Page 1056 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

H aemolymphatic system 1031

VetBooks.ir of PCV. Initially, PCV should be monitored every Clinical presentation
Horses are typically presented with vague signs of
few days. The goal should be to maintain a PCV of
<0.50 l/l (50%). Secondary polycythaemia caused
argy, inappetence, weight loss and weakness. More
by hypoxia should not be treated by aggressive disease that may include poor performance, leth-
phlebotomy because the increased PCV is a com- specific signs indicating haematological involvement
pensatory mechanism. If the PCV is not markedly may include pale mucous membranes and petechial
elevated (<0.60 l/l [<60%]), removal of smaller vol- or ecchymotic haemorrhages. Intermittent fever
umes of blood is indicated. The primary cause of may be present when opportunistic infections occur
hypoxia must be addressed. In humans and small due to neutropenia.
animals, hydroxyurea has been used to induce
reversible bone marrow suppression; however, its Differential diagnosis
use has not been reported in horses. If the presence of circulating atypical cells is con-
firmed, the primary differential diagnoses are myelo-
Prognosis proliferative and lymphoproliferative disorders.
The prognosis for relative polycythaemia is good
if appropriate fluid therapy is administered and the Diagnosis
cause of dehydration can be addressed. Cases of sec- Diagnosis of AML is based on the observation of
ondary polycythaemia carry a more guarded prog- atypical cells on blood smears or in bone marrow
nosis, especially those due to neoplasia. (Fig. 9.34), while MPNs can be recognised based on
inappropriate accumulation of mature cells. MDS
MYELOPROLIFERATIVE DISEASE usually results in one or more peripheral blood cyto-
(MYELOID LEUKAEMIA) penias, with accompanying dysplastic features in

Definition/overview
Tumours of developing non-lymphocytic haema- 9.34
topoietic cells including those derived from eryth-
rocytes, granulocytic cells, monocytic cells and
platelets are classified into acute myeloid leukaemia
(AML), myeloproliferative neoplasms (MPNs) and
myelodysplastic syndrome (MDS). These diseases
are often collectively referred to as leukaemias and
are rare in horses.

Aetiology/pathophysiology
Neoplastic transformation of any cell line may occur,
but the underlying cause is usually unknown in
horses. Unregulated proliferation of a cell line at a
certain stage of development results in the increased
autonomous production of these cells in bone mar-
row, which can then infiltrate other tissues and be
found in circulating blood. The tumour replaces nor-
mal haematopoietic cells, resulting in cytopenias in Fig. 9.34 Blood smear from a horse with acute
other cell lines due to lack of production. Neoplastic myeloid leukaemia. A monotypic population of
cells are often observed in circulation, as they are large round cells with round-to-indented nuclei
released from bone marrow. Other tissues affected and multiple prominent nucleoli is pictured
may include the spleen, liver and lymph nodes. (Wright’s stain). (Photo courtesy RM Jacobs)