Canine DM on the NAVLE almost always comes down to three things: knowing your breeds, picking the right insulin, and not getting fooled by the Somogyi effect. The exam loves this topic because there are so many distinct testable facts — glucose thresholds, syringe matching, DKA electrolytes, and the intact female angle. Get these locked in and you will not lose easy points.
Pathophysiology: Why Dogs Are Different from Cats
Dogs almost exclusively get Type 1 (insulin-deficient) diabetes. Beta cells are destroyed — by immune-mediated processes, chronic pancreatitis, or idiopathic atrophy — and they do not come back. This is fundamentally different from cats, where Type 2 insulin resistance predominates and remission is actually possible. In dogs, remission is rare, insulin is lifelong, and oral hypoglycemics do nothing.
The one reversible exception worth knowing: diestrus diabetes in intact females. Progesterone drives growth hormone secretion from mammary tissue during diestrus. That GH causes insulin resistance and, eventually, beta cell exhaustion. OHE early in the course can reverse the diabetes if beta cells have not been permanently damaged yet. This is why the intact-female-with-DM question almost always has "perform OHE" as part of the correct answer.
Breed Predisposition
The Samoyed carries the highest risk — roughly 35 times baseline. Other high-risk breeds: Australian Terrier, Cairn Terrier, Yorkshire Terrier, Keeshond, Miniature Schnauzer, Miniature and Toy Poodle, Bichon Frise. Terriers, Toys, and Nordic breeds are all overrepresented.
One unique case: the Keeshond. These dogs can develop juvenile-onset diabetes at 2–6 months of age from congenital beta cell aplasia. If you see a young Keeshond with PU/PD on a board question, think inherited diabetes — not Cushing's, not another endocrinopathy.
Sex matters too. Intact females are roughly twice as likely to develop diabetes as males, driven entirely by that progesterone-GH axis during diestrus. Middle-aged to older dogs (peak 7–9 years) are most commonly affected.
Clinical Signs
The four P's: polyuria, polydipsia, polyphagia, and progressive weight loss. The mechanism is straightforward — glucose exceeds the renal threshold (~180–200 mg/dL in dogs), osmotic diuresis kicks in, and the dog compensates with polydipsia. Despite high circulating glucose, cells cannot take it up without insulin, so they signal starvation — hence polyphagia alongside muscle and fat catabolism.
About 25% of diabetic dogs actually present with decreased appetite rather than polyphagia, especially when concurrent pancreatitis or DKA is present. Do not let an anorexic dog knock DM off your differential.
Cataracts are nearly universal in canine DM — 50% within 6 months, 80% within 16 months of diagnosis. The mechanism: glucose enters the lens freely and aldose reductase converts it to sorbitol. Sorbitol cannot exit the lens. Osmotic water influx swells and disrupts lens fibers rapidly. This happens regardless of how well-controlled the diabetes is — tell owners upfront.
Diagnosis
You need two things to diagnose DM: persistent fasting hyperglycemia AND glucosuria, in a dog with compatible clinical signs. One elevated glucose is not enough — stress hyperglycemia can transiently push glucose above 200 mg/dL in dogs.
Serum fructosamine solves the stress hyperglycemia problem. It reflects average blood glucose over the prior 2–3 weeks and is unaffected by acute stress. Normal fructosamine with high blood glucose = stress. Elevated fructosamine (≥ 400 μmol/L) = true persistent hyperglycemia = diabetes.
Minimum database for a new diabetic dog: CBC, chemistry panel, urinalysis with culture (UTIs are common and can destabilize control), fructosamine, Spec cPL (pancreatitis is a major concurrent finding), and consider a cortisol or LDDS test if Cushing's is on the differential.
