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==Description==
==Description==
[[Platelet|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'''). [[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]] (ITP) is one common cause of thrombocytopaenia and this may be a primary or secondary disease. '''Thrombocytosis''' refers to an increase in the blood platelet concentration above the normal level.
[[Platelet|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. These breaches are then sealed more completely by the formation of a fibrin clot mediated by the coagulation factor cascade.
Thrombocytopaenia and thrombocytopathia lead to disorders of primary haemostasis but, in general, this is less serious than the disorders of secondary haemostasis caused by deficiencies in the [[Coagulation Factor Deficiency|clotting factors]].
Bleeding disorders may occur if platelets are deficient ('''thrombocytopaenia''') or if the platelets are unable to function adequately ('''thrombocytopathia'''). [[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]] (ITP) is one common cause of thrombocytopaenia and this may be a primary or secondary disease. '''Thrombocytosis''' refers to an increase in the blood platelet concentration above the normal level.
Thrombocytopaenia and thrombocytopathia lead to disorders of primary haemostasis but, in general, this is less serious than the disorders of secondary haemostasis caused by deficiencies in the [[Coagulation Factor Deficiency|coagulation factors]].
==Thrombocytopaenia==
==Thrombocytopaenia==
Thrombocytopaenia is the most common haematological abnormality as platelet numbers are subject to fluctation in a large number of different diseases however care should be taken to ensure that this finding is not caused by an artefact of sampling. The normal blood platelet concentration of the dog is 175-500x10^9 but clinical disease is not usually observed until this level falls below 50x10^9 and spontaneous haemorrhage is observed only when the level falls below 20x10^9. Reductions in the number of platelets may be caused by failure to produce adequate numbers in the bone marrow in the process of megakaryopoiesis, increased destruction of existing platelets or sequestration of platelets outside of the circulation.
Thrombocytopaenia is the most common haematological abnormality as platelet numbers are subject to fluctuation in a large number of different diseases. Care should be taken however to ensure that this finding is not caused by an artefact of sampling that leads to the formation of platelet clumps. The normal blood platelet concentration of the dog is 175-500x10^9 but clinical disease is not usually observed until this level falls below 50x10^9 and spontaneous haemorrhage is observed only when the level falls below 20x10^9. Reductions in the number of platelets may be caused by a failure to produce adequate numbers in the bone marrow in the process of megakaryopoiesis, increased destruction of existing platelets or sequestration of platelets outside of the circulation.
*Diseases that cause '''[[Bone Marrow - Anatomy & Physiology|bone marrow]] suppression'''
*Diseases that cause '''[[Bone Marrow - Anatomy & Physiology|bone marrow]] suppression'''
**'''Aplastic anaemia''' caused by ingestion of bracken, administration of oestrogens or of [[Chloramphenicol|chloramphenicol]] or [[Sulphonamides|sulphonamide]] antibiotics. Prolonged use of phenylbutazone or salicylate may cause the same disease.
**'''Aplastic anaemia''' caused by ingestion of bracken or administration of oestrogens, [[Chloramphenicol|chloramphenicol]] or [[Sulphonamides|sulphonamide]] antibiotics. Prolonged use of phenylbutazone or salicylate may cause the same disease.
**'''Infectious diseases''' that reduce stem cell function, including [[Distemper|canine distemper virus]], [[Canine Parvovirus|canine parvovirus]] and [[Feline Panleucopaenia|feline panleucopaenia virus]].
**'''Infectious diseases''' that reduce stem cell function, including [[Distemper|canine distemper]], [[Canine Parvovirus|canine parvovirus]] and [[Feline Panleucopaenia|feline panleucopaenia virus]].
**'''Myelophthisis''', the displacement of the normal cell lines 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.
**'''Myelophthisis''', the displacement of the normal cell lines of the bone marrow by another cell or tissue type may also reduce the function of the megakaryocytes. Possible diseases in this category include '''myelofibrosis''' and '''immunoproliferative''' or '''myeloproliferative''' neoplastic disease.
**'''Radiotherapy''' or myelosuppressive '''chemotherapy''' may cause reversible bone marrow suppression.
**'''Radiotherapy''' or myelosuppressive '''chemotherapy''' may cause reversible bone marrow suppression.
*Diseases that cause '''increased destruction of platelets'''
*Diseases that cause '''increased destruction of platelets'''
**'''[[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]]''' is a common autoimmune disease that leads to the destruction of platelets by cell of the monocyte phagocyte system (MPS).
