Page 1049 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1024 CHAPTER 9

VetBooks.ir 9.25 9.26

Fig. 9.25 Bone marrow core biopsy with a normal Fig. 9.26 Bone marrow core biopsy from a horse
cellularity (approximately 50% cellular and 50% fat) with aplastic anaemia. There is complete absence of all
(H&E stain). haematopoietic cells, with only fat remaining (H&E stain).

IMMUNE-MEDIATED Most clinically affected animals have platelet
THROMBOCYTOPENIA counts <10 × 10 /l.
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Definition/overview Clinical presentation
IMTP is inappropriate destruction of mature plate- Horses exhibit bleeding from multiple mucosal sur-
lets because of the presence or exposure of a per- faces and often have petechial or ecchymotic haem-
ceived foreign antigen on the surface of the cell, orrhages. Epistaxis, melaena and hyphaema may be
with subsequent removal of these cells by the mono- present, and excessive bleeding following trauma,
nuclear phagocytic system. surgery or venipuncture may be noted. Other clini-
cal signs are not usually present, and horses are
Aetiology/pathophysiology alert and afebrile unless severe haemorrhage has
IMTP may be truly autoimmune (meaning the per- occurred or IMTP is secondary to an underlying
ceived foreign antigen is a self-antigen), secondary disease.
due to the ability of some drugs to act as haptens or
because an infectious agent exposes or has a cross- Differential diagnosis
reactive antigen. IMTP secondary to lymphoma has Other causes of petechial haemorrhage from throm-
been reported. Neonatal alloimmune thrombocy- bocytopenia, including DIC and infectious diseases,
topenia may also occur in foals from multiparous need to be ruled out.
mares.
Thrombocytopenia develops because of the Diagnosis
formation of antigen–antibody complexes on Diagnosis is made by observing appropriate clini-
the platelet surface, with subsequent removal cal signs in conjunction with typical CBC findings,
by the mononuclear phagocytic system in the including severe thrombocytopenia. Macroplatelets
spleen and liver. Haemostatic abnormalities may may be noted by the time clinical signs are identified,
develop depending on the severity of thrombo- indicating bone marrow release of younger plate-
cytopenia. In general, spontaneous haemorrhage lets (Fig. 9.27). Since platelets are usually destroyed
will not occur until <30 × 10 /l platelets remain. outside the bone marrow, megakaryocytes should be
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