Definition

Microbial colonization of the upper urinary tract including the renal pelvis, collecting diverticula, renal parenchyma, and ureters; because it is not usually limited to the renal pelvis and parenchyma, a more descriptive term is upper urinary tract infection; this chapter is limited to bacterial pyelonephritis.

Pathophysiology

• Infection of any portion of the urinary tract usually requires some impairment of normal host defenses against urinary tract infection (see Lower Urinary Tract Infection chapters); normal defenses against ascending urinary tract infection include mucosal defense barriers, ureteral peristalsis, ureterovesical flap valves, unidirectional flow of urine and an extensive renal blood supply. Pyelonephritis usually occurs by ascension of microbes causing lower urinary tract infection. In dogs and cats, hematogenous seeding of the kidneys does not usually cause pyelonephritis. Regardless of the route of infection, an upper urinary tract infection is frequently accompanied by lower urinary tract infection.
• Pyelonephritis can develop secondarily to infection of metabolic nephroliths. Upper urinary tract infection with urease-producing bacteria can predispose to formation of struvite nephroliths (see Urolithiasis, Struvite-Dogs).
• Obstruction of an infected kidney or ureter can rapidly cause septicemia (so-called urosepsis).

Systems Affected

• Renal/Urologic
• Can cause urosepsis, thus affecting any body system

Incidence/Prevalence

• Unknown.
• Probably occurs much more commonly than is recognized clinically, because many animals with pyelonephritis are asymptomatic or have signs limited to lower urinary tract infection.

Signalment

Signs

Causes

Usually, ascending urinary tract infection caused by aerobic bacteria; most common isolates are Escherichia coli and Staphylococcus spp.; other bacteria, including Proteus, Streptococcus, Klebsiella, Enterobacter, and Pseudomonas spp., which frequently infect the lower urinary tract, may ascend into the upper urinary tract. Anaerobic bacteria, ureaplasma, and fungi uncommonly infect the upper urinary tract.

Risk Factors

• Ectopic ureters, vesicoureteral reflux, congenital renal dysplasia, and lower urinary tract infection.
• Conditions that predispose to urinary tract infection-e.g., diabetes mellitus, hyperadrenocorticism, exogenous steroid administration, renal failure, transurethral catheterization, urine retention, uroliths, urinary tract neoplasia, perineal urethrostomy.
• In cats with experimentally induced lower urinary tract disease, indwelling urinary catheters combined with administration of exogenous steroids frequently resulted in pyelonephritis.

Differential Diagnosis

• Clinical diagnosis of pyelonephritis is usually presumptive, based on results from CBC, biochemical analysis, urinalysis, urine culture, and diagnostic imaging; definitive diagnosis is not usually required for planning treatment.
• Since many dogs and cats lack specific symptoms attributable to pyelonephritis, any patient with urinary tract infection could potentially have pyelonephritis; the best methods for differentiating between upper and lower urinary tract infection are ultrasonography or excretory urography. Remember, most patients with pyelonephritis are asymptomatic.
• Consider pyelonephritis as a rule-out in dogs or cats with fever of unknown origin, PU/PD, chronic renal failure, and/or lumbar/abdominal pain.

CBC/Biochemistry/Urinalysis

• CBC-results often normal with chronic pyelonephritis; leukocytosis and immature neutrophilia may be detected in some patients.
• Biochemistry-values usually normal unless chronic pyelonephritis leads to chronic renal failure (azotemia with an inappropriate urinary specific gravity).
• Urinalysis reveals hematuria, pyuria, proteinuria, bacteriuria, and leukocyte casts in some animals. Leukocyte casts are diagnostic for renal inflammation, but unfortunately are very uncommon. Observe dilute urine specific gravity in patients with nephrogenic diabetes insipidus, which may occur secondary to pyelonephritis. Absence of these abnormalities does not rule out pyelonephritis.

Other Laboratory Tests

• Quantitative urine culture to confirm urinary tract infection; see Lower Urinary Tract Infection chapters for interpretation.
• Dogs with chronic pyelonephritis may have a negative urine culture and require multiple urine cultures to confirm urinary tract infection.

Imaging

• Ultrasonography and excretory urography are the best methods for presumptively differentiating between upper and lower urinary tract infection. Ultrasonography is more sensitive than excretory urography for identification of mild-to-moderate acute pyelonephritis.
• Ultrasonographic findings supporting pyelonephritis include dilation of the renal pelvis and proximal ureter and a hyperechoic mucosal margin line within the renal pelvis and/or proximal ureter.
• Intravenous urography may reveal decreased opacity of the nephrogram phase of the IVU, dilation and blunting of the renal pelvis with lack of filling of the collecting diverticula, decreased opacity of contrast media in the collecting system, and dilation of the proximal ureter.
• In patients with acute pyelonephritis, the kidneys may be large; in patients with chronic pyelonephritis, the kidneys may be small, with an irregular surface contour.
• Concomitant nephroliths detected in some patients by survey radiography, ultrasonography, or excretory urography.

