative disorders. An increased total platelet count can occur in all of these myeloproliferative disorders but in each an increase in a particular cell type usually predominates. These cell types are the platelets in essential thrombocytosis, red blood cells in polycythemia vera, myeloid cell in chronic myeloid leukemia and fibroblasts in myelofibrosis.

The blood disorder that is mostly characterized by an increased total platelet count is primary thrombocytosis - also called essential thrombocythemia. Essential thrombocythemia is one of the myeloproliferative disorders in which the major feature is a thrombocytosis; when the bone marrow is examined there is an increase in the megakaryocytes.

Some dogs with essential thrombocythemia can experience bleeding or clotting problems due to their disease but others have no signs at all. If signs occur, signs may include enlarged spleen, distended abdomen or blood in the stool.