When to see a vet urgently
A suspected kidney infection needs prompt assessment. Contact an emergency vet immediately if:
- your cat collapses, responds poorly or cannot get up;
- repeated vomiting is accompanied by marked weakness or dehydration;
- your cat tries to urinate but passes no urine;
- breathing becomes difficult or your cat's overall condition deteriorates dramatically.
The absence of fever is no reason to wait with these signs: find a vet near you.
Overview
Pyelonephritis develops when bacteria reach the kidneys by travelling up from the lower urinary tract, such as from cystitis or urethritis, or through the bloodstream from another site of infection. The infection damages the renal pelvis and the tissue between the kidney's functional structures, reducing its ability to filter the blood.
In cats, the disease occurs on its own or as a complication of urinary stones, chronic cystitis or diabetes.
Despite its name, the problem is not confined to the lining of the renal pelvis: infection can extend into the kidney tissue. It can cause acute kidney injury or worsen existing chronic kidney disease. In some cases, no symptomatic bladder infection has been noticed before the kidney infection is diagnosed.
However, raised kidney blood values do not always result from a bacterial infection. Other kidney diseases or an obstruction to urine flow can also impair kidney function. Bacterial findings, symptoms, blood tests and imaging are therefore assessed together. Pyelonephritis cannot be diagnosed from raised creatinine alone.
Causes
- an ascending bacterial infection, most often E. coli and less often Proteus, Klebsiella or Staphylococcus;
- urinary stones, which obstruct urine flow and encourage infection;
- obstruction of the ureters;
- diabetes and other hormonal diseases;
- chronic bladder inflammation;
- reduced immunity and stress.
Bladder inflammation does not always mean a bacterial infection. Feline idiopathic cystitis, for example, is a different disease. Stress-related urinary signs alone do not show that an infection has travelled up to the kidneys. Stress or a general reduction in immunity in the list above cannot therefore be used to establish the diagnosis. Tests are used to look for factors that encourage infection.
Risk factors
- older cats;
- cats with chronic bladder inflammation;
- cats with diabetes or other hormonal diseases;
- cats with urinary stones;
- female cats, whose short, wide urethra makes ascending infection easier.
Bacterial urinary tract infections are diagnosed more often in older cats and females, but other cats can also develop pyelonephritis. Diabetes or stones do not automatically mean that a cat has a kidney infection. An underlying disease helps guide investigations and may affect how treatment is given. Detailed evidence on the importance of each individual predisposing factor in cats is limited.
Typical symptoms
- fever or a raised body temperature;
- tenderness when the lower back is examined by touch;
- increased urination, known as polyuria, and thirst, known as polydipsia;
- strong-smelling urine, sometimes containing blood or pus;
- vomiting and refusing food;
- lethargy and listlessness;
- in chronic cases, gradual wasting, a dull coat and weight loss.
Fever and pain in the abdomen or kidney area can occur, but they are not common or easy to detect in every patient. The disease may mainly cause a reduced appetite, vomiting or subdued behaviour. Cats may also hide discomfort. A normal temperature or an apparently painless back therefore does not rule out a kidney infection.
Urine smell alone cannot tell you whether there is a bacterial infection or whether the problem is in the kidneys. Increased drinking and urination can also accompany other diseases. In severe acute kidney injury, a cat may instead pass little or no urine. Tell the vet about reduced urine output as well as increased thirst.
Complications and consequences
- chronic kidney failure;
- sepsis, as infection spreads into the bloodstream;
- enlargement of the renal pelvis due to an obstruction;
- death in severe untreated cases.
If pus accumulates behind an obstruction, the condition is called pyonephrosis. This may require an urgent procedure because antibiotics alone cannot correct an obstruction to urine flow. Sepsis is a serious disturbance affecting the whole body in response to infection. The cat's overall condition and circulation are central to its assessment.
Diagnosis at the vet
- routine urine testing for white blood cells, bacteria, protein and casts;
- a urine culture to identify the organism and determine its antibiotic susceptibility;
- a complete blood count and blood biochemistry, looking for raised creatinine and urea and changes associated with inflammation;
- kidney ultrasound to look for enlargement of the renal pelvis, signs of inflammation and stones;
- X-rays if urinary stones are suspected;
- ruling out other causes, such as tumours and glomerulonephritis.
For a urine culture, a sterile sample is usually collected directly from the bladder with a needle, a procedure called cystocentesis. Where safe, the sample is taken before antibiotics are started. However, essential treatment for a seriously ill cat is not delayed simply to collect a sample or wait for results. Tell the vet about previous antibiotics and when they were given.
A positive culture from bladder urine supports the presence of bacteria but does not, on its own, locate the infection in the kidneys. Bacteria in the urine without symptoms must be distinguished from an infection that needs treatment. Sometimes bacteria are present in the upper urinary tract even though the bladder sample is negative. In particular circumstances, the vet may consider taking a sample from the renal pelvis; this is not done routinely in every cat.
Ultrasound is used to assess the kidneys, renal pelvises, ureters and any stones. However, enlargement of the renal pelvis is a non-specific finding: it can occur in conditions other than bacterial infection. Blood tests may also show no inflammatory changes. A normal or abnormal result from one test therefore does not settle the whole diagnosis.
The inflammatory marker SAA, or serum amyloid A, can sometimes provide additional information. In a study in cats, a raised concentration supported suspicion of pyelonephritis but was not entirely specific to the disease. It therefore does not replace urine culture, imaging and a clinical examination, and is not automatically included in every set of tests.
Treatment
- antibiotics chosen according to urine culture results, with the course and follow-up agreed by the vet in line with guidance such as the 2019 ISCAID recommendations;
- fluid therapy to correct dehydration and the effects of toxin build-up;
- pain relief and anti-inflammatory medicines;
- a special diet to support the kidneys;
- treatment of the underlying disease, such as urinary stones, diabetes or an obstruction;
- hospitalisation and intensive care in severe cases.
Treatment is chosen according to the overall situation. The recommended antibiotic course is based on guidance supported by limited direct research evidence in animal patients. Its length and the follow-up checks are agreed individually. Do not shorten the course when your cat seems brighter or extend it using an old prescription without an assessment.
For a kidney infection, the medicine must work in kidney tissue. A high concentration in bladder urine alone is therefore not a sufficient reason to choose it. Culture susceptibility results are interpreted according to the location of the infection. If serious clinical suspicion requires treatment before results are available, the plan is reviewed when they arrive. Antibiotics are nevertheless not justified for every cat with abnormal kidney blood values.
Pain is relieved in a way that suits the patient's condition. The anti-inflammatory medicine listed above is not an automatic choice for a dehydrated cat or one with acute kidney injury. Fluid therapy requires assessment of both need and response: giving large amounts of fluid without monitoring is not a safe way to flush the kidneys. The vet also monitors urine output and the risk of fluid overload.
A kidney diet is not automatically part of treatment for every acute case of pyelonephritis. The need for one is assessed according to any confirmed chronic kidney disease and the rest of the cat's condition. Early on, adequate energy intake and controlling nausea are essential. Prolonged failure to eat also puts cats at risk of fatty liver disease, so report difficulty eating promptly.
What to monitor at home
Record appetite, drinking, urination, vomiting and general activity. Tell the vet if your cat cannot keep the medicine down, you cannot give it or symptoms worsen. If there is no response, possible explanations include an obstruction, an incorrect initial diagnosis or another factor preventing recovery. A medicine is not changed simply because the cat does not become brighter immediately.
Follow-up assesses how the cat is doing and, where necessary, kidney blood values, a urine sample and culture. If the cat has recovered clinically, the significance of any remaining bacteria is considered separately. A positive result alone does not automatically mean another antibiotic course is needed. Permanent kidney damage may need monitoring even after the infection is controlled.
Prognosis
The outlook depends on previous kidney function, any obstruction, the severity of the infection and the response to treatment. Creatinine raised by an acute infection does not, on its own, show how much kidney function will remain permanently. Assessment of chronic kidney disease becomes clearer once the cat's condition and fluid balance have stabilised. Some cats need long-term treatment.
Prevention
- Have bladder inflammation and urinary tract infections treated promptly.
- Have urinary stones treated and monitored.
- Take older cats for regular health checks.
- Encourage adequate drinking with options such as water fountains and wet food.
- Monitor blood glucose in cats with diabetes.
Prevention does not mean treating every bacterial finding in a cat without symptoms. Most cases of asymptomatic bacteriuria are not treated with antibiotics. Any exceptions are assessed individually. However, a new deterioration in general health, difficulty urinating or increased thirst warrants investigation for an infection or another disease.
Important for cat owners
Pyelonephritis is a serious disease that can lead to kidney failure and sepsis if untreated. Remember that frequent urination, thirst, lower back pain and fever are warning signs that require a veterinary visit. Starting antibiotics and supportive care early substantially improves the chances of recovery.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Can a cat have pyelonephritis without fever?
Yes. Fever and pain around the kidneys do not occur in every cat. Subdued behaviour, loss of appetite or vomiting may be the only signs you notice, so suspected cases need investigation.
Does an enlarged renal pelvis on ultrasound confirm the diagnosis?
No. Enlargement can also occur with other kidney and urinary tract problems. The finding is considered alongside symptoms, kidney blood values, urine testing and, where necessary, culture.
Does a cat need a kidney diet after the infection?
Not automatically. The diet is chosen according to kidney function, any chronic disease and nutritional status. Not eating is not acceptable simply because the food on offer is labelled as a kidney diet.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Pyelonephritis in Small Animals, Merck Veterinary Manual, accessed 2026-11-03.
- ISCAID guidelines for the diagnosis and management of bacterial urinary tract infections in dogs and cats, ISCAID / The Veterinary Journal, accessed 2026-11-03.
- 2025 iCatCare Consensus Guidelines on the Diagnosis and Management of Lower Urinary Tract Diseases in Cats, iCatCare / Journal of Feline Medicine and Surgery, accessed 2026-11-03.
- 2026 iCatCare Consensus Guidelines on the Diagnosis and Management of Chronic Kidney Disease in Cats, iCatCare / Journal of Feline Medicine and Surgery, accessed 2026-11-03.
- Chronic Kidney Disease, Cornell Feline Health Centre, accessed 2026-11-03.
- Hepatic Lipidosis, Cornell Feline Health Centre, accessed 2026-11-03.
- Initial Triage and Resuscitation of Small Animal Emergency Patients, Merck Veterinary Manual, accessed 2026-11-03.
- Diagnostic Testing, AAFP / AAHA, accessed 2026-11-03.
- Usefulness of serum amyloid A for the diagnosis of pyelonephritis in cats: A prospective evaluation, Journal of Veterinary Internal Medicine, accessed 2026-11-03.