Determination of red blood cell potassium content in horses with diarrhoea: a practical approach for therapy

Changes of the erythrocyte phenotype and blood biochemistry in dairy calves during the first ten weeks of age

Background

Bovine erythrocytes undergo important changes in their morphology and chemical composition during the first weeks of age, which must be understood to accurately interpret hematology results in calves. The objectives of this prospective cohort study were to describe physiological changes of calf erythrocytes and to investigate mechanisms potentially causing these changes.

Methods

Blood samples from 30 clinically healthy dairy calves were obtained from birth to the tenth week of age in weekly intervals. Hematological and plasma biochemical parameters as well as the mineral electrolyte content of erythrocytes were determined and followed over time. The changes of parameters characterizing the erythrocyte phenotype over time were compared to the changes of plasma and erythrocyte biochemical parameters and possible associations were investigated using correlation and stepwise regression analyses.

Results

Although the erythrocyte mean corpuscular volume (MCV) declined from 43.6 ± 3.7 fL to 35.6 ± 3.2 fL between the first and seventh week, the red blood cell count (RBC) increased from 7.2 ± 1.1 × 10¹²/L to 9.3 ± 1.0 × 10¹²/L until the fifth week of age. The blood hemoglobin (Hb) concentration increased from 0.96 ± 0.16 g/L to 1.16 ± 0.11 g/L in the first three weeks of age and remained at this level until the end of the study. Changes in MCV were accompanied by a decline of the erythrocyte potassium content (K_ERY) from 91.9 ± 13.5 to 24.6 ± 7.2 mmol/L and a concomitant increase of the erythrocyte sodium content from 45.0 ± 32.0 to 102.7 ± 26.5 mmol/L. MCV was found to be associated with K_ERY, the primary determinant of the intra-erythrocyte osmotic pressure from the sixth week of age and with blood hemoglobin, the primary determinant of the intra-erythrocyte oncotic pressure from the eighth week of age, when K_ERY, blood Hb and MCV already had reached or approached normal levels of adult cattle. The plasma iron concentration was not found to be associated to any of the studied hematological parameters.

Conclusion

A volume reduction of 20% in bovine neonatal erythrocytes is a physiological change occurring during the first weeks of age and is neither associated with sideropenia nor with anemia in healthy calves. The mechanism driving the observed erythrocyte volume change could not be identified. Results of the correlation and regression analyses indicate that changes in intra-erythrocyte osmotic or oncotic pressure are improbable underlying causes. Results reported here show that K_ERY is an unreliable indicator for the K homeostasis of the intracellular space in neonatal calves and that a decrease in MCV in early life per-se is an unreliable indicator for the development of microcytic anemia.

Description of 14 cases of bovine hypokalaemia syndrome

The records of 14 cases of bovine hypokalaemia observed between 1983 and 1996 were reviewed. The most common history included a protracted, often infectious, disease. All age groups were represented. Although previously reported as a risk factor, isoflupredone acetate had not been administered to five of the cases. The following clinical signs were recorded in 10 cases: abnormal position of the head and neck, severe weakness, rumen hypomotility or atony, abnormal faeces, anorexia and tachycardia. Cardiac dysrhythmia was observed in six cases. Acid-base imbalance (alkalosis in 10 cases), hyperglycaemia and increased activities of aspartate aminotransferase and creatine kinase were associated with hypokalaemia ranging from 1.35 to 2.49 mmol/litre. Treatments included symptomatic treatment, supportive care and potassium chloride given intravenously and orally at an average total daily dose of 42 g/100 kg bodyweight (26 g by mouth and 16 g intravenously) for an average of five days. Eleven cases recovered after an average of three days.

Electrolyte and acid-base disturbances in the horse

Laboratory electrolyte and acid-base analyses are important for the characterization and assessment of the severity of disorders of fluid balance, and they enable the veterinarian to institute appropriate corrective therapeutic interventions. Abnormalities of electrolytes or acid-base rarely define the diagnosis, but certain diseases are characterized by predictable trends in these parameters. Important clinical situations in which assessment of electrolyte and acid-base status should be regarded as important to the equine practitioner include diarrhea, severe colic, peritonitis, pleuritis, dysphagia (inability to drink water or ingest food), neurologic dysfunction, exhaustion, renal failure, and rhabdomyolysis.

