Even though it seems to be a human disease, it is quite common in dogs and cats.
Diabetes Mellitus is a disease where the body cannot produce or use insulin to transport sugar (glucose) into the cells for energy.
Untreated it can lead to: cataracts (clouding of the lens of the eye), urinary tract infections, pancreatic disease, renal disease, Cushing disease, anemia and thyroid disease. Any animal could develop diabetes at any time but we see it mostly between 7 – 9 year of age. Other factors that can increase the risk of DM are: obesity, NOT spaying  your dog and a genetic predisposition. (Poodle, Westie, Alaskan Malamute an Persian cats.)
How would you know if your pet might have diabetes? Excessive drinking (polyipsia), excessive urinating (polyuria), excessive appetite but losing weight and lethargy.
Your vet will ask you to bring a urine sample and if there is glucose in the urine, proceed with blood tests. Then insulin treatment will be started. It is possible that your pet will need insulin for the rest of its life. A strict diet an insulin injections daily or twice daily, regular blood checks and urine tests, will ensure that your pet can lead a fairly normal life.
If you have pet insurance they will cover the DM as a chronic condition.