**'''[[Immune Mediated Thrombocytopaenia|Immune-mediated thrombocytopaenia]]''' is a common autoimmune disease that leads to the production of antibodies against platelets and their subsequent destruction by cells of the monocyte phagocyte system (MPS).
**'''Infectious diseases''' that cause destruction of platelets include [[Bovine Viral Diarrhoea Virus|bovine viral diarrhoea]], [[Classical Swine Fever|classical swine fever]] and [[Infectious Canine Hepatitis|infectious canine hepatitis]]. The parasites ''[[Ehrlichia 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 endotoxin (e.g. [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas spp.'']] or [[Salmonella|''Salmonella spp.'']]) 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 endotoxin (e.g. [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas spp.'']] or [[Salmonella|''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:
*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
**[[Haemangiosarcoma]] of the spleen and liver
**Splenic enlargement under general anaesthesia maintained with agents such as [[Barbiturates|barbiturates]] and [[Phenothiazines|phenothiazines]]
**Splenic enlargement under general anaesthesia maintained with agents such as [[Barbiturates|barbiturates]] and [[Phenothiazines|phenothiazines]]
==Thrombocytopathia==
==Thrombocytopathia==
Defects in platelet function may be congenital defects or they may be acquired with a number of diseases. Congenital thrombocytopathias are rare inherited diseases which are characterised by defects in platelet adhesiveness, aggregation and factor release. The defects are usually associated with particular breeds, such as Chediak-Higashi syndrome in blue smoke Persian cats.
Defects in platelet function may be congenital defects or they may be acquired with a number of diseases. Congenital thrombocytopathias are rare inherited diseases which are characterised by defects in platelet adhesiveness, aggregation or factor release. The defects are usually associated with particular breeds, such as '''Chediak-Higashi syndrome''' in blue smoke Persian cats.
Causes of acquired thrombocytopathia include:
Causes of acquired thrombocytopathia include:
*Infection with ''[[Angiostrongylus vasorum]]'', the canine lungworm, which also causes a consumptive coagulopathy.
*Infection with ''[[Angiostrongylus vasorum]]'', the canine lungworm, which also causes a consumptive coagulopathy.
*'''Hypergammaglobulinaemia''' as occurs with multiple myeloma and some forms of (B-cell) lymphoma may affect platelet function.
*Administration of large volumes of some '''[[Colloids|colloid solutions]]'''.
*Administration of large volumes of some '''[[Colloids|colloid solutions]]'''.
*Administration of certain '''pharmaceutical products''', including [[NSAIDs|non-steroidal anti-inflammatory drugs]] and [[Cephalosporins|cephalosporins]].
*Administration of certain '''pharmaceutical products''', including [[NSAIDs|non-steroidal anti-inflammatory drugs]] and [[Cephalosporins|cephalosporins]].
**'''Splenic contraction''' pushes sequestered platelets into the circulation. This phenomenon is especially marked in horses which have a muscular splenic capsule.
**'''Splenic contraction''' 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 constantly increased levels of platelets.
**'''Splenectomy''' prevents the sequestration of platelets in the spleen, resulting in constantly increased levels of platelets.
**As part of a response to '''anaemia''', a '''reactive thrombocytosis''' is often documented and this may precede signs of regeneration.
**As part of a response to '''[[Anaemia – Introduction|anaemia]]''', a '''reactive thrombocytosis''' is often documented and this may precede signs of regeneration.
*Pathological
*Pathological
**'''Essential thrombocythaemia'''
**'''Essential thrombocythaemia''' is a rare myeloproliferative disease that results in the excessive production of platelets which function abnormally. Affected animals suffer from bouts of spontaneous haemorrhage.
==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:
*'''Petechial''' or '''ecchymotic''' haemorrhages on the skin or mucous membranes.
*'''Haemorrhages from the mucous membranes''', producing haematuria, haematochezia, haematemesis, haemoptysis and melaena.
*'''[[Anaemia – Introduction|Anaemia]]''' with 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.
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.
[[Category:Haemorrhagic Diseases]]
[[Category:Haemorrhagic Diseases]]
[[Category:To Do - James]][[Category:Haematology Changes]]
[[Category:To Do - James]][[Category:Haematology Changes]]