Diabetes mellitus and the resulting changes in the body's metabolism affect all systems of the body. Some systems are more capable of adapting than others. Long term complications of diabetes vary between species. In cats, these complications are infrequent, but kidney disease and disease of the nervous system (neuropathy) are the most common. Cats are much less likely than dogs and humans to develop retinal disease. 

Diabetic nephropathy

'Diabetic nephropathy' is the term used to describe changes in the kidney resulting from diabetes mellitus. The changes occur in the glomeruli, which are the filtering structures in the kidney. The changes may be mild enough such that signs of disease do not occur, and the condition can only be diagnosed by biopsy of the kidney. Diabetic nephropathy is one of the most serious complications of diabetes in people.

Diabetic neuropathy

'Diabetic neuropathy' is the term used to describe changes in the nervous system resulting from diabetes mellitus. One type of diabetic neuropathy is described as a condition in which cats walk or stand with their hocks touching the ground. This condition may occur as one of the early signs of diabetes in the cat, or it can occur later in the stage of the disease, especially if the cat is poorly regulated. 
 
 

Infections

Cats with diabetes mellitus appear more prone to develop infections. Recurrent urinary tract infections are often a signal that the cat is not properly regulated.

Hepatic lipidosis

Feline hepatic lipidosis is a condition in cats in which the liver is injured, accumulates a large amount of fat and cannot function normally. It has been associated with diabetes mellitus. It is a very serious disease, and unless treated aggressively, can be fatal.

Gastrointestinal disease

Cats with diabetes mellitus may develop certain gastrointestinal conditions including diarrhea and vomiting. A review of cases at the University of Colorado Veterinary Teaching Hospital and reported by Dr. Kelly Diehl, revealed that 30% of cats with diabetes mellitus showed some gastrointestinal disorders, and 50% of them vomit intermittently.

Other pancreatic diseases

Depending upon the contributing factors to the development of diabetes, continued injury to the pancreas can occur. An inflammation of the pancreas, pancreatitis, can be a painful condition and can come on suddenly (acute) or be a long-term (chronic) condition. Chronic pancreatitis can lead to maldigestion syndrome in which the pancreas fails to produce enough digestive enzymes. The food that is eaten is not properly broken down, the nutrients are not absorbed and the cat can virtually starve even though she is eating.

Prevention of longterm complications

The risk of longterm complications of diabetes mellitus in the cat can be reduced by strictly controlling the blood glucose level. High or widely fluctuating blood glucose levels can contribute to long term complications as well as increase the risk of diabetic ketoacidosis and hypoglycemia.
 

References
 

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Ford, SL. NIDDM in the cat: Treatment with the oral hypoglycemic medication, glipizide. In Greco, DS; Peterson, ME (eds) The Veterinary Clinics of North America Small Animal Practice: Diabetes Mellitus. WB Saunders Co. Philadelphia, PA; 1995: 599-616.

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