Thrombocytopenia

65 Thrombocytopenia



Astrid Nielssen



1. What is thrombocytopenia?

Thrombocytopenia in the dog is defined as a platelet count of less than 200 × 1012/L.


2. What can cause thrombocytopenia?

The causes of thrombocytopenia can be divided into the following three major categories:


1. Decreased production

2. Increased consumption or destruction

3. Abnormal platelet distribution

In addition there are dog breeds that can have low platelet counts as a variation of normal, one example being the Cavalier King Charles Spaniel. Approximately one third of the Cavalier King Charles Spaniel breed may have platelet counts that are lower than 100 × 1012/L but greater than 50 × 1012/L. The platelets are typically macrocytic in these dogs. There is no recognized increase in bleeding tendency in affected dogs. Greyhound and Shiba Inu dogs may also have lower platelet counts than those usually seen with other dog breeds.


3. What are some causes of decreased platelet production?

Platelet production defects are an uncommon cause of thrombocytopenia in the dog and when present are typically present in association with other cytopenias. Potential causes of marrow production defects in the dog include infectious agents (canine parvovirus, distemper virus, ehrlichia), vaccination (typically a mild transient response arising 3-5 days after vaccination and not associated with any bleeding tendencies), drug therapy (chemotherapeutics, immunosuppressants, estrogens, sulfa antibiotics), irradiation, toxins, dysthrombopoiesis associated with myelodysplasia, and immune-mediated destruction of megakaryocytes.


4. What are some causes of increased platelet consumption or destruction?

Thrombocytopenia due to increased platelet consumption can be seen in cases of severe trauma or hemorrhage. The thrombocytopenia is usually mild and transient. Clinical conditions that cause generalized activation of the coagulation cascade or extensive damage of the endothelium, potentially even resulting in disseminated intravascular coagulation, can also cause a thrombocytopenia. These mechanisms for platelet consumption likely play a role in the thrombocytopenias identified in dogs with many systemic inflammatory diseases and infections including babesiosis, ehrlichiosis, Rocky Mountain spotted fever, and adenovirus type I infection.


Increased platelet destruction can arise from direct injury by certain infectious agents (distemper virus) or drugs or may be due to immune-mediated mechanisms.


5. What is immune-mediated thrombocytopenia?

Immune-mediated thrombocytopenia (IMT) typically results from antibody mediated platelet destruction by the mononuclear-phagocyte system. Primary (idiopathic, autoimmune) IMT does not have an apparent etiology, is not associated with any underlying conditions, and arises when antibodies develop against platelet autoantigens. Although dogs of many breeds and ages have been diagnosed with IMT, the mean age of dogs with this disorder is approximately 6 to 7 years, and females and Cocker Spaniels appear to be overrepresented. Secondary IMT arises when an underlying associated condition or therapy triggers the immune-mediated platelet destruction.


6. What are some potential causes of secondary IMT?

Secondary IMT may arise as a result of an antibody response directed toward non-self antigens that are nonspecifically adsorbed to the surface of circulating platelets. Infectious agents, drugs, novel antigens associated with malignancies, and circulating immune complexes have the capacity to become adsorbed onto the platelet surface and trigger this response. High levels of circulating immune complexes can be seen as a result of some infectious diseases, vaccination, drug therapies, neoplasia, or in association with other autoimmune disorders.


Alloimmune thrombocytopenia is another form of immune-mediated platelet destruction that can result in significant thrombocytopenia. The only form described in the dog is posttransfusion alloimmune thrombocytopenia (posttransfusion purpura). The thrombocytopenia may arise shortly after the transfusion, possibly as the result of passively transfused donor antiplatelet alloantibodies, or it may occur approximately 1 week after the transfusion, possibly as the result of the triggering of the production of an antidonor platelet antibody that shares reactivity against the recipient platelets.

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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Thrombocytopenia

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