Pyelonephritis in Dogs: Kidney Infection Symptoms & Treatment

Last reviewed April 2026 · Veterinary reference article

Lower UTI vs. Pyelonephritis: A Critical Distinction

A lower urinary tract infection (UTI) affects the bladder and urethra. Pyelonephritis is a kidney infection. Both are bacterial, both can cause straining and bloody urine — but pyelonephritis is a systemic illness with the potential to permanently damage kidney function. Treating a kidney infection the same as a bladder infection (1–2 weeks of antibiotics) is one of the most common reasons pyelonephritis fails to resolve and causes lasting harm.

How Pyelonephritis Develops

The most common pathway is ascending infection: bacteria from the lower urinary tract travel up the ureters to the kidneys. In a healthy dog, the ureteral valves and regular urine flow prevent this — urine should flow one way, from kidney to bladder. When that system is disrupted — by vesicoureteral reflux, anatomical abnormalities, or sustained high bacterial counts in the bladder — bacteria can reach the kidney.

Less commonly, bacteria reach the kidney hematogenously — traveling through the bloodstream from a distant infection site (e.g., skin infection, dental disease, diskospondylitis). This route is more common in immunocompromised dogs.

Risk Factors

• Recurrent or inadequately treated lower UTIs — the most common risk factor; each infection is an opportunity for ascent
• Bladder stones — uroliths create a surface for bacterial biofilm and interfere with complete sterilization; recurrent UTIs on stones drive pyelonephritis
• Cushing's disease (hyperadrenocorticism) — excess cortisol suppresses immunity and makes dogs susceptible to UTIs that frequently ascend to the kidneys; Cushing's and pyelonephritis commonly co-occur
• Diabetes mellitus — high urine glucose promotes bacterial growth; diabetic dogs have higher rates of UTI and pyelonephritis
• Long-term corticosteroid use — immunosuppression facilitates ascending infection
• Female anatomy — shorter urethra in females makes ascending infection easier; pyelonephritis is more common in female dogs
• Anatomical abnormalities — ectopic ureters, perivulvar skin folds, vestibular abnormalities
• Urinary catheterization — particularly long-term catheters, which bypass normal protective mechanisms

Symptoms of Pyelonephritis in Dogs

Pyelonephritis can present acutely or as a more subtle, chronic condition. The classic presentation includes:

• Fever — often above 39.5°C (103.1°F); one of the most reliable signs distinguishing pyelonephritis from lower UTI
• Lethargy and weakness — often pronounced in acute cases
• Loss of appetite
• Vomiting — from systemic illness and uremic toxin accumulation if kidney function is compromised
• Polyuria and polydipsia (PU/PD) — increased thirst and urination; the infected kidneys lose concentrating ability
• Flank or lower back pain — dogs may flinch or vocalize when the kidney region is palpated; may appear hunched
• Lower UTI signs — straining, blood in urine, frequent urination — may or may not be present

Diagnosing Pyelonephritis

Urinalysis

Urinalysis should be performed on a urine sample collected by cystocentesis (direct bladder puncture) for an uncontaminated sample. Key findings:

• White blood cell (WBC) casts — the hallmark finding suggesting kidney involvement; casts form in the kidney tubules and indicate active inflammation within the kidney itself
• Pyuria (WBCs in urine) and bacteriuria (bacteria)
• Hematuria (blood)
• Proteinuria — protein leaking into urine from damaged kidney tissue
• Inappropriately low specific gravity — infected kidneys cannot concentrate urine normally

Urine Culture and Sensitivity

A urine culture is non-negotiable — treating pyelonephritis empirically (without knowing what bacteria is present and which antibiotics kill it) risks treatment failure and antibiotic resistance. The culture identifies the organism; the sensitivity panel indicates which antibiotics will be effective. Results take 48–72 hours; initial antibiotic choice is usually a broad-spectrum option that is adjusted once results are available.

Bloodwork

A full chemistry panel and CBC provide essential information. The chemistry panel assesses kidney function (creatinine, BUN, SDMA, phosphorus) and screens for underlying conditions that increase risk: elevated glucose (diabetes), elevated ALP and cortisol (Cushing's). The CBC typically shows elevated white blood cells (neutrophilia) reflecting the active infection. If kidney values are elevated, the vet must determine whether the pyelonephritis is the cause (potentially reversible with treatment) or whether underlying CKD is present.

Abdominal Ultrasound

Ultrasound assesses kidney architecture: swollen kidneys with dilation of the renal pelvis (pyelectasia) and surrounding inflammation are consistent with pyelonephritis. Ultrasound also detects concurrent problems — bladder stones, structural abnormalities, obstructions — and evaluates both kidneys to determine if infection is unilateral or bilateral. Bilateral pyelonephritis carries a higher risk of significant acute kidney dysfunction.

Treatment: Why 4–6 Weeks Matters

Bacteria embedded within kidney tissue are protected from antibiotics by the kidney's architecture, making eradication genuinely difficult. A short course creates the appearance of improvement (bacteria are suppressed but not eliminated), followed by relapse when antibiotics stop. Full 4–6 week treatment is not excessive — it is the minimum needed to reliably clear the infection.

Antibiotic Selection

The antibiotic must achieve adequate concentration within kidney tissue — not all antibiotics that work for bladder infections penetrate the kidney adequately. Culture and sensitivity results guide selection. Commonly used antibiotics for canine pyelonephritis include:

Supportive Care

Dogs with significant dehydration, elevated kidney values, or severe systemic illness are often hospitalized initially for IV fluid therapy — this restores kidney perfusion and helps flush bacterial toxins. Anti-nausea medication controls vomiting and maintains hydration. Pain management addresses kidney/flank discomfort. Once stabilized and eating, treatment continues at home with oral antibiotics.

Monitoring During and After Treatment

The treatment protocol requires structured follow-up:

1. Day 5–7: Recheck urine culture — confirms the chosen antibiotic is clearing the bacteria. If still positive, the antibiotic may need to change.
2. Day 14–21: Clinical recheck — assess improvement in systemic signs, repeat kidney bloodwork
3. End of course: Repeat kidney values and urinalysis
4. 5–7 days after completing antibiotics: Final urine culture — confirms eradication. A negative culture here means the infection is resolved.
5. Long-term: Address underlying risk factors — treat bladder stones, manage Cushing's disease or diabetes, schedule regular urinalyses

Complications of Untreated or Recurrent Pyelonephritis

Each episode of pyelonephritis leaves scar tissue where functional kidney tissue was. Over time:

• Chronic kidney disease (CKD) — progressive loss of nephron mass from repeated scarring; the most common long-term consequence
• Hydronephrosis — if infection causes enough scarring at the ureter-kidney junction, urine drainage can be obstructed, leading to the kidney swelling with urine and further damaging the tissue
• Sepsis — in severe acute cases, bacteria can enter the bloodstream and cause systemic infection (septicemia/sepsis) — a life-threatening emergency
• Renal abscess — a walled-off pocket of infection within the kidney; may require surgical drainage or, in severe cases, nephrectomy (kidney removal)

Frequently Asked Questions