EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

G u i d e t o e q u i n e c l i n i c a l p a t h o l o g y
Cardiac troponin (cTnI) levels are measured
in serum samples (lower results may be
found in plasma samples). Clinically
normal horses have serum cTnI levels
of less than 0.2 ng/ml. Experience so
far suggests that greater than 0.3 ng/
ml is abnormal, i.e. suggests myocardial
pathology and 0.2-0.3 ng/ml is currently
a ‘grey’ zone. Levels of 0.9-5.4 ng/ml
have been measured in horses with
ultrasound-confirmed cardiomyopathy.
Nevertheless, results greater than 0.2 ng/
ml are considered ‘red flags’ for expert
cardiological appraisals.
Sorbitol Dehydrogenase (SDH)
This is an enzyme found in the cytoplasm
of hepatocytes and is therefore virtually
liver-specific, although rises are sometimes
seen in horses with skin conditions and
enteropathy. It is useful for the identification
of acute hepatocellular damage for in-house
laboratory conditions, but the enzyme is
highly labile. Therefore, samples must be
handled with care and assays must be
performed within 8-12 hours of sampling
thus SDH assays are unsatisfactory for
transported samples. SDH is not assayed at
our laboratories - the more stable GLDH assay
is now used more frequently than SDH.
Glutamate Dehydrogenase (GLDH)
Elevations are seen in the presence of
acute hepatocellular damage. This is a
mitochondrial enzyme found mainly in
liver, heart muscle and kidney. It is a
relatively stable enzyme and is a suitable
replacement for sorbitol dehydrogenase
(SDH) (see left) in transported samples.
GLDH rises are sometimes seen in horses
with skin conditions and enteropathy.
L-Gamma
Glutamyltransferase (GGT)
GGT is found in cell membranes of
hepatocytes and biliary epithelial cells,
but the enzyme is also found in the
pancreas and kidney. Elevations in serum
levels are seen in the presence of acute
hepatitis, chronic liver cirrhosis and in very
rare cases of pancreatitis. Nephropathy
does not usually result in significantly
raised serum GGT levels so high levels
measured in the horse are usually a sign
of biliary or cholestatic disease. Chronic
pyrrolizidine alkaloid toxicity (ragwort, i.e.
Senecio jacobea poisoning) causes bile
duct hyperplasia and biliary stasis and
therefore typically results in raised serum
GGT and SAP (see below) levels. This
remains an important cause of hepatopathy
in horses and ponies in UK, who ingest
the plant unknowingly in poor-quality hay.
Toxicity is uncommon in well-managed
horses.
Idiopathic GGT elevations are sometimes
seen in horses in training that appear
otherwise healthy, but perform poorly. The
cause of these GGT rises has not yet been
satisfactorily defined although plant and
fungal hepatotoxins have been suspected.
In most cases, other liver enzymes are
27