Diabetes mellitus can lead to a secondary myocarditis (malfunction of the heart muscles). This can eventually cause heart failure. The signs that this is happening include a decrease in activity or weakness associated with normal activities, difficulty breathing or increased respiratory rate, decrease in appetite and sometimes pain or paralysis of the rear legs. Unfortunately, most of these signs can also occur for other reasons, including other complications of diabetes, like diabetic neuropathy leading to hind limb weakness or decrease in appetite associated with a loss of control of insulin regulation. With diabetes, it is important to work very closely with your vet to monitor the treatment. Teamwork makes a huge difference in the successful maintenance of a dog or cat with diabetes.