Changes

[[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.  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 Deficiencies|clotting factors]].   

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]].   

==Thrombocytopaenia==

==Thrombocytopaenia==

**'''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, administration of oestrogens or of [[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 virus]], [[Canine Parvovirus|canine parvovirus]] and [[Feline Panleucopaenia|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.

**'''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.

**'''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.

**'''[[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).

**'''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]]'', ''[[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]], [[Classical Swine Fever|classical swine fever]] and [[Infectious Canine Hepatitis|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 [[: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:

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

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

*Physiological

*Physiological

**'''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. 

**As part of a response to '''anaemia''', a rebound thrombocytosis is often documented and this may precede signs of regeneration.

**'''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.

*Pathological

*Pathological

**'''Essential thrombocythaemia'''

**'''Essential thrombocythaemia'''