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 30 Nephrosis and nephritis are important conditions in neonatal and older foals and are occasionally seen in other age groups. Creatinine Creatinine is formed in muscles from creatine breakdown and is excreted via the kidneys. In normal horses, daily production and excretion are remarkably constant, leading to its use as an arithmetic constant for use with urinary fractional excretion rates (see below). Therefore serum elevations reflect renal malfunction (reduced glomerular filtration), levels being controlled by excretion rate. This may occur in horses with pre-renal (e.g. circulatory disturbances, dehydration or shock), renal (insufficiency) or post-renal conditions. Measurement of urine specific gravity and fractional excretion of electrolytes may help to differentiate pre-renal and renal azotaemia. When uroperitoneum (postrenal azotaemia) is suspected it can be useful to compare peritoneal fluid and serum creatinine concentrations as a ratio of greater than 3:1 confirms uroperitoneum. For both renal and post-renal azotaemia, ultrasonography can be very helpful. Urinary Fractional Electrolyte and Mineral Clearance Ratios In horses with normal renal tubular function, urinary excretion rate of creatinine is almost constant. It can therefore be used as an arithmetic constant to produce a measure of the fractional excretion of electrolytes and minerals in equine urine. Concentrations of electrolytes or minerals are measured in serum and urine samples collected at the same time or at least within 1 hour of one another. Diuretics must not be used to stimulate urination or spurious results will be obtained. The percentage excretion of an electrolyte is calculated from the following equation:- Where: (E)u = Concentration of electrolyte in urine (E)u (Cr)s Fractional urinary X X 100 = (E)s (Cr)u excretion (%) (E)s = Concentration of electrolyte in serum (Cr)u = Concentration of creatinine in urine (Cr)s = Concentration of creatinine in serum Fractional excretion rates for sodium, potassium, chloride, magnesium, calcium and phosphate are most commonly measured. The result provides an assessment of the horse’s homeostatic regulatory status for that electrolyte or mineral, e.g. sodium and chloride may be selectively excreted at an increased rate because of excessive dietary salt intake. Phosphate may be excreted at an increased rate to try to maintain a normal serum calcium:phosphate ratio in the face of inadequate calcium intake. The use of this method applies only to horses with normal renal function. Impaired renal tubular function results in high urine excretion rates for all the electrolytes and minerals,
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 from failure of retention - in such cases, creatinine is not a valid arithmetic constant. Electrolyte imbalance may predispose exercise-induced myopathy in horses in training and so fractional clearance ratios may sometimes be helpful in the investigation and management of recurrent cases. Dietary deficiencies, excesses or imbalances can be corrected and return to normal excretion rates monitored, sometimes with useful results in terms of resolution of myopathy. In secondary nutritional hyperparathyroidism, which occurs in horses on a high phosphate diet, phosphate excretion rate is high, indicating the need for oral calcium supplementation and phosphate reduction, to restore balance. Experience has shown that in UK, many healthy, fit, stabled and well performing horses, receiving high cereal training rations, have urinary fractional phosphate excretion rates in excess of 9%, emphasising the need for calcium supplementation. Higher excretion rates are seen in horses with clinical manifestations of secondary nutritional hyperparathyroidism, which include shifting lameness, periosteal thickening, facial and mandibular swelling. Urinary phosphate clearance ratios are useful measures of calcium:phosphate balance in weanlings and yearlings, important for bone growth and development. Calcium: phosphate imbalance can predispose to physitis. Calcium, Potassium and Chloride Electrolyte imbalance and fluid loss may occur with diarrhoea, endotoxaemia, intestinal crises and exertional exhaustion. The latter is of particular importance for endurance horses and for other horses performing in hot and humid weather conditions. Serial assays are helpful with intensive care cases. In other cases, fractional urinary electrolyte excretion rates (see page 30) are a much better assessment than single serum assays. Electrolyte assays are very important to the assessment of neonatal foals under critical care, with specific supplementation, where indicated and monitoring of return to normality. Calcium, Phosphate and Magnesium Mineral analysis may be helpful in young horses, i.e. yearlings and two-year-olds coming into training, with signs suggesting abnormalities of bone metabolism. As homeostatic mechanisms are efficient, serum levels are often normal even in the face of whole body abnormality, and thus urinary fractional excretion rates (see page 30) are of greater value. Hypocalcaemia is a cause of synchronous diaphragmatic flutter in performance horses and of uterine inertia in pregnant mares at full term, requiring cautious corrective therapy. 31
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EQUINE CLINICAL PATHOLOGY - Rossdale & Partners

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