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 28 within normal range as are urea and creatinine levels, and liver biopsy reveals insignificant histopathological findings. It is therefore not certain that primary hepatopathy or nephropathy is involved. Some cases have raised muscle enzyme levels suggesting an association with myopathy, either directly or secondarily, again perhaps via respiratory virus-induced increased muscle cell membrane fragility. Most cases respond (GGT levels return to normal) to a period of reduction in the training programme. Urine GGT:creatinine ratios are elevated (>4.0) in renal tubular pathology. Alkaline Phosphatase (SAP) Elevations in this brush border enzyme are most commonly seen in the presence of chronic biliary obstructive liver pathology (e.g. chronic pyrrolizidine alkaloid toxicity). High levels are also seen with abnormalities of bone metabolism and intestinal malfunction, a useful assay in growing foals and yearlings with clinical signs of developmental orthopaedic disease, where SAP results may be very high and will respond to restricted exercise and mineral, vitamin and trace element supplementation. SAP levels are age-dependent to maturity and appropriate reference ranges must be consulted when interpreting results for young horses (see pages 56-69). Intestinal Phosphatase (IAP) Elevations in IAP relative to total SAP are seen in the presence of intestinal pathology. References ranges for serum SAP and IAP levels are very age-related and apparently high results must be interpreted carefully in foals, yearlings and immature performance horses. Bilirubin The analysis of bilirubin levels is seldom useful in the horse but may aid the classification of anaemia and jaundice in some cases. Owing to the horse's unusual biliary excretion system, indirect (unconjugated) bilirubin levels, may be higher than those in other species, without clinical disease, and the significance of elevations without other abnormalities may therefore be difficult to interpret. A period of anorexia, inanition or intestinal malfunction typically increases indirect bilirubin levels spuriously. Bile acids This is a much better guide to hepatobiliary status than bilirubin assays. High bile acid levels occur with embarrassed hepatic function and are a useful diagnostic and prognostic liver function and prognostic guide in horses. It is important to remember that none of the liver enzymes, measured singly or in a profile, give useful information
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 about liver function. The liver has a large functional reserve and compensatory capacity. Liver enzyme rises suggest hepatopathy which can be differentiated to a degree into acute, chronic, biliary obstructive or mixed pathology, but bile acid assays and bromsulphalein clearance test results reveal either adequate (normal levels) or impaired (high levels) functional compensation. Impaired hepatic function suggests a guarded prognosis. Liver biopsy and ultrasound examinations are required to confirm an etiologic diagnosis and to refine prognosis. Amylase Elevations occur in the presence of pancreatitis, but this condition is rarely diagnosed in the horse. Glucose Other than for oral glucose and xylose absorption tests, useful for the diagnosis and evaluation of intestinal malabsorption cases, the value of this assay in adult horses is limited to cases of pituitary pars intermedia dysfunction (equine Cushing’s syndrome) which are frequently hyperglycaemic. Measurement of blood glucose is invaluable in the management of critically ill foals as profound hypoglycaemia is often seen in neonatal septicaemia. Samples for glucose assay must be taken into fluoride anticoagulant or must reach the laboratory within hours of collection. Oral Glucose Absorption Test The horse to be tested should be fasted for 18-24 hours. 1 g glucose per kg body weight is administered by stomach tube. Blood samples are collected into fluoride anticoagulant at times 0, 30, 60, 90, 120, 150, 180, 210 and 240 mins. Glucose levels should peak at double resting (0 mins) levels at 60-120 mins and return to resting levels by 240 mins. Cholesterol and Triglycerides Elevations are seen in the presence of abnormal lipid metabolism and hyperlipidaemia. These conditions are typically seen in the Shetland and other small ponies, donkeys and occasionally as a secondary complication in horses that are anorexic or unable to eat. Pregnant Shetland pony mares appear particularly susceptible following a managemental and nutritional change/challenge. Urea Urea is produced in the liver from the metabolism of ammonia. Elevations are seen in the presence of abnormal renal function. Urea levels may also rise in the haemoconcentrated and ‘over-trained’ horse, associated with fluid balance shifts rather than renal disease. Many cases of equine dysautonomia (‘grass sickness’) have a degree of uraemia but this is usually caused by catabolism. A period of anorexia can have a similar result. 29
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EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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