EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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T h e B e a u f o r t c o t t a g e l a b o r a t o r i e s Creatine Kinase (CK, CPK) Elevations are specifically seen in the presence of acute myopathy. CK isoenzyme analysis was used in human medicine to help differentiate cardiac and skeletal myopathy from brain pathology, but has never become routinely established in equine sports medicine. Cardiac troponin (see page 25) assays are now used to differentiate skeletal from cardiac myopathy in horses. CK levels peak at 6-12 hours and return to baseline by 3-4 days, assuming that no further myopathy occurs. When measured alongside AST (see page 22), which takes longer to rise, peak and return to normal, the timing and response to treatment of myopathy in horses can be usefully monitored (Fig.10). Paired CK assays taken before and 2-3 hours after strenuous exercise, can form a useful diagnostic test for exercise-induced myopathy, in horses where the diagnosis may be in doubt. Some respiratory virus infections, notably Equine Herpesvirus-1, appear to increase muscle cell membrane fragility and predispose to exercise-induced myopathy in horses in training. This condition is sometimes associated with clinical signs of fatigue and stiffness in performance horses. Significant myopathy, demonstrable by higher serum CK and AST levels, occurs more often after exercise in unfit rather than in fit horses. The conditioning process protects equine muscle cells from exerciseinduced injury. Very high CK levels are seen in young foals with nutritionally associated myopathy (‘white muscle disease’) associated with selenium deficiency. Lactate Dehydrogenase (LD) A variety of disease conditions can cause elevations in total LD and more useful differentiation can sometimes be provided by isoenzyme analysis (Fig.11):- ■ LD isoenzyme 1 - most dramatically increased by intravascular haemolysis (Fig.12). Fig.11: serum ld isoenzymes normal horse Test : LD Gel 2 – 5 16/01/2002 Fraction Rel% IU/L LD 1 9.2 46 LD 2 26.6 133 LD 3 41.6 208 LD 4 18.4 92 LD 5 4.2 21 Total IU/L 500 LD1/LD2: 0.35 24
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 ■ LD isoenzyme 2 - elevated in some cases of cardiac pathology, an indication for cardiac troponin assay (see below) (Fig.13). ■ LD isoenzyme 3 – no known disease association in the horse. ■ LD isoenzyme 4 - most commonly elevated by intestinal pathology (Fig.14). ■ LD isoenzyme 5 - rises seen with skeletal myopathy and hepatopathy, requiring further differentiation with CK (see page 24) and liver enzyme (see pages 24-28) assays (Fig.15). Total LD levels are age-dependent to maturity and reference ranges must be consulted when interpreting results for young horses (see pages 56-69). Cardiac Troponin (cTnI) Two proteins (tropomyosin and troponin) working in concert with calcium, regulate muscle contraction. Troponin is a globular protein complex composed of three single chain polypeptide subunits: TnI (troponin inhibitory component), which prevents muscle contractions in the absence of calcium; TnT (tropomyosinbinding component), which connects the troponin complex with tropomyosin; and TnC (calcium binding component), which binds calcium. The cardiac musclespecific isoform cTnI (24 kDa) exhibits approximately 60% homology with the skeletal isoforms (sTnI) and has a unique 31 amino acid extension of the N-terminus. Experience in human medicine has shown that after acute myocardial infarction (AMI), elevated cTnI levels appear in the circulation within 3-6 hours. Serum levels peak within 14-20 hours and return to normal after 5-7 days. The measurement of cTnI can therefore be a useful diagnostic aid for AMI and an aid for the monitoring of recovery. Myocardial infarction is rarely diagnosed in the horse but myocardial necrosis is seen in conditions such as atypical myoglobinuria. Myocarditis is sometimes suspected on the basis of arrhythmias, echocardiographic abnormalities and/or raised serum lactate dehydrogenase isoenzyme 2 levels. This can follow upper respiratory viral infections. Horses who have suffered in this way will need more rest and supportive treatment followed by follow-up to normality before return to strenuous exercise if potentially serious complications are to be avoided. Raised cTnI levels are an indication for cardiac ultrasound examinations and ambulatory ECG monitoring. 25
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EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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