Making the diagnosis
Clinical diagnosis of pyelonephritis is often presumptive - based on results from CBC, serum chemistry profile, urinalysis,
urine culture and imaging procedures.
A definitive diagnosis is not usually required for planning treatment. Because many dogs lack specific signs attributable
to pyelonephritis, any dog with urinary tract infection could potentially have pyelonephritis.
Always consider the possibility of pyelonephritis as a differential diagnosis for any dog with fever of unknown origin, polydipsia/polyuria,
chronic renal failure and/or lumbar/abdominal pain. The CBC results are often normal with chronic pyelonephritis, but leukocytosis
and neutrophilia with a left shift may be detected in some dogs. The serum chemistry profile is usually normal unless chronic
pyelonephritis is contributing to chronic renal failure (azotemia with an inappropriate urinary specific gravity). The urinalysis
may reveal hematuria, pyuria, proteinuria, bacteriuria and leukocyte casts. Leukocyte casts are diagnostic for renal inflammation
and usually result from pyelonephritis.
Remember that dilute urine specific gravity in dogs with nephrogenic diabetes insipidus may occur secondary to pyelonephritis
and absence of abnormalities does not rule out pyelonephritis.
Dogs with chronic pyelonephritis may have a negative urine culture and require multiple urine cultures to confirm urinary
tract infection. The new IndicatoRx device (IDEXX) is excellent for obtaining immediate bacterial urine culture results –
results are obtained overnight and done in-hospital. Small numbers of bacteria may be recovered with this system.
Ultrasonography and excretory urography are the preferred imaging procedures done for presumptively differentiating between
upper and lower urinary tract infection.
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. Excretory urography may show dilation and blunting of
the renal pelvis with lack of filling of the collecting diverticula, and dilation of the proximal ureter. In dogs with acute
pyelonephritis, the kidneys may be large; in dogs with chronic pyelonephritis, the kidneys may be small with an irregular
surface contour. Concomitant nephroliths may be seen in some dogs evaluated by survey radiography, ultrasonography or excretory
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 and immediately transferred
to the IndicatoRx device. To confirm the diagnosis the biopsy specimen should include the renal cortex and medulla. Recurrent
pyelonephritis may be asymptomatic. Unresolved chronic pyelonephritis may lead to chronic renal failure, and diagnostic follow-up,
therefore, is important to document resolution of the pyelonephritis. In dogs with nephroliths, resolution is unlikely unless
the nephroliths are removed.
One should preferentially base the antibiotic selection on urine culture and sensitivity testing. Antibiotics used should
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;
therefore, chronic pyelonephritis may be difficult to cure.
Do give orally administered antibiotics at full therapeutic dosages for at least six weeks. Do not use aminoglycosides unless
no other alternatives exist on the basis of urine culture and sensitivity testing. The trimethoprim/sulfa combinations can
cause significant side effects (keratoconjunctivitis sicca, blood dyscