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Feline pyelonephritis
Pyelonephritis is a bacterial infection that affects the kidney itself, not just the bladder. Bacteria usually travel up from the bladder along the ureter to the area of the kidney where urine is collected. It is a serious disease: unlike...
What is it?
Pyelonephritis is a bacterial infection that affects the kidney itself, not just the bladder.
Bacteria usually travel up from the bladder along the ureter to the area of the kidney where urine is collected.
It is a serious disease: unlike simple cystitis, it can destroy kidney tissue and leave permanent after-effects.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Fever, often the first sign, but it can be missing.
Marked depression, cat hiding and no longer reacting as usual.
Refusal of food, weight loss.
Pain in the back and sides: the cat complains or tenses up when touched in this area.
Increased thirst and urine volume.
Urine sometimes tinged with blood, sometimes with an unusual odor.
Straining to urinate if the bladder is also infected.
Vomiting and dehydration when kidney function deteriorates.
In some older cats, only a decline in fitness and weight loss attracts attention.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Bacteria, most often of digestive origin, colonize the bladder then move up to the kidney.
They can also reach the kidney through the blood during a generalized infection, but this route is rarer.
Anything that interferes with the normal flow of urine promotes infection: stone blocked in a ureter, stricture, malformation.
A weakened immune system makes it easier for bacteria to take hold.
Pre-existing chronic kidney disease.
Diabetes, which makes urine sweet and encourages the multiplication of bacteria.
Hyperthyroidism and other diseases weakening the defenses.
Advanced age.
Urinary stones, especially when they interfere with the flow of urine.
Urinary catheterization, especially after an obstruction.
Treatments reducing immune defenses, given over long periods.
Being a female: the shorter urethra makes it easier for bacteria to come up.
Transmission
Pyelonephritis is not a contagious disease: it is not transmitted from one cat to another, nor to humans.
The bacteria in question come from the flora of the cat itself, generally from its digestive tract, and ascend through the urinary tract.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Urinalysis is the central test: it shows white blood cells, bacteria and blood.
Urine culture with antibiogram is essential. It identifies the bacteria responsible and indicates which antibiotics are really effective. The sample is taken directly from the bladder with a fine needle, to avoid any contamination.
A blood test looks for inflammation and evaluates kidney function.
Ultrasound visualizes the affected kidney, often modified in its shape, and looks for an obstacle such as a stone.
Temperature is taken systematically and blood pressure is checked.
Treatment
Treatment is based on an antibiotic chosen according to the antibiogram, capable of reaching the renal tissue.
The duration of treatment is long, much longer than a bladder infection, and it must be completed even if the cat seems cured after a few days. Treatment stopped too early promotes relapse and resistant bacteria.
Pain is managed because kidney damage is painful.
Hospitalization with drip is necessary when the cat is dehydrated, vomiting or its kidneys are suffering.
If a stone blocks the flow of urine, it must be treated: otherwise the infection will return.
A control urine test is performed after treatment to check that the infection has cleared.
Never reuse leftover antibiotics from a previous treatment: poorly chosen or poorly dosed, it masks the signs without healing and distorts the tests.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Treat and check any lower urinary tract infection to prevent bacteria from coming back up.
Rigorously follow the checks in a cat with diabetes or kidney failure: they are the most exposed.
Ensure good water consumption: copious urine regularly flushes the urinary tract.
Maintain a sufficient number of clean litter boxes so that the cat can urinate without holding back.
Have known urinary stones managed.
Complete any prescribed antibiotic treatment, without stopping it as soon as it improves.
Possible complications
Progressive destruction of kidney tissue and progression to chronic kidney failure.
Acute renal failure when the attack is sudden or bilateral.
Diffusion of bacteria in the blood, with a potentially fatal state of shock.
Accumulation of pus in the kidney, serious situation which may require surgery.
High blood pressure secondary to kidney damage.
Frequent relapses when an obstacle, such as a stone, has not been treated.
When to see a veterinarian
Your cat has a fever, is hiding and is no longer eating.
He whines or tenses up when you touch him on his back or sides.
He drinks and urinates a lot more than before.
Their urine contains blood or gives off an unusual odor.
He is vomiting and appears dehydrated: consult without delay.
He is diabetic or is being monitored for his kidneys and his general condition is deteriorating, even without obvious urinary signs.
Prognosis
Pyelonephritis diagnosed early and treated with the appropriate antibiotic for the full duration prescribed is most often cured.
The prognosis is more guarded when both kidneys are affected, when the cat already suffered from kidney disease, or when the diagnosis was delayed.
Permanent kidney damage is possible: this is why monitoring of kidney function is recommended after recovery.
Frequently asked questions
What is the difference with simple cystitis?
Cystitis affects the bladder, pyelonephritis affects the kidney. The first is painful but rarely dangerous for the cat's life; the second can destroy kidney tissue and affect the general condition, with fever and depression.
Why does the treatment take so long?
Because the antibiotic must penetrate deep into the kidney tissue and remain effective there long enough to kill all the bacteria. Shortened treatment allows the most resistant to survive and the infection starts again.
Can I get this infection from caring for my cat?
No. The bacteria in question come from the flora of the cat itself. However, washing your hands after handling the litter remains a good habit in general.
Is a simple urine test enough?
It provides guidance, but it is not enough. The culture with antibiogram is what allows you to choose the right antibiotic. Without it, treatment is done haphazardly, with a risk of failure and relapse.
References
AAFP — American Association of Feline Practitioners
WSAVA — World Small Animal Veterinary Association
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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