Pyelonephritis — a bacterial infection inside the kidneys — is uncommon in cats but serious when it happens. It can develop on its own or as a complication of bladder infection, kidney stones, or chronic kidney disease. Untreated, it causes permanent scarring of the kidneys and can progress to sepsis within days.
Last reviewed: May 2026
What Pyelonephritis Is
Pyelonephritis is infection of the kidney tissue itself — specifically the renal pelvis and surrounding parenchyma. It usually develops when bacteria ascend from the bladder up the ureter, or less commonly when bacteria spread through the bloodstream. Cats most at risk are those with diabetes, chronic kidney disease, immunosuppression, urinary stones, or recent urinary catheterization. The bacteria are most often E. coli, followed by Staphylococcus, Proteus, and Enterococcus.
In a healthy cat, urinary tract infection of any kind is uncommon — feline lower urinary tract disease (FLUTD) in young cats is almost always sterile inflammation. So when an older cat or a diabetic cat presents with urinary signs and fever, pyelonephritis moves to the top of the list, as outlined in the AAFP-AAHA Feline Life Stage Guidelines, 2021.
Signs Owners Notice
Early pyelonephritis can look like a bladder infection: frequent urination, straining, blood in urine, and licking the genital area. But cats with kidney involvement usually appear systemically sick — fever (above 102.5°F), lethargy, decreased appetite, vomiting, and dehydration. Increased thirst is common because infected kidneys lose concentrating ability. Pain along the back near the kidneys may cause the cat to flinch when picked up under the chest.
Why Pyelonephritis Damages Kidneys Long-Term
Untreated kidney infection causes inflammation and scarring of renal tubules. Even after the bacteria are killed, the damaged nephrons don't regenerate. Cats that survive pyelonephritis frequently progress to chronic kidney disease — about 30 to 40 percent of treated cats show measurable CKD on follow-up bloodwork within 6 to 12 months, as per the IRIS CKD Staging Guidelines, 2023.
How Vets Diagnose Pyelonephritis
The cornerstone is urine culture from a cystocentesis sample (a needle aspirate of the bladder, which avoids skin contamination). Bloodwork shows elevated white cell count, often with a left shift; kidney values (BUN, creatinine, SDMA) are often elevated; and urinalysis shows white blood cells, bacteria, and sometimes granular casts. Abdominal ultrasound is the imaging test of choice — affected kidneys may appear enlarged or asymmetric, with a dilated renal pelvis (pyelectasia) and increased echogenicity. Ureteral or kidney stones may be visible.
Treatment — Long Course of the Right Antibiotic
Pyelonephritis requires 4 to 6 weeks of antibiotics chosen by urine culture and sensitivity, not by guess. First-line empirical choices while awaiting culture include amoxicillin-clavulanate or enrofloxacin. Supportive care includes IV fluids, anti-nausea medication, and addressing any underlying issue (stones, obstructed ureter, diabetes management). Follow-up urine culture 1 week after starting therapy and 1 to 2 weeks after finishing therapy is standard — recurrent or persistent infection is common if treatment is shortened.
Cats that recover need annual monitoring: bloodwork, urinalysis, blood pressure, and ultrasound to track kidney function. Many will eventually need a kidney-supportive diet.
Prevention
There's no specific prevention beyond managing risk factors. Diabetic cats need tight glucose control. Cats with prior UTIs need urine recheck cultures. Cats with bladder stones need stone-specific diets or removal. Regular hydration (water fountain, wet food) supports kidney health overall. Senior wellness exams help catch the gradual creep of kidney disease before it becomes pyelonephritis-prone.
When to See a Vet
Suspected bladder infection plus fever or lethargy is not minor — pyelonephritis can progress to sepsis in 24 to 48 hours.
Call your vet today if:
- Frequent urination, straining, or blood in urine
- Increased thirst combined with reduced appetite
- Lethargy or hiding in a senior cat
- Diabetic cat with new urinary signs
- Vomiting alongside any urinary changes
Go to the ER immediately if:
- Fever, vomiting, and back pain together
- Cat completely stops eating for 24 hours
- Severe lethargy, collapse, or unresponsiveness
- Decreased urine output despite ongoing drinking
- Pale gums, weakness, or rapid breathing
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Frequently Asked Questions
How is pyelonephritis different from a regular UTI?
A regular UTI is confined to the bladder; pyelonephritis is infection inside the kidney. UTIs cause urinary signs without systemic illness; pyelonephritis adds fever, vomiting, dehydration, and elevated kidney values. Treatment is also longer — UTIs typically need 7 to 14 days of antibiotics while pyelonephritis needs 4 to 6 weeks. Diagnosis distinction matters because treatment length and prognosis differ significantly.
How much does treating pyelonephritis cost?
Outpatient treatment with culture, oral antibiotics, and rechecks typically runs $400 to $900. Hospitalized cats needing IV fluids and monitoring add $1,500 to $3,500 for 2 to 4 days. Complex cases with ureteral stones or sepsis can exceed $5,000. Long-term kidney monitoring adds about $150 to $300 per recheck visit.
Can pyelonephritis go away on its own?
No. Without targeted antibiotics, pyelonephritis either progresses to sepsis or becomes chronic, gradually destroying kidney function. Even mild-appearing cases need 4 to 6 weeks of culture-directed antibiotics. Stopping antibiotics early because the cat looks better is the most common reason for recurrence.
Are some cats more prone to kidney infections?
Yes. Cats with diabetes, chronic kidney disease, hyperthyroidism (especially methimazole-treated), urinary stones, congenital kidney abnormalities, or recent urinary catheterization are all at increased risk. Female cats are slightly more prone than males because of shorter urethras. Senior cats over 10 are higher risk than young cats.
What's the prognosis after pyelonephritis?
With appropriate treatment, most cats clear the infection and live for years. However, residual kidney damage is common — 30 to 40 percent develop or worsen CKD over the following year. Long-term outlook depends more on the residual kidney function than on the infection itself. Annual senior bloodwork and urinalysis are essential after a kidney infection.
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