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The Doctor is: IN…..

                                                                                            by David K. Qualls, D.V.M.

Why is my dog drinking so much water?

In the day to day world of veterinary medicine, excessive thirst (polydypsia or “PD“) and excessive urination (polyuria or “PU“) are common complaints we receive from pet owners. When a household companion begins to consume water in amounts that lead to the loss of housebreaking capabilities, it may translate into a true life or death decision for the pet owner.

In absence of the typical symptoms of a common urinary tract infection; i.e. visible blood, a sense of urgency and discomfort, determining whether PD/PU is factually present, and it’s origin, may necessitate the evaluation of several organ systems. While board certified internists may pursue a differential list of twenty or more potential causes of PD/PU, most primary care small animal practitioners will want to first rule out the few but most commonly encountered medical conditions responsible for most complaints of PD/PU.

The first step is determining that PD/PU is actually present. Owners commonly complain that their pet is consuming excessive amounts of water when the pet may be merely drinking sufficient water to meet it’s daily needs, i.e. what is required to maintain hydration in the face of water loss through metabolism, respiration, and excretion of normal volumes of urine and feces. While we may need to measure daily water intake for accurate diagnosis of some diseases, a more simple method to determine if excessive water is truly being consumed is to measure the specific gravity of the pet’s urine.

Because distilled water contains no solutes (dissolved solids), it has been established as  the standard for measuring specific gravity, and has been designated as having a specific gravity of 1.000. Specific gravity is measured and compared to distilled water to help us compare the amount of solutes contained in the liquid of question. Blood plasma usually has a specific gravity of about 1.020.  A dog’s urine, concentrated under what we would consider to be situations of normal activity and hydration usually has a specific gravity ranging from about 1.020 to 1.040. Urine specific gravity remains at or above the specific gravity of  plasma, as the body attempts to conserve fluid while maintaining the ability to concentrate and excrete the waste products of daily metabolism via urine production.

While urine specific gravity may vary slightly from day to day, according to diet, activity and ambient temperatures, it should remain somewhere in this range in a normal, active and healthy dog. When urine specific gravity exceeds 1.040, physiological dehydration is present. When urine specific gravity remains below 1.020, the urine is not concentrated and there is evidence that excessive water intake has occurred. Once PD/PU has been documented by testing urine specific gravity, (sometimes on repeated evaluations), we need to determine the most likely cause of this symptom. Now let us review some of  the most commonly encountered disease conditions responsible for PD/PU.

In pets over eight years of age, “chronic” renal failure is probably the most commonly diagnosed cause of documented PD/PU. Chronic renal failure refers to the gradual, and what is usually considered to be normal, loss of the kidney’s efficiency in the task of filtering the blood of wastes which are then excreted through the urine. Chronic renal failure is usually very gradual in onset, and it occurs naturally with ageing, as opposed to acute renal failure which may occur in response to a toxin or an infectious agent. Renal failure also refers to a decreased ability of the kidneys to concentrate urine sufficiently enough to prevent dehydration in situations of normal water consumption. Because the kidneys have significant reserve capacity, chronic renal disease (failure) is often not evident on commonly used laboratory tests until renal function has been reduced to 25 to 30 percent of the previously normal, or full capacity. This renal reserve is the reason we are able to survive following the loss or donation of a kidney. Although chronic renal disease or failure cannot be cured, other than through renal transplantation, it may be managed for variable amounts of time through dietary control and medical support.

Another commonly encountered disease condition leading to the symptoms of PD/PU is diabetes mellitus. In dogs this is usually an insulin dependent disease. Insufficient secretion of insulin by specific pancreatic cells leads to inadequate transfer of glucose from the bloodstream to the cells of the body. Feedback mechanisms sense resultant low intracellular glucose and, reasoning that blood glucose levels must also be low, stimulate the body to release more glucose into the blood, merely aggravating the already high blood glucose levels. The kidneys can effectively filter out and conserve glucose in the blood stream until the blood glucose level exceeds about 100mg/dl. Once blood glucose exceeds this level, the threshold of the filtration units in the kidneys is exceeded and glucose begins to “spill over” into the urine. Glucose acts as an osmotic diuretic as it draws more fluid (water) from the blood flowing through the kidneys into the urine. This increases urine output, decreases urine specific gravity, leads to net water loss, and necessitates increased water intake if dehydration is to be avoided. Diabetes mellitus is diagnosed by the presence of high glucose levels in the urine and the blood. Diabetes mellitus can frequently be controlled by the administration of insulin and also by dietary modification.

The next most common cause of PD/PU is Cushing’s disease or hyperadrenocorticism. Most people realize that corticosteroids, such as prednisone, administered to a dog or taken by a person will result in increased thirst and consequently increased urine output. Cushing’s disease is an increased production of corticosteroids by the body. This usually results in increased thirst, increased urination, and a multitude of other, but sometimes subtle symptoms associated with excessive levels of corticosteroids. Recurrent urinary tract infections, recurrent staphylococcal skin infections, thinning of the hair coat, and loss of abdominal muscular tone, leading to a “pot-bellied” appearance are also symptoms of Cushing’s disease.

Corticosteroids are essential for a multitude of metabolic functions, and as such they are produced in specific but low quantities by specialized cells of the adrenal glands. In a case of Cushing’s disease, excessive corticosteroids are produced as a result of abnormal stimulation of the adrenal glands to increase their daily production of cortisol, the body’s prevalent corticosteroid. This excessive stimulation of the adrenal glands may be the result of either a primary adrenal tumor, or a result of either a benign or a malignant tumor of the pituitary gland. The pituitary gland is located at the base of the brain, above the hard palate or “roof” of the mouth, and among other things, it is responsible for stimulating and controlling production and release of cortisol by the adrenal glands.

Cushing’s disease does require special testing for proper diagnosis. Basically, the body’s resting cortisol level is measured, then a special medication that stimulates the adrenal glands to produce and release their full capacity of cortisol is administered, and then the cortisol level is again measured. By measuring the cortisol  producing capacity of the adrenal glands, we are indirectly determining that these glands are enlarged. This is most commonly the result of excessive stimulation by a benign pituitary tumor, (pituitary dependant Cushing’s), although it may occasionally be the result of either a malignant pituitary tumor, or of a primary (frequently malignant) adrenal tumor. Most clinical cases of Cushing’s disease are the result of a benign pituitary tumor, so they can fortunately be controlled and managed by the appropriate administration of oral chemotherapeutic medications.

Pituitary dependant Cushing’s disease can usually be controlled by the administration of Mitotane (Lysodren) and periodic repeat cortisol stimulation testing. While this is not an inexpensive disease to diagnose, monitor, or to treat, if it is properly controlled, pets with pituitary dependant Cushing‘s disease may remain healthy for years. Unfortunately, adrenal dependant Cushing’s disease, or disease resulting from a malignant pituitary tumor do not carry such a favorable prognosis.      

This article has been a very simplified introduction to three specific disease conditions that may offer a reasonable explanation of why a pet’s water intake could suddenly or gradually change. If you think your dog is drinking excessive amounts of water, or it’s urination habits have changed, see your regular veterinarian for further advice.

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Dr. Qualls received his veterinary degree in 1981 from the University of Florida. He is a practicing veterinarian in Jacksonville, Florida.   As an owner and breeder of Siberian Huskies for over 30 years, he and his wife Sheila, have bred over 100 AKC Champions under the Indigo prefix, and have Owner-Handled their Siberians to 33 Best-in-Shows.

Questions for Dr. Qualls may be addressed to him at <IndigoSiberians@gmail.com>