In most dogs, the first thing I think of when I hear that they're drinking a lot of water is that they could be diabetic. Polyuria (increased urination) and polydipsia (increased drinking) are two of the hallmark signs for diabetes.
The symptoms you describe: polydipsia (excessive water drinking) of course can be due to a number of different causes. The most common causes of PU/PD in dogs include kidney disease, Cushing's disease (excessive blood cortisol levels), and diabetes mellitus (excessive blood glucose levels). In an 15 year old dog, any one of these disease processes could be occurring.
Getting her to your vet for a thorough physical examination and blood work is the best thing you can do. Try to collect a urine sample as well so that your vet can evaluate kidney function and look for signs of urinary tract infection. Since your dog is a female, allow your dog to begin urinating and when you are sure she is peeing you can slip a shallow bowl or soup ladle into the urine stream to catch your sample. Put the urine in a clean, dry container and refrigerate until your vet appointment. It's best if the sample is less than 24 hours old.
Other possible causes for PU/PD include inflammation of the prostate (rare in a neutered male), liver problems, or diabetes insipidus (a disorder of water metabolism in which part of the brain does not secrete the hormone which tells the kidneys to re-absorb water, or the kidneys do not respond to the hormone). In addition, there is a condition called psychogenic polydipsia which means the excessive water drinking is all in his head. If he exhibits other symptoms like vomiting, diarrhea or lethargy, other causes should also be investigated. Blood work often helps narrow the field.
I hope this helps!!
Frequent urination can be caused by urinary stones, Cystitis, Kidney Disease, Bladder Cancer, or Urinary Bacterial Infections. Urinary Stones are common and found in the kidneys, bladder and urethra though usually they are in the bladder. The formation of crystals or stones in urinary tract can be caused by the following factors usually working together: mineral crystals in the urine, bacteria, diseases, some medications, excess Vitamin C, imbalance in pH of urine. Typical symptoms are straining to urinate, frequent urination of small amounts and blood in the urine. Some breeds are known to have problems with stones. These are Miniature Schnauzer, Schnauzer, Lhasa Apso, Shih Tzu, Bichon Frise, Miniature poodle, and Yorkshire terrier. Cystitis is when your dog's bladder becomes inflamed. It is believed that infections, polyps, tumors and stones contribute to this condition, but the actual cause is unclear. Typical symptoms are straining to urinate, frequent urination, straining to urinate, difficulty urinating and blood in the urine. Kidney Disease is a common cause of non-accidental death in dogs. It is not clear what causes kidney disease but it is believed that dehydration, blood loss, impaired heart function, tumors, infections and toxins contribute to it. Some medications for kidney disease may cause blood in the urine. Bladder cancer occurs in middle-aged and older dogs. Bladder cancer is not as common as other causes such as Urinary tract infections, though it is not recognized early for that same reason. Typical symptoms are frequent urination, urinating small amounts, blood in the urine, and infections. Urinary bacterial infections can occur in bladder (most common), kidneys, urethra, and prostate. They are easily treated once diagnosed. They are the major cause of the prostate disease in male dogs. The prostate becomes inhabited by bacterial and it becomes difficult to eliminate. Some other conditions such as diabetes, cancer and viral diseases affect a dog's immune system and may contribute to this condition.
The best thing to do is to get your pup in to see a vet as soon as you can. Some of the possibilities for what is going on can end up being very painful or life threatening if left untreated.
If it's a bladder infection, treatment is absolutely successful.
You will need to catch a urine sample. Generally we suggest you collect the first morning's voided sample. If your appointment isn't until later in the day, you can refrigerate the sample until you go to the vet (just make sure you remember to take it out and bring it to the vet with you!).
Neutering is a very good idea. It cuts down on unwanted puppies, free ranging to find a female and makes them less hormonal. It also cuts the possibility of contracting prostate cancer.