Insulin Selection
This is one of the most heavily tested sections. Know the insulins, their concentrations, and critically — which syringe to use.
| Insulin | Concentration | Starting Dose | Duration | Key Points |
|---|---|---|---|---|
| Vetsulin / Caninsulin (porcine lente) | U-40 | 0.25–0.5 U/kg q12h | 10–14 hr | FDA-approved for dogs; identical amino acid sequence to canine insulin; shake vigorously before use; U-40 syringes only |
| NPH (Humulin N / Novolin N) | U-100 | 0.25–0.5 U/kg q12h | 6–12 hr | Intermediate-acting; less expensive; shorter duration often requires strict q12h; roll gently to mix; U-100 syringes |
| Glargine (Lantus) | U-100 | 0.25–0.5 U/kg q12–24h | 12–24 hr | Long-acting analog; peakless in humans but variable in dogs; do NOT shake; U-100 syringes |
| Detemir (Levemir) | U-100 | 0.1 U/kg q12h | 12–24 hr | More potent than other insulins — lower starting dose is critical; often used for dogs poorly regulated on Vetsulin or NPH; U-100 syringes |
First-line in most dogs: Vetsulin at 0.25–0.5 U/kg SC q12h with meals. It is FDA-approved, matches canine insulin structure, and has predictable pharmacokinetics. NPH is a valid, less expensive alternative.
Blood Glucose Curve Interpretation
The glucose curve is how you assess whether your insulin is actually working. Measure blood glucose every 2 hours for 10–12 hours after the morning injection and feeding. What you are looking for:
| Timepoint | Expected Finding | Target / Concern |
|---|---|---|
| Hour 0 (pre-dose) | Baseline glucose | < 250–300 mg/dL ideal |
| Hours 2–4 | Declining glucose | Should be clearly falling |
| Hours 4–8 (nadir) | Lowest glucose of curve | 80–150 mg/dL — <80 = overdose; >150 = underdose |
| Hours 8–10 | Rising glucose | Gradual rise back toward pre-dose value |
| Hour 12 (pre-next dose) | Back near baseline | < 250–300 mg/dL — if very high, duration too short |
The Somogyi effect is the classic trap. If the nadir drops below about 80 mg/dL, the body counterregulates — glucagon, epinephrine, and cortisol all dump glucose back in. The resulting rebound hyperglycemia looks identical to inadequate dosing. If you increase the dose based on that high pre-dose number, you make the hypoglycemia worse and the rebound more severe. The fix: reduce the dose, not increase it.
Diabetic Ketoacidosis
DKA happens when absolute insulin deficiency is severe enough that fat metabolism goes completely uncontrolled. The liver produces ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) faster than tissues can clear them. The result: metabolic acidosis, severe dehydration, and electrolyte disaster.
Classic DKA presentation: vomiting, anorexia, lethargy, sweet or fruity breath, sometimes tachypnea (Kussmaul breathing as respiratory compensation for the acidosis). Blood glucose is often above 500 mg/dL. Ketonuria confirms it; blood ketones > 3.8 mmol/L are diagnostic.
The electrolyte piece is critical on the exam. Total body potassium is depleted in DKA — always. Vomiting, osmotic diuresis, and poor intake all contribute. Serum K+ may look normal or even high initially because acidosis pushes K+ out of cells, but as you correct the acidosis and give insulin, K+ slams intracellularly and serum levels can crash. Supplement potassium based on serum levels and monitor closely.
DKA treatment sequence: IV fluids (0.9% NaCl to restore volume, correct dehydration over 12–24 hours), regular insulin by CRI at 2.2 U/kg/day or hourly IM injections, potassium supplementation, and dextrose added to fluids when glucose drops below 250 mg/dL — you need to keep giving insulin to clear ketones even as glucose normalizes. Always identify the trigger: pancreatitis, UTI, pneumonia, hyperadrenocorticism.
Cataracts and Lens-Induced Uveitis
Diabetic cataracts in dogs develop fast. The aldose reductase pathway converts excess lens glucose to sorbitol, sorbitol accumulates (it cannot exit the lens), and osmotic swelling disrupts lens fibers — sometimes within weeks. Glycemic control does not meaningfully slow this process: 80% of diabetic dogs have cataracts within 16 months regardless of how well-managed they are.
As cataracts mature, lens proteins leak through the capsule and cause phacolytic uveitis. If the capsule ruptures (phacoclastic uveitis), the inflammation is severe and can lead to secondary glaucoma, lens luxation, and retinal detachment. Start topical NSAIDs as soon as cataracts are detected — avoid topical steroids in diabetic dogs since they dysregulate glucose. Refer to ophthalmology early: phacoemulsification has a 90–95% success rate when done before significant uveitis develops.