Defined as a harmful reduction of the amount of water in the body, dehydration results from either a lack of water going in, either in the form of fluids or in the food, or too much water leaving the body in sweat, urine, faeces (diarrhoea) milk and breath. The water loss may be accompanied by loss of salts (electrolytes). Dehydration of 15% is potentially fatal to adult horses.
In horses, the most common causes of dehydration are heavy sweating associated with exercise, and diarrhoea. In both cases electrolyte ions are lost with the water, principally Chloride and Sodium in sweat and those plus Potassium in diarrhoea. Lost electrolytes may well need replenishment as well as water.
Mild dehydration stimulates thirst, but more severely dehydrated horse may refuse to drink and require drenching with water and electrolytes, or intravenous fluid therapy.
The simplest test for dehydration is the skin pinch test where a fold of skin on the side of the neck is pinched between fingers and thumb – the fold should flatten out and return to normal within 2 seconds in a normally hydrated horse. Extension of that time indicates dehydration.
Provision of adequate supplies of drinking water is fundamental to prevent dehydration and where water losses from sweating or other causes are increased, supplementary salt and electrolytes may also be required.
See also Water Intake