Illness places the body under stress, which can raise blood glucose levels. It also goes to work trying to rid itself of the problem; this causes some hormonal release of some of the same counter-regulatory hormones the body releases when it feels it's being threatened with hypoglycemia.  This can make it difficult to keep in good control using the same amount of insulin when there's no illness. Humans with diabetes have "sick day" plans; on those days, more insulin, more blood glucose testing and checking for ketones may be needed.
When you see the word "occult"  in medical reports, abstracts and articles, it has nothing to do with the paranormal. It describes anything which is hidden and therefore difficult to spot under normal conditions.  For example, a widely-used colon cancer test for people looks for occult blood in stool. This can be an early sign of colon cancer in humans, but without the test the sign may not be noticed until later stages. Similarly, having occult blood in the urine (not visible, but may be detected by testing) can mean a urinary tract infection.
Infections and diseases can also be referred to as 'subclinical'; this means the condition is present but there are no easily discerned signs or symptoms of it. Using urinary tract infections as an example, it is possible for one to be present, but not evoke the traditional signs which tell us it's there. A traditional urinalysis might not turn it up, but doing futher testing and performing urine culture tests may be the only way to spot it. 
- ↑ What Makes My Blood Glucose Rise?. Abbott Diabetes UK.
- ↑ Hypoglycemia in Dogs. Petplace.com.
- ↑ Hypoglycemia. Cornell University.
- ↑ A.G. Pöppl, F. Müller, L. Queiroga, I. Oliveira (2009). Insulin Resistance Due to Periodontal Disease in an Old Diabetic Female Poodle. WSAVA.
- ↑ Brooks, Wendy C.. Diabetes Mellitus Center. Veterinary Partner.
- ↑ Forrester SD, Troy GC, Dalton MN, Huffman JW, Holtzman G. (1999). Retrospective Evaluation of Urinary Tract Infection in 42 Dogs with Hyperadrenocorticism (Cushing's Disease) or Diabetes Mellitus or Both. Journal of Veterinary Internal Medicine.
- ↑ Schermerhorn, Thomas (2001). Persistent Hyperglycemia in Dogs and Cats-pages 10-11. Standards of Care-Compendium.
- ↑ When You're Sick. American Diabetes Association.
- ↑ Occult UTI and Urine Culture. Antech Diagnostics.
- ↑ McGuire NC, Schulman R, Ridgway MD, Bollero G. (2002). Detection of Occult Urinary Tract Infections in Dogs With Diabetes Mellitus. Journal of the American Animal Hospital Association.
- ↑ Understanding Urine Culture. Lab Tests Online.
- ↑ Understanding Susceptibility. Lab Tests Online.
- Detection of occult urinary tract infections in dogs with diabetes mellitus McGuire NC, Schulman R, Ridgway MD, Bollero G., 2002, Journal-American Veterinary Medical Association
- Cultures and Sensitivity Testing for Infection Pet Education
- Cultures and Sensitivity Tests-What, When, Why Pet Education
- Vetsulin-Insulin Requirement Changes in Dogs Intervet
- Vetsulin-Insulin Control-Insulin Resistance Intervet
- Infectious Diseases and the Kidney Goldstein, Richard E., 2010, WSAVA