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→Thrombocytopaenia
Thrombocytopaenia is a common haematological abnormality as platelet numbers are subject to fluctuation in a number of diseases. Care should be taken, however to ensure that this finding is not caused by an artefact of sampling where platelet clumps have formed. 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 platelet numbers may be caused by a failure to produce adequate amounts in the bone marrow during [[Thrombopoiesis|megakaryopoiesis]], or an increased destruction of existing platelets or sequestration of platelets outside of the circulation.
Thrombocytopaenia is a common haematological abnormality as platelet numbers are subject to fluctuation in a number of diseases. Care should be taken, however to ensure that this finding is not caused by an artefact of sampling where platelet clumps have formed. 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 platelet numbers may be caused by a failure to produce adequate amounts in the bone marrow during [[Thrombopoiesis|megakaryopoiesis]], or an increased destruction of existing platelets or sequestration of platelets outside of the circulation.
===Causes of bone marrow suppression===
*'''Aplastic anaemia''' which can be caused by the ingestion of bracken or administration of oestrogens, [[Chloramphenicol|chloramphenicol]] or [[Sulphonamides|sulphonamide]] antibiotics. Prolonged use of [[NSAIDs#Phenylbutazone|phenylbutazone]] or salicylates may cause the same disease.
*'''Aplastic anaemia''' which can be caused by the ingestion of bracken or administration of oestrogens, [[Chloramphenicol|chloramphenicol]] or [[Sulphonamides|sulphonamide]] antibiotics. Prolonged use of [[NSAIDs#Phenylbutazone|phenylbutazone]] or salicylates may cause the same disease.
*'''Infectious diseases''' that reduce stem cell function include [[Distemper|canine distemper]], [[Canine Parvovirus|canine parvovirus]] and [[Feline Panleucopaenia|feline panleucopaenia virus]].
*'''Infectious diseases''' that reduce stem cell function include [[Distemper|canine distemper]], [[Canine Parvovirus|canine parvovirus]] and [[Feline Panleucopaenia|feline panleucopaenia virus]].
*'''Radiotherapy''' or myelosuppressive '''chemotherapy''' may cause reversible bone marrow suppression.
*'''Radiotherapy''' or myelosuppressive '''chemotherapy''' may cause reversible bone marrow suppression.
===Causes of increased platelet destruction===
*'''[[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 [[Monocytes|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 [[Monocytes|monocyte]] phagocyte system (MPS).
*'''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 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 endotoxins (e.g. [[:Category:Pseudomonas and Burkholderia species|''Pseudomonas spp.'']] or [[Salmonella|''Salmonella spp.'']]) may also result in the destruction of platelets.
===Causes of platelet sequestration===
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. A common example would be [[Haemangiosarcoma|haemangiosarcoma]] of the spleen and liver. Splenic enlargement under general anaesthesia maintained with agents such as [[Barbiturates|barbiturates]] and [[Phenothiazines|phenothiazines]] can also lead to sequestration 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. A common example would be [[Haemangiosarcoma|haemangiosarcoma]] of the spleen and liver. Splenic enlargement under general anaesthesia maintained with agents such as [[Barbiturates|barbiturates]] and [[Phenothiazines|phenothiazines]] can also lead to sequestration of platelets.
==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.
*'''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.
*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.