Thrombocytopenia in Dogs
Dr. Leah Cohn
Thrombocytopenia does not always lead to excessive bleeding. Normally, there are more than 600,000 platelets per microliter of blood, which is about one millionth of a quart. Platelet numbers in blood must fall to about 10,000 to 40,000 per microliter before spontaneous bleeding becomes likely. Moderate thrombocytopenia may be found in animals with no evidence of bleeding and this finding often serves as a clue to an important underlying disease process. Vasculitis. This inflammatory disease of blood vessels is characterized by tiny defects in the lining that can allow blood to leak from the vessels. Platelets are attracted to these defects and attempt to plug the holes. Consequently, vasculitis often also results in thrombocytopenia. Several different disease processes can cause vasculitis, including some types of infection and abnormal immune system function that results in an attack against the vessel walls.
Thrombocytopathia is a term that refers to abnormal function of platelets. Animals with thrombocytopathia have adequate numbers of platelets in their blood, but the platelets do not function properly. Thrombocytopathia may be inherited or acquired. Acquired defects in platelet function may be due to drugs (such as aspirin), cancer, or organ failure (such as kidney failure, liver failure).
Coagulation refers to the clotting ability of blood, and normal coagulation arises from the combined effects of properly functioning platelets, blood vessel lining cells (endothelium) and protein clotting factors found in the blood and tissues. Deficiency or abnormal function of any of these components can cause spontaneous bleeding. Occasionally, a disease causes defects in more than one of these components simultaneously.
The following disease processes may cause symptoms similar to those seen in dogs with severe thrombocytopenia:
Von Willebrand's disease. The is an inherited defect in a body protein that normally facilitates the attachment of platelets to each other and to damaged vessel walls. Animals with this disorder have prolonged bleeding times and sometimes experience excessive bleeding after minor injury or surgical procedures. It is an inherited trait in several breeds of dogs, notably Doberman pinschers.
Insufficient quantities of coagulation proteins. Inherited deficiency of a coagulation protein is known as hemophilia. Certain disease processes such as liver failure can result in acquired deficiency of coagulation proteins.
Warfarin. Ingestion of this compound, a common ingredient in rat poisons, can result in a life-threatening bleeding disorder. Some of the currently available rat poisons contain ingredients that have the same effect as warfarin but are much more potent and longer acting. These poisons affect vitamin K metabolism such that coagulation proteins cannot be properly activated.
Disseminated intravascular coagulation (DIC). This refers to life-threatening bodywide coagulation that occurs as a complication of several serious diseases including infections and cancer. During development of DIC, tiny clots form throughout the body in small blood vessels. Both platelets and coagulation proteins are consumed in the formation of these small clots. Widespread life-threatening bleeding occurs when the body's platelets and coagulation proteins become depleted.
Disease in specific organs or body locations. Diseases can cause bleeding that is localized to those areas. For example, bladder stones can cause blood in the urine, nasal tumors can cause bleeding from the nostrils (often primarily on one side), gum disease can cause bleeding around the teeth, and trauma can cause bruising or bleeding.
Skin rashes can be mistaken for the tiny pinpoint hemorrhages called petechiae that are found in animals with severe thrombocytopenia. When finger pressure is placed on the red spots in a rash, the redness typically blanches out whereas with petechiae, the redness stays.