Like all mammals, felines lose a certain amount of required fluids daily through natural functions such as producing urine, saliva, and waste. These much-needed liquids are easily replaced through consuming the proper amounts of water.

Certain circumstances may cause cats to lose too much of these valuable fluids, which may lead to dehydration, a dangerous and potentially life-threatening complication. The common causes of dehydration in cats include: an ongoing illness which may cause the cat to stray away from food and water, bouts of vomiting and or diarrhea, increased urination (generally caused by medical conditions such as diabetes and kidney failure), heat stroke, shock, blood loss, and fever.

Fortunately for pet owners, cats typically present with one or more symptoms of dehydration, which may include:

Dryness in Mouth
Sudden dryness of the mouth may indicate dehydration in felines. The cat’s mouth may develop a sticky or filmy texture, and the animal may repeatedly rub its tongue over the roof of its mouth and lips.

Difference in Skin Elasticity
Changes in skin elasticity often indicates a health concern such as dehydration in felines. To perform the test, simply grasp the scruff of the back of the cat’s neck and release. In healthy animals, the skin will retract quickly. Slower retraction may indicate dehydration, though this test may be difficult for some pet owners to judge properly.

Mood Changes
Felines suffering from dehydration often display quite noticeable mood changes such as lethargy or an unusually ill-temper. Some cats may become extremely sleepy or difficult to keep awake.

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Absence of Urination and or Bowel Movements
Any time in which eight to ten hours have passed in which a feline has failed to produce either urine or waste, dehydration is likely present.

Sunken Eyes
Perhaps the most obvious of all signs of dehydration in cats is the appearance of sunken and or swollen eyes.

Capillary Refill Time
When a cat is dehydrated, its blood circulation is greatly affected. Testing the animal’s capillary refill time is a nearly fool-proof way of diagnosing the animal with dehydration. This can be done by raising the cat’s upper lip and pressing your finger firmly on the top gums for at least five seconds. After removing the finger, a white spot will be revealed where the pressure was placed. Using a watch with a second hand, time how long it takes for the white spot to disappear and for the gum tissue to return to its normal pinkish shade. In healthy felines, capillary refill time is typically no more than two seconds. It is recommended to perform this test at least twice in order to obtain the correct timing.

Dehydrated felines require the immediate replacement of all lost fluids, typically through an IV, in order to prevent serious complications and even death. If you suspect that your cat is dehydrated, it is absolutely necessary to seek veterinary help as soon as possible.

Sources:
http://www.cat-world.com/au/dehydrationincats.htm
http://www.petplace.com/cats/dehydration-in-cats/page1.aspx