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→Clinical Significance
[[Platelet|Platelets]] (or thrombocytes) are responsible for primary haemostasis by the formation of a temporary platelet plug that initially seals any breach to a blood vessel wall. These breaches are then sealed more completely by the formation of a fibrin clot induced by the coagulation factor cascade.
[[Platelet|Platelets]] (or thrombocytes) are responsible for primary haemostasis by the formation of a temporary platelet plug that initially seals any breach to a blood vessel wall. These breaches are then sealed more completely by the formation of a fibrin clot induced by the coagulation factor cascade.
===Terminology===
===Terminology===
*'''Thrombocytopaenia''' refers to low absolute numbers of platelets. [[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]] (ITP) is one common cause of thrombocytopaenia and this may be a primary or secondary disease.
*'''Thrombocytopaenia''' refers to low absolute numbers of platelets. Infections, neoplasia and [[Immune Mediated Thrombocytopaenia|immune-mediated thrombocytopaenia]] (ITP) are common causes of thrombocytopaenia, which is frequently a secondary disease.
*'''Thrombocytopathia''' refers to platelets that are unable to function adequately.
*'''Thrombocytopathia''' refers to platelets that are unable to function adequately.
*'''Thrombocytosis''' refers to an increase in the blood platelet concentration above the normal level.
*'''Thrombocytosis''' refers to an increase in the blood platelet concentration above the normal level.
===Causes of increased platelet destruction===
===Causes of increased platelet destruction===
*'''Infectious diseases''' that cause destruction of platelets include [[Bovine Viral Diarrhoea Virus|bovine viral diarrhoea (BVD)]], [[Classical Swine Fever|classical swine fever]] and [[Infectious Canine Hepatitis|infectious canine hepatitis]]. The parasites [[Ehrlichia platys|''Anaplasma platys'']], [[Ehrlichia phagocytophila|''Anaplasma phagocytophilum'']] and ''[[Ehrlichia canis]]'' may also cause infectious thrombocytopaenia. Any severe bacterial infection, including those caused by [[:Category:Staphylococcus species|''Staphylococci'']] and the Gram negative bacteria that produce endotoxins (e.g. [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas sp.'']] or [[Salmonella|''Salmonella sp.'']]) may also result in the destruction of platelets.
*'''Infectious diseases''' that cause destruction of platelets include [[Bovine Viral Diarrhoea Virus|bovine viral diarrhoea (BVD)]], [[Classical Swine Fever|classical swine fever]] and [[Infectious Canine Hepatitis|infectious canine hepatitis]]. The parasites [[Ehrlichia platys|''Anaplasma platys'']], [[Ehrlichia phagocytophila|''Anaplasma phagocytophilum'']] and ''[[Ehrlichia canis]]'' may also cause infectious thrombocytopaenia. Any severe bacterial infection, including those caused by [[:Category:Staphylococcus species|''Staphylococci'']] and the Gram negative bacteria that produce endotoxins (e.g. [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas sp.'']] or [[Salmonella|''Salmonella sp.'']]) may also result in the destruction of platelets.
*'''[[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]]''' is an autoimmune disease that leads to the production of antibodies against platelets and their subsequent destruction by cells of the [[Monocytes|monocyte]] phagocyte system (MPS). Platelet numbers can be very low in animals with this condition, but infection and neoplasias are more common causes of thrombocytopaenia.
===Causes of platelet sequestration===
===Causes of platelet sequestration===
===Artefactual or spurious thrombocytopaenia===
===Artefactual or spurious thrombocytopaenia===
*'''Cavalier King Charles spaniels''' have a genetic abnormality which produces a small number of giant platelets (macrothrombocytes) in the circulation with a corresponding reduction in free platelets; they do not suffer from bleeding disorders as they maintain a similar total platelet mass as do dogs of other breeds. Automated blood counts will include the macrothrombocytes in the white or red cell count and the apparent thrombocytopenia will be exaggerated.
*'''Cavalier King Charles spaniels''' have a genetic abnormality which produces a small number of giant platelets (macrothrombocytes) in the circulation with a corresponding reduction in free platelets; they do not suffer from bleeding disorders as they maintain a similar total platelet mass as do dogs of other breeds. Automated blood counts will include the macrothrombocytes in the white or red cell count and the apparent thrombocytopenia will be exaggerated.
*'''Greyhounds''' and other sight hounds frequently have platelet counts at the lowest end of the reference range (as well as increased haematocrit, mild neutropaenia and reduced T4 levels) but this is a normal finding for this breed.
*If venepuncture is traumatic (using small viens with excessive suction or patient agitation) '''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. Time, temperature and contact with glass can also cause clumping of platelets once the sample has been taken.
*If venepuncture is traumatic (using small viens with excessive suction or patient agitation) '''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. Time, temperature and contact with glass can also cause clumping of platelets once the sample has been taken.
*'''Splenic contraction''' which pushes sequestered platelets into the circulation. This phenomenon is especially marked in '''horses''' which have a muscular splenic capsule.
