Kidney infection in dogs – pyelonephritis symptoms and treatment

Overview

Infection extends into kidney tissue

Despite its name referring to the renal pelvis, pyelonephritis is not confined to that area but involves kidney tissue infection. It can be acute or chronic. Acute disease can cause acute kidney injury. Chronic disease may involve recurrent infections.

The precise incidence in dogs is poorly understood because confirming the disease is difficult. The absence of typical bladder infection signs does not exclude kidney infection.

In a study of 47 dogs with histologically confirmed disease, 18/47, or 38%, had no local signs suggesting urinary tract infection. The case group was weighted towards disease identified at post-mortem examination: 35/47, or 75%. These figures describe this selected patient group, rather than all dogs.

Causes

Bacteria can ascend from the bladder

Infection often begins when bladder bacteria ascend to the kidneys. It can also arise from bacteria spreading through the bloodstream. A preceding lower urinary tract infection is not always identifiable. Escherichia coli is the most common organism found on culture.

Of 129 bacterial isolates collected in studies of dogs with pyelonephritis, 70, or 54%, were E. coli. Staphylococci accounted for 10%, or 13/129 canine isolates. Proteus accounted for 7%, or 9/129. Enterococci accounted for 6%, or 8/129. This distribution concerns bacterial isolates, rather than individual dogs.

Impaired urine flow can sustain infection

Kidney stones and recurrent urinary tract infections can predispose to kidney infection. Ureteral obstruction can involve pus accumulating in the renal pelvis. This complication is called pyonephrosis. The accumulated pus can itself obstruct the ureter. Ureteral obstruction and kidney tissue infection are then investigated together.

Risk factors

Pyelonephritis can affect any dog, but older dogs and females are more frequently affected. In the study of 47 histologically confirmed cases, median age was 7.7 years. Of these dogs, 28/47, or 60%, were over 7 years old. This does not exclude young dogs from risk.

Dogs in the same study ranged from 5 months to 15 years old.

Chronic kidney disease can predispose to infection. Diabetes, Cushing's disease and reduced immune defences are also risk factors. Urinary tract structural abnormalities, such as a ureter ending in the wrong place, can increase risk. Cancer is another possible predisposing condition.

Urinary incontinence and urine flow obstructions occur in many affected dogs. Identifying predisposing diseases forms part of assessment.

Typical symptoms

Severity varies, and some dogs show only mild signs. Possible signs at home include:

• Reduced appetite.
• Greater tiredness than usual.
• Vomiting.
• Increased drinking.
• Increased urination.
• Urinary accidents indoors.
• Visible blood in urine.
• Straining during urination.
• Weight loss.

Diarrhoea can also occur with kidney infection. Fever and abdominal pain are possible but do not occur in every patient. Severe acute disease can reduce or stop urine production. Signs alone do not always distinguish kidney infection from other systemic diseases.

Concurrent lower urinary tract infection can involve needing to urinate more frequently. Urination may look painful. Urine may also smell stronger than usual.

Complications and consequences

Infection can threaten kidney function and the circulation

Pyelonephritis can cause life-threatening acute kidney injury. Bacteria can also enter the bloodstream. Pyonephrosis, or pus accumulating in an obstructed renal pelvis, generally needs an urgent procedure.

Pyelonephritis can acutely worsen existing chronic kidney disease. An infection treated late or unresponsive to treatment can leave kidney damage. Recurrent infections or those caused by resistant bacteria can worsen the outlook.

Dogs without predisposing diseases generally recover well with treatment. After acute kidney injury, recovery can take days to months. The initial creatinine level alone does not determine whether injury is reversible.

In the study of histologically confirmed cases, sepsis was identified in 7 dogs, or 15%. With acute kidney injury, a marked rise in blood potassium can predispose to heart rhythm disturbances.

Diagnosis at the vet

Culture results are considered alongside the whole clinical picture

Your vet uses blood tests to assess general condition and kidney function. Increased blood neutrophils can support suspected inflammation but are not consistently found. Increased creatinine or SDMA supports impaired function but does not establish bacteria as the cause of injury.

Urine bacterial culture and susceptibility testing are performed in suspected pyelonephritis. The sample is generally collected directly from the bladder with a needle. Clean sampling and freshness reduce the risk of misleading bacterial findings. The sample is examined and cultured as soon as possible.

A positive culture alone does not confirm kidney infection. Asymptomatic bacteriuria and kidney tissue infection need different assessment. Blood culture is also recommended for a patient with fever or impaired immunity.

Urinalysis includes specific gravity, dipstick testing and sediment examination. Urinary casts can directly suggest kidney tubular damage. Ultrasound during needle collection from the bladder can show inflammatory bladder wall changes.

Ultrasound helps find predisposing changes

Ultrasound and, where needed, X-rays look for kidney changes. Renal pelvic dilation can relate to obstruction, high urine output or chronic kidney disease as well as infection. Dilation is therefore not used as a stand-alone confirmatory test for pyelonephritis. Ultrasound can also detect urinary stones predisposing to signs.

In the study of histologically confirmed cases, 2/29, or 7%, of dogs examined by ultrasound had an apparently normal urinary system. Normal imaging therefore does not end investigation if other findings support infection.

