Page 1042 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

H aemolymphatic system 1017

VetBooks.ir presence of red maple trees. Evidence of haemolytic 9.19
anaemia on the CBC and finding Heinz bodies on
the blood smear are other supportive findings, as are
haemoglobinaemia and haemoglobinuria. Blood can
be stained with new methylene blue to highlight the
inclusions, which can sometimes be difficult to dis-
cern on Romanowsky-stained preparations. Plasma
methaemoglobin concentration may be increased,
resulting in a chocolate brown discolouration of
the blood. Elevations in bilirubin, predominantly
the unconjugated form, are present on a serum bio-
chemical profile. Elevations in urea and creatinine
will be present if pigment-associated renal failure
has developed. Hypoxic damage to the liver may
result in elevations in hepatocellular enzyme activ- Fig. 9.19 Left: dark brown-black urine due to
ity. Microscopic or macroscopic haemoglobinura methaemoglobinuria. Right: two blood tubes with similar
may be present (Fig. 9.19). discolouration due to methaemoglobinaemia from a horse
with red maple toxicosis. (Photo courtesy L Arroyo)
Management
There is no specific treatment for this disease.
Further exposure to wilted red maple leaves should haemoglobin on kidney function are possible com-
be prevented. Intravenous fluid therapy may be use- plications, as is DIC.
ful in dehydrated animals and for diuresis to pre-
vent haemoglobin-associated kidney injury. Blood OTHER CAUSES OF HEINZ BODY
transfusion may be required, depending on clini- HAEMOLYTIC ANAEMIA
cal signs and PCV. Exercise should be restricted.
Severely anaemic animals may benefit from intra- Definition/overview
nasal oxygen therapy. Addition of ascorbic acid Although red maple leaf toxicosis is the most com-
(30–50 mg/kg i/v q12 h) may be useful as an anti- mon cause of Heinz body haemolytic anaemia in
oxidant. Oral ascorbic acid is less useful because of horses, other oxidant compounds also have the
the time required to achieve therapeutic tissue lev- capacity to denature haemoglobin. These include
els. Methaemoglobinaemia resulting from wilted red onions and garlic, and the drug phenothiazine.
maple toxicosis should not be treated with methylene
blue because of the limited efficacy and potential Aetiology/pathophysiology
for further Heinz body formation. Affected horses Onions contain compounds that if consumed in suf-
should be closely monitored for complications such ficient quantity can result in Heinz body formation
as laminitis. and subsequent haemolysis. Suspect compounds
Red maple trees should not be accessible to horses include allyl-propyl disulphide, di-n-propyl disul-
and existing trees should be removed or adequate phide and phenol. Garlic belongs to the same genus
fencing provided so that horses do not have access to as onions and probably has similar toxic components.
leaves. Horses should also not have access to leaves High doses of phenothiazine result in oxidative dam-
and branches from pruned red maples. age due to production of phenothiazine disulphide in
the intestinal tract.
Prognosis The toxic compounds cause oxidative damage to
Intravascular haemolysis for any reason warrants a equine RBCs. Oxidation denatures haemoglobin,
guarded to poor prognosis. If the disease is detected with subsequent Heinz body formation. Cells with
early and the dose of toxin was minimal, the ani- Heinz bodies are removed by macrophages in the
mal may recover. Tissue hypoxia and the effects of spleen or lysed intravascularly.