Diagnostic Procedures

• Definitive diagnosis requires urine cultures obtained from the renal pelvis or parenchyma, or histopathology from a renal biopsy. Pyelocentesis can be performed percutaneously using ultrasound guidance or during exploratory surgery; can obtain specimen for culture from the renal pelvis (or from nephroliths) during nephrotomy.
• To confirm the diagnosis, the biopsy specimen must include the renal cortex and medulla; thus renal biopsy should be performed by an individual very familiar with the technique of ultrasound guided renal biopsy, or by open surgery and only if necessary.
• The renal lesions may be patchy in distribution and therefore the renal biopsy sample may not be representative of the light microscopic lesions.

Pathologic Findings

• Kidneys affected by chronic pyelonephritis may have areas of infarction and scarring on the capsular surface. The renal pelvis and collecting diverticula may be dilated and distorted from chronic infection and inflammation. Purulent exudate is occasionally observed in the renal pelvis.
• Light microscopic findings include papillitis, pyelitis, interstitial nephritis, and leukocyte casts in tubular lumens.

Appropriate Health Care

Outpatient unless animal has septicemia or symptomatic renal failure.

Activity

Unlimited

Diet

Modified renal diet (e.g., Prescription Diet k/d for dogs or cats) recommended in cats or dogs with concomitant chronic renal failure or nephrolithiasis.

Client Education

• Recurrent pyelonephritis may be asymptomatic. Unresolved chronic pyelonephritis may lead to chronic renal failure; diagnostic follow-up is important to document resolution or progression of pyelonephritis.
• In patients with nephroliths, resolution is unlikely unless the nephroliths are removed.

Surgical Considerations

• Complete obstruction of the upper urinary tract of a patient with pyelonephritis may rapidly progress to septicemia and therefore should be regarded as a medical emergency. The cause of the obstruction should be corrected by surgery (or lithotripsy for nephroliths).
• Infected nephroliths-surgically remove, medically dissolve (struvite), or fragment by extracorporeal shock wave lithotripsy; use periprocedural antibiotics to reduce the risk of urosepsis when manipulating infected nephroliths.
• Unilateral nephrectomy is usually not effective for elimination of suspected unilateral pyelonephritis.

Drug(s) Of Choice

• Base antibiotic selection on urine culture and susceptibility testing.
• Antibiotics should be bactericidal, achieve good serum and urine concentrations, and not be nephrotoxic.
• High serum and urinary antibiotic concentrations do not necessarily ensure high tissue concentrations in the renal medulla; thus chronic pyelonephritis may be difficult to eradicate.
• Give orally administered antibiotics at full therapeutic dosages for 4–6 weeks.
• Do not use drugs that achieve good concentrations in urine but poor concentrations in serum (e.g., nitrofurantoin).

Contraindications

Do not use aminoglycosides unless no other alternatives exist on the basis of urine culture and susceptibility testing.

Precautions

Trimethoprim/sulfa combinations can cause side effects (keratoconjunctivitis sicca, blood dyscrasias, and polyarthritis) when administered for more than 4 weeks.

Patient Monitoring

Perform urine cultures and urinalysis during antibiotic administration (∼5–7 days into treatment) and 1 and 4 weeks after antibiotics are finished.

Prevention/Avoidance

Eliminate factors predisposing to urinary tract infection; correct ectopic ureters.

Possible Complications

Renal failure, recurrent pyelonephritis, struvite nephrolithiasis, septicemia, septic shock, metastatic infection (e.g., endocarditis, polyarthritis).

Expected Course and Prognosis

• Patients with acute or subacute pyelonephritis-fair to good, with a return to normal health unless the patient also has nephrolithiasis, chronic renal failure, or some other underlying cause for urinary tract infection (e.g., obstruction or neoplasia).
• Established chronic infection of the renal medulla may be difficult to resolve because of poor tissue penetration of antibiotics.
• Patients with chronic renal failure caused by pyelonephritis-prognosis determined by the severity and rate of progression of the chronic renal failure.
• Recurrent pyelonephritis is likely if infected nephroliths are not removed.

Associated Conditions

Hyperadrenocorticism, exogenous glucocorticoid administration, chronic renal failure, hyperthyroidism (cats), and diabetes mellitus are associated with lower urinary tract infection, which can ascend into the ureters and kidneys.

Pregnancy/Fertility/Breeding

Use antibiotics that are safe for the pregnant bitch or queen.

Synonyms

Upper urinary tract infection, pyelitis

See Also

• Lower Urinary Tract Infection chapters
• Nephrolithiasis
• Renal Failure, Chronic
• Urinary Tract Obstruction
• Urolithiasis, Struvite-Cats
• Urolithiasis, Struvite-Dogs

Abbreviations

• IVU = intravenous urogram
• PU/PD = polyuria and polydipsia

Client Education Handout Available Online