A negative culture is assessed in context

If bladder urine culture is negative, sampling directly from the renal pelvis may be considered. However, puncturing the renal pelvis is not a routine initial test. A kidney biopsy can miss infection because it generally targets the cortex rather than where disease starts. Testing for leptospirosis using antibodies and PCR is also considered in culture-negative dogs.

In the case study, bacterial culture was positive in 22/28 tested dogs, or 79%. Of the 6 dogs with negative cultures, 4, or 67%, had previously received antibiotics. If culture grows several bacteria, interpretation considers their species and colony counts.

Treatment

The antibiotic must work in kidney tissue

Your vet selects an antibiotic using culture and susceptibility results. With strong suspicion, treatment begins while results are pending. Not every acute kidney injury therefore requires an antibiotic.

The laboratory is told that kidney tissue infection is suspected. Susceptibility is interpreted using concentrations achievable in blood and tissue, rather than urine concentration alone. A medicine suitable for cystitis may therefore be insufficient for kidney infection.

Finding an enterococcus may change medicine choice, as fluoroquinolones often have only moderate activity against it. Aminoglycoside antibiotics, which can harm the kidneys, are not recommended in suspected pyelonephritis. Treating multidrug-resistant bacteria may require specialist assessment.

Severe disease needs supportive care

A patient who is otherwise well and eating normally can receive oral medication. A dehydrated dog, or one eating poorly or feeling tired, may need intravenous treatment. Hospital care for severe kidney injury may include fluids and other supportive treatment.

Fluids are matched to urine volume and other losses. Fluid overload is avoided because injured kidneys cannot always remove excess water. Underlying diseases are treated alongside the kidney infection. Pyonephrosis may require a procedure that restores urine flow while preserving the kidney.

Checking the response

If systemic signs or laboratory findings do not improve within 72 hours despite an appropriate antibiotic, the diagnosis and predisposing causes are reassessed. A ureteral stone or tumour, for example, may underlie the problem. A follow-up examination is recommended 1–2 weeks after antibiotic treatment ends. This includes physical examination, creatinine, urinalysis and bacterial culture.

Finding the same bacterium again prompts assessment of why infection persists. However, a positive culture in a dog whose signs and blood values have recovered may represent asymptomatic bacteriuria. The result then does not automatically mean further medication is needed.

Acute kidney injury can progress from mild to severe within hours or days. Early kidney status is therefore monitored with daily tests if needed.

Nutrition and further kidney monitoring

Nutritional support is indicated if loss of appetite has lasted or is expected to last over 48 hours. Protein restriction has no demonstrated benefit during acute kidney injury. Kidney values can continue improving for several months after discharge. For a dog with acute kidney injury, kidney function checks are initially recommended at least every 3 months for 1 year. Further monitoring is indicated even after apparently complete recovery.

Prevention

Preventing recurrence targets confirmed predisposing diseases. Managing factors underlying recurrent bacterial cystitis is essential for a lasting outcome. Recurrent signs therefore prompt investigation for conditions such as stones and urinary tract structural abnormalities.

Repeated antibiotic use without investigating the underlying cause generally does not produce lasting recovery. Unnecessary antibiotic treatment can promote drug resistance. Asymptomatic bacteriuria is therefore not equated directly with a kidney infection needing treatment. Prevention and monitoring are adapted to your dog's own medical history.

In a repeat infection, a different bacterium suggests a new infection. If the previous bacterium persists, possibilities including drug resistance and urinary stones are checked.

Important for dog owners

Notice small changes too

Record drinking, urination, appetite and activity before and during treatment. Your observations and knowledge of your dog help identify mild urinary signs. Bring previous culture results and a current medication list.

Report if your dog vomits their medicine or is not eating normally. Poor appetite and dehydration affect whether intravenous treatment is needed. Also confirm when follow-up is due and which samples will be collected.

If culture finds resistant bacteria, report this before the next appointment. At home, careful hand hygiene generally suffices with an ESBL carrier, and isolation is not usually needed. Carrying a resistant bacterium alone is not the same as active infection.

Frequently asked questions

Can a kidney infection occur without fever?

Yes. Fever and abdominal pain are possible but inconsistent findings in pyelonephritis. Typical urinary problems can also be absent. Assessment therefore combines your dog's condition, urine tests and imaging. Fever is not required before necessary investigations begin.

Does a dilated renal pelvis prove infection?

No. Dilation can also occur with ureteral obstruction, increased urine output or chronic kidney disease. Ultrasound is therefore one part of the overall assessment. Signs and bacterial culture are considered too. The changes also help identify diseases predisposing your dog to infection.

Why is urine culture needed before choosing medication?

Culture and susceptibility testing form part of investigating suspected pyelonephritis. Treating kidney tissue uses susceptibility thresholds based on blood concentrations. A high urine concentration alone therefore does not show that a medicine is sufficiently effective deeper in the kidney tissue where the bacterial infection is being treated.

Can kidney infection need hospital care?

Yes. Severe infection can cause acute kidney injury and require intravenous fluids and other supportive care. Dehydration, poor appetite and tiredness are reasons to consider intravenous treatment. Care also includes investigating predisposing underlying disease.

Does a positive follow-up culture always mean another antibiotic?

Not automatically. Once signs and elevated kidney values resolve, bacterial growth may represent asymptomatic bacteriuria. Finding the same bacterium, however, can prompt investigation of persistent infection causes. The decision therefore also depends on your dog's condition and other test results.