*'''Splenic contraction''' which pushes sequestered platelets into the circulation. This phenomenon is especially marked in '''horses''' which have a muscular splenic capsule.
*'''Splenectomy''' prevents the sequestration of platelets in the spleen, resulting in consistently increased levels of platelets.
*'''Splenectomy''' prevents the sequestration of platelets in the spleen, resulting in consistently increased levels of platelets.
*As part of a response to [[Anaemia - Introduction|anaemia]] a '''reactive thrombocytosis''' is often documented and this may precede signs of regeneration.
*As part of a response to [[Regenerative and Non-Regenerative Anaemias|anaemia]] a '''reactive thrombocytosis''' is often documented and this may precede signs of regeneration.
Pathological
Pathological
==Clinical Significance==
==Clinical Significance==
Other than essential thrombocythaemia, thrombocytosis has no clinical importance except as an indicator of another disease process. Thrombocytopaenia and thrombocytopathia both result in reductions in the effectiveness of primary haemostasis, producing bleeding disorders. Since bleeding points are usually sealed by a fibrin clot, disorders of primary haemostasis tend to be less severe than those caused by deficiencies of the coagulation factors. Common signs of a disorder of primary haemostasis include:
Other than essential thrombocythaemia, thrombocytosis has no clinical importance except as an indicator of another disease process. Thrombocytopaenia and thrombocytopathia both result in reductions in the effectiveness of primary haemostasis, producing bleeding disorders. Since bleeding points are usually sealed by a fibrin clot, disorders of primary haemostasis tend to be less severe than those caused by deficiencies of the coagulation factors. Common signs of a disorder of primary haemostasis include:
*Petechial or ecchymotic [[Haemorrhage - Pathology|haemorrhages]] on the skin or mucous membranes.
*Petechial or ecchymotic [[Haemorrhage|haemorrhages]] on the skin or mucous membranes.
*[[Haemorrhage - Pathology|Haemorrhages]] from the mucous membranes, producing haematuria, haematochezia, haematemesis, haemoptysis and melaena.
*[[Haemorrhage|Haemorrhages]] from the mucous membranes, producing haematuria, haematochezia, haematemesis, haemoptysis and melaena.
*[[Anaemia - Introduction|Anaemia]] with a reactive [[Neutrophilia|neutrophilia]] and [[Monocytosis|monocytosis]] if the haemorrhage is severe.
*[[Regenerative and Non-Regenerative Anaemias|Anaemia]] with a reactive [[Neutrophilia|neutrophilia]] and [[Monocytosis|monocytosis]] if the haemorrhage is severe.
Severe platelet deficiencies may be managed with transfusions of whole blood or, in the USA, with transfusions of platelet cryoprecipitate. With both techniques however, it is likely that the transfused platelets have only a short half life in the recipient.
Severe platelet deficiencies may be managed with transfusions of whole blood or, in the USA, with transfusions of platelet cryoprecipitate - this helps to control the anaemia which occurs as a consequence of haemorrhage and which is the life threatening sympton that required correction (rather than the thrombocytopenia). With both techniques however, it is likely that the transfused platelets have only a short half life in the recipient.
The use of Vincristine (a drug also used in chemotherapy for neoplasia) has been advocated in cases of thrombocytopaenia as it causes the release of immature platelets from the bone marrow.
The use of Vincristine (a drug also used in chemotherapy for neoplasia) has been advocated in cases of thrombocytopaenia as it increases the rate of fragmentation of megakaryocytes and decreases the descruction of platelets by macrophages. A single intravenous dose is usually given.
{{Learning
|literature search = [http://www.cabdirect.org/search.html?q=title%3A%28thrombocytopenia%29+OR+title%3A%28thrombocytopaenia%29&fq=sc%3A%22ve%22 Thrombocytopaenia publications]
[http://www.cabdirect.org/search.html?q=title%3A%28thrombocytopenia%29+OR+title%3A%28thrombocytopaenia%29&fq=sc%3A%22ve%22 Thrombocytopaenia publications]
[http://www.cabdirect.org/search.html?q=title%3A%28thrombocytopathia%29+ Thrombocytopathia publications]
[http://www.cabdirect.org/search.html?q=title%3A%28thrombocytopathia%29+ Thrombocytopathia publications]
[http://www.cabdirect.org/search.html?q=title%3A%28thrombocytosis%29&fq=sc%3A%22ve%22 Thrombocytosis publications]
[http://www.cabdirect.org/search.html?q=title%3A%28thrombocytosis%29&fq=sc%3A%22ve%22 Thrombocytosis publications]
|flashcards = [[Small Animal Emergency and Critical Care Medicine Q&A 21]]
}}
[[Category:Haemorrhagic Diseases]]
[[Category:Haemorrhagic Diseases]]
[[Category:Haematology Changes]]
[[Category:Haematology Changes]]
[[Category:Lymphoreticular and Haematopoietic Diseases - Dog]]
[[Category:Cardiology Section]]