Dehydration

There are two major causes of dehydration (decrease in free water):

• Inadequate water intake.
• Excessive fluid loss.

Deprivation of water, a lack of thirst caused by toxemia and the inability to drink water e.g. in esophageal obstruction are examples of dehydration from inadequate water intake.

The most common cause of dehydration is when excessive fluid is lost. Diarrhea is the most common reason for excessive fluid loss, although vomiting, polyuria, and loss of fluid from extensive skin wounds or by copious sweating may be important in sporadic cases.

Severe dehydration also occurs in acute carbohydrate engorgement in ruminants, acute intestinal obstruction and diffuse peritonitis in all species, and in dilatation and volvulus of the abomasum.

In most forms of dehydration, deprivation of drinking water being an exception, the serious loss, and the one that needs correction, is not the fluid but the electrolytes.

The ability to survive for long periods without water in hot climates represents a form of animal adaptation that is of some importance. This ability depends on a number of factors, including: the insulating ability of the hair/fleece, the ability to carry water reserves in the rumen and extracellular fluid space, the ability of the kidneys to conserve water, and the ability to maintain blood circulation with a lower plasma volume.


PATHOGENESIS

The initial response to negative water balance is the withdrawal of fluid from the tissues and the maintenance of normal blood volume. The fluid is drained primarily from the intracellular and interstitial fluid spaces. Major loss occurs from connective tissue, muscle, and skin. Essential organs, including the CNS, heart, and skeleton, contribute little.
The loss of fluid from the interstitial and intracellular spaces results in loss of skin elasticity, dryness of the skin and mucosa, and a reduction and retraction of the eyeball (enophthalmia) caused by reduction in the volume of the postorbital fat deposits.

The secondary response to continued negative water balance is a reduction in the fluid content of the blood causing a reduction in circulating blood volume and an increase in the concentration of the blood (hemoconcentration). Because of the hemoconcentration, there is an increase in the viscosity of the blood, which impedes blood flow and may accelerate peripheral circulatory failure. The loss in circulating blood volume also contributes to the mental depression of dehydrated animals.

In deprivation of water and electrolytes or in deprivation of water alone, the kidney compensates effectively by decreasing urine output and increasing urine osmolality. In addition, water is preserved by reduced fecal output and increased absorption which in turn results in dry, scant feces.

Dehydration exerts important effects on tissue metabolism. There is an increase in the breakdown of fat, then carbohydrate, and finally protein to produce water of metabolism. The increased endogenous metabolism under relatively anaerobic conditions results in the formation of acid metabolites and the development of metabolic acidosis. Urine formation decreases because of the restriction of renal blood flow and this, together with the increased endogenous metabolism, causes a moderate increase in plasma concentration of urea nitrogen.

Dehydration leads to death, especially in acute intestinal obstruction, vomiting, and diarrhea, but it is chiefly a contributory cause of death when combined with other systemic states, such as acidosis, electrolyte imbalances, toxemia, and septicemia.


CLINICAL FINDINGS

The first and most important clinical finding in dehydration is dryness and wrinkling of the skin, which gives the body and face a shrunken appearance. The eyes recede into the sockets, and the skin subsides slowly after being picked up into a fold. The dehydration is usually much more marked if water and electrolyte losses have been occurring over a period of several days.

Here are the finding in different animals:

• Dogs: Sunken eyes, dark urine, lethargy, dry mouth, raspy barks, appetite loss, thick saliva, loss of skin elasticity, depression
  
  
• Cats: Panting, lethargy, sunken eyes, loss of skin elasticity, elevated heart rate, dry mouth, appetite loss
  
  
• Pet Birds: Sunken or dull eyes, sticky or dry mouth, dry droppings, lack of droppings, lethargy, weakness
  
  
• Chickens: Panting, labored breathing, drooped wings or fluffing feathers, limp posture, unresponsive behavior, convulsions
  
  
• Horses: Elevated heart rate, rapid shallow breathing, reddened gums, dry eyes, thick saliva, loss of skin elasticity
  
  
• Goats: Low body temperature, listless behavior, lethargy, sunken eyes, sticky mouth, dry membranes, weakness
  
  
• Cattle: Dry membranes, loss of skin elasticity, depression, appetite loss, less urine, strong-smelling urine
  
  
• Pigs: Constipation, skin irritation, ear twitching, appetite loss, listless wandering, disorientation, circling, convulsions
  
  
• Sheep: Dry mouth, sunken eyes, dry eyes, loss of skin elasticity, thin appearance, low body temperature, constipation