Fluids, electrolytes, and bicarbonate

In an attempt to enhance performance, primarily by delaying the onset of fatigue, a variety of formulations of fluids, electrolytes, and sodium bicarbonate are administered to performance horses. Some current practices of fluid and electrolyte supplementation are well justified; others have no basis to support their use. In addition, occasional combined administration of certain agents (i.e., furosemide and sodium bicarbonate) can have detrimental effects on performance.

Electrolytes: clinical applications

Many factors can influence electrolytes in the horse. With the major electrolytes (sodium and potassium), alterations in serum or plasma values do not reflect changes in total body status. However, estimates of electrolyte alterations may be made by combining assessments of body weight changes with plasma sodium values by using the formula of Edelman et al. In planning electrolyte therapy, it has to be remembered that access to green feed or hay is important in correcting any total body potassium depletion and that diets marginal in sodium may interfere with exercise capacity by limiting sweat production. Free access to salt would seem to be the simplest method to ensure adequate sodium availability. Despite the widespread use of various commercial electrolyte supplements administered in horses' feed, most would appear to be unwarranted. Extensive electrolyte deficits are most likely in endurance horses in which substantial electrolyte losses can occur in the sweat. In such cases and when access to grass or hay is limited, supplementation with sodium and potassium chloride may be useful. Bicarbonate administration appears to have no clinical role in endurance or pleasure horses. However, its use as a buffer to limit the extent of lactic acidosis during galloping exercise has not been resolved.

Dietary requirements and athletic performance of horses

There is no clear evidence that the chronic requirement for any non-energy yielding nutrient rises in proportion as the energy requirement increases with hard work. The need for protein, and probably that for calcium, remain a function of bodyweight daily. Some proportionality with energy may exist for certain nutrients, although the evidence has not been adduced. For example, because of an increase in both the proportion and amount of propionic acid in the volatile fatty acids of caecal contents, the tissue requirement for vitamin B12 may rise with an increase in the rate of energy metabolism. Exercise influences appetite and therefore voluntary intake, and consequently the daily intake of nutrients. Although that intake is not just a function of dietary bulk and weight, it is necessary to increase energy concentration of diets to achieve an adequate chronic intake of energy where work intensity and energy expenditure are considerable. Acute nutrient requirements paint a different picture from chronic requirements. An increase in total feed intake, or the density of that feed, would neither satisfy these requirements nor be a desirable means of doing so. The acute needs of water, electrolytes and soluble carbohydrates should be met by dosing when the need arises. The timing of the consumption of energy yielding substrates relative to that of exercise may be critical to performance. An inevitable postprandial consequence of a meal of starch or protein by the resting horse, is an increase in the activity of plasma insulin. This increase decreases blood glucose, depriving muscles of a critical substrate, but the assertion has not been resolved by experiment in horses. Experiments are required to ascertain the optimum feeding regime during the 24 h preceding extreme exertion. Whereas exhaustion in sprint work is largely a function of elevated blood lactate concentration, that of extended work is a consequence of a decline in glycogen reserves and losses of body fluid and electrolytes. Glycogen loading is of benefit to many long distance human athletes, but no advantage has yet been established for this practice in horses, and without modification it could render them subject to laminitis and endotoxaemia. Nevertheless supplementation of horses with water, glucose and electrolytes during work may benefit their endurance. The provision of 5 litres water every 2 h with 30 g salt, or twice as much of mixed electrolytes, and 15 g sucrose or glucose, is recommended for a 500 kg horse during periods of extreme sweating.(ABSTRACT TRUNCATED AT 400 WORDS)

Colitis: differential diagnosis and treatment

Equine colitis characterised by diarrhoea and/or pain may be caused by a wide variety of bacterial, viral, protozoal agents and toxins. The causative agent of Potomac horse fever, Ehrlichia risticii, is the most recently recognised cause of colitis. Salmonella, the agent typically associated with colitis, also causes abdominal pain (colic) of variable intensity. Acute colitis is also caused by colitis X, various antibiotics, endotoxic shock and peritonitis caused by arterial infarction. The principles of therapy for each of these, together with means of differential diagnosis, are presented.