Platelet Abnormalities

Description

Platelets (or thrombocytes) are responsible for primary haemostasis, the formation of a temporary platelet plug that initially seals any breach to a blood vessel wall. Bleeding disorders may occur if platelets are deficient (thrombocytopaenia) or if the platelets are unable to function adequately (thrombocytopathia). Thrombocytopaenia may be a primary disease, relating to a failure to produce platelets in the bone marrow, or it may occur secondary to another insult. Thrombocytosis refers to an increase in the blood platelet concentration above the normal level.

Thrombocytopaenia

Primary Thrombocytopaenic Disease

The aetiology of primary thrombocytopenic disease is often uncertain and it is therefore also known as idiopathic thrombocytopaenia. Some cases of primary thrombocytopaenia may occur due to autoimmune responses directed at platelets and, although the bone marrow may contain adequate numbers of normal megakaryocytes, this disease results in a reduced peripheral blood thrombocyte count. Immune-mediated thrombocytopaenia occurs most commonly in young animals.

Secondary Thrombocytopaenic Disease

Secondary thrombocyopaenic disease is fairly common in adult domestic animals and it may be associated with diseases that depress the production of platelets or increase their consumption or sequestration:

• Diseases that cause bone marrow suppression
  • Aplastic anaemia caused by ingestion of bracken, administration of oestrogens or of chloramphenicol or sulphonamide antibiotics. Prolonged use of phenylbutazone or salicylate may cause the same disease.
  • Infectious diseases that reduce stem cell function, including canine distemper virus, canine parvovirus and feline panleucopaenia virus.
  • Myelophthisis, the displacement of the normal tissues of the bone marrow by another cell or tissue type may also reduce the function of the megakaryocytes. Possible diseases include myelofibrosis and immunoproliferative or myeloproliferative neoplastic disease.
  • Radiotherapy or myelosuppressive chemotherapy may cause reversible bone marrow suppression.
• Diseases that cause increased destruction of platelets
  • Infectious diseases that cause destruction of platelets include bovine viral diarrhoea, classical swine fever and infectious canine hepatitis. The parasites Ehrlichia platys, Anaplasma phagocytophilum and Ehrlichia canis may also cause infectious thrombocytopaenia. Any severe bacterial infection, including those caused by Staphylococci and the Gram negative bacteria that produce endotoxin (e.g. Pseudomonas spp. or Salmonella spp.) may also result in the destruction of platelets.
• Diseases that cause sequestration of platelets usually involve some enlargement of the spleen as this is the major organ where platelets are stored outside of the circulation. Examples include:
  • Haemangiosarcoma of the spleen and liver
  • Splenic enlargement under general anaesthesia maintained with agents such as barbiturates and phenothiazines
• Artefactual or spurious thrombocytopaenia
  • Cavalier King Charles spaniels have a relatively small number of giant platelets (macrothrombocytes) but they do not suffer from bleeding disorders as they maintain a similar total platelet mass as do dogs of other breeds.
  • If venepuncture is traumatic, platelet clumps may form and these will not be counted by automated machines. The presence of clumps can be investigated by making a blood smear and examining the feathered edge for large agglomerations of platelets.

Thrombocytopathia

Von Willebrands, Angiostrongylus, colloids, Pelger Huet anomaly?.

• A rare inherited condition.
• Recorded in the dog.
• Characterised by defective thrombocyte formation.
  • Poor adhesiveness.
  • Poor aggregations.
  • Poor platelet factor release.

Thrombocytosis

Increases in platelets above the normal level may occur due to physiological or pathological processes.

• Physiological
  • Splenic contraction pushes sequestered platelets into the circulation. This phenomenon is especially marked in horses which have a muscular splenic capsule.
  • As part of a response to anaemia, a rebound thrombocytosis is often documented and this may precede signs of regeneration.
• Pathological
  • Essential thrombocythaemia
  • Megakaryocytic leukaemia