Dehydration occurs when loss of fluid from the body, via feces, urine, sweat and water vapor in exhaled air, exceeds fluid intake from food and water. As dehydration occurs, fluid is lost from the blood, which becomes more concentrated. When the fluid lost from the blood is not replaced, the volume of blood in the body decreases. Blood carries oxygen and nutrients to the body, and removes waste products. If there is less blood, it is unable to circulate to all of the tissues as frequently as normal. Consequently, the heart beats faster in an attempt to circulate the blood around the body more quickly, attempting to compensate for the decreased blood volume.
Causes: Dehydration may occur as a consequence of colic, diarrhea, choke, excessive sweating, poor appetite (insufficient amount of food – malnutrition) or blood loss. The commonest causes of dehydration in the horse are diarrhea, sweating, colic and performance on drugs like lasix.
An average adult horse (15.2 h) produces approximately 125 litters of saliva and digestive juices each day. These usually pass through the small intestine as it digests food, and are then reabsorbed in the large intestine. In surgical colic cases where the intestines are blocked, fluid is continually produced in saliva and digestive juices but is prevented from reaching the large intestine by the blockage and so cannot be re-absorbed. Instead it sits in the small intestine causing dehydration and pain as it stretches the intestinal wall. In cases of diarrhea, inflammation off the large intestine reduces its ability to re-absorb fluid, and so the horse passes out very loose, watery feces (diarrhea).
Prolonged or extreme exercise (work), such as in endurance rides, long days at shows combined with a long trip, racing (especially on lasix), may cause excessive amounts of fluid to be lost in sweat, particularly if the weather is very hot. This will cause dehydration, if not replaced. In addition, whenever large amounts of blood are lost, the amount of fluid in the body is markedly depleted and dehydration and shock follow.
Lack of appetite will cause dehydration through the failure to take in fluid. This also occurs in choke, where fluid is also lost in saliva, which cannot be swallowed.
Signs: As dehydration develops, affected animals will become progressively duller and more shocked. The amount of urine produced will decrease as the body tries to conserve fluid, and urine will also become more concentrated. Animals will have tacky or dry mouth, and their lower limbs and ears will become cold.
Treatment: Treatment involves replacing the fluid that is lost, which contains water and electrolytes, and it is important that both are replaced, using specifically formulated solutions. In mildly effected animals where there is no evidence of choke or intestinal blockage, it may be possible to replace the fluids by inserting them directly into the stomach via a stomach tube. The more serious conditions require sterile fluids to be administered directly into the bloodstream via drip. In some cases the volumes may be huge, and in horses with severe diarrhea 100 – 200 liters of fluid may be needed each day.
A proper “water management” in the individual horse care can prevent the dehydration caused by fluids lost due to sweating.