Oral Electrolytes Stimulate Water Drinking by Dehydrated Horses

1. Introduction

Prolonged exercise, food and water deprivation and acute enterocolitis all produce substantial depletion of body fluid and electrolyte stores. Based on changes in body weight, Carlson¹ estimated sweat water loss to be about 10 l/h during endurance exercise and suggested that greater losses occur during competitions under conditions of high heat and humidity. Thus, a typical 50-mile endurance ride lasting 5 hours could result in fluid losses approaching 50 l (,10% of body weight). Similar fluid losses are often experienced by horses dehydrated by severe enterocolitis.² In both instances, fluid losses are partially replaced by water drinking, but a substantial loss of body fluid (3–5% of body weight) may persist despite free access to water. Because the major stimulus for thirst is an increase in plasma osmolality (or plasma sodium concentration), replacement of water without electrolytes leads to dilution of remaining body fluid electrolyte stores (manifested by mild to severe hyponatremia) and a lack of thirst. In humans, this phenomenon of persisting weight loss in the absence of thirst has been termed involuntary dehydration.³ To avoid or minimize involuntary dehydration, human endurance athletes and patients suffering from diarrhea can force themselves to drink a variety of isotonic to hypotonic carbohydrate/electrolyte rehydration solutions. In horses, administration of large volumes of isotonic rehydration solutions via a nasogastric tube is also effective in limiting involuntary dehydration consequent to prolonged exercise or diarrhea.^4-6 Unfortunately, forced drinking is not an option for dehydrated horses, and frequent nasogastric intubation may not be acceptable (during endurance exercise) or well tolerated by all horses with diarrhea. In an attempt to stimulate drinking and replace electrolytes lost in sweat, endurance riders routinely supplement horses with oral electrolyte pastes before, during and after the ride. Similarly, oral electrolytes are sometimes used as an adjunct treatment with intravenous fluids in horses with diarrhea. […]