Updated: May 8
Under normal conditions, fecal water loss is the largest component of water loss. In the normal horse, the fluid contents of the intestine amount to about 35 liters, 75 percent of which is found in the large bowel. The water content of the bowel is likely to be less during anorexia and substantially higher during diarrhea.
The only quantitative data on diarrhea was a study in a horse that was induced with aloin. In this study, the most substantial water loss observed was in one horse which put out 40 kg of feces containing an estimated 37 liters of water. The study suggested that horses with acute diarrhea may lose between 50 and 100 liters of water a day, and others estimate a range as high as 80 liters a day. Other conditions that are therapeutically equivalent to diarrhea are early colitis X and high intestinal obstruction. In both of these conditions, there is a rapid and substantial loss of fluid and electrolytes into the intestine without any overt diarrhea. As in diarrhea, there are substantial losses of gastric and pancreatic intestinal secretions which contain Na+ , K+ , and HCO3 ions.
Substantial K+ deficits are likely to develop in horses with acute diarrhea. Since horses with acute diarrhea are often anorectic, their potassium intake is greatly reduced, while fecal losses are greatly increased. In addition, changes in the acid-base balance of these horses may affect the potassium balance, a decrease in plasm pH being associated with a movement of K+ from intracellular to extracellular sites. Serum potassium levels of less than 3.0 mEq/L are suggestive of severe hypokalemia in the horse. Studies show K+ deficits of horses with acute diarrhea and hypokalemia may range up to several thousand mEq. Since attempts to replace K+ deficits of this magnitude intravenously are not advisable, oral administration of K+ chloride is the only logical approach, with serum K+ levels being used to guide the replacement therapy.
Studies show horses with profuse diarrhea tend to have significant decreases in the concentrations of all important electrolytes. In treating these horses, the studies find the best results by using Ringer's lactate with either 5 gms of NaHCO3 or 5 gms of KHCO3 or, in some cases, 5 gms of each added per liter. It is recommended a total of 40 liters of this fluid in twenty-four hours, the first 10 liters IV, and the remainder orally in twelve-hour intervals.
The rapid administration of solutions with such high concentrations of K+ and bicarbonate is not without danger, and this danger is greatly increased if renal function is impaired. In this case, the initial treatment of horses with suspected renal failure with the less dangerous Ringer's lactate is to be followed with more potent corrective solutions when evidence of good kidney function is obtained.