Short answer: Cat starves to death, painfully, eating lots but peeing out all the goodness, until it gets dehydrated and dies of either that, or Diabetic Ketoacidosis.
If you're willing to step up and be a responsible human though, it's very treatable and can totally restore your cat to normal. You'll need a change of diet, twice-daily insulin injections, and a glucose meter from your local drugstore. If you do it right, there's even a 75% or so chance of getting the cat off insulin after 2 or 3 months.
See http://www.felinediabetes.com/ for more.
Endocrinologists treat diabetes.
diabetes symptons in house cats
helping it from diabetes
Diabetes in cats is not typically painful, but it can lead to other health issues if not managed properly. Symptoms may include increased thirst, urination, and weight loss. Regular veterinary care and monitoring are important for managing diabetes in cats.
There are various ways to treat diabetes. One way to treat diabetes is by eating properly and following your doctor's orders. You should also exercise regularly. If you take insulin, it is also important to monitor your glucose levels.
Dogs will develop diabetes more often then cats. As with humans getting diabetes all depends on predisposing factors such as obesity, hereditary factors and illness.
with insulin
Diabetes
with antihypertensives and antidiabetics
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There is no cure for Diabetes yet. However, diabetes can be treated. There are a couple different ways to treat diabetes, but the most common way is to count out your insulin-to-carb ratio, and inject yourself with the proper amount of lantus and humalog or novolog (insulin). Ask your local endocrinologist for how to specifically treat diabetes and how to count out your personal insulin-to-carb ratio.
Endocrinologists treat hormone disorders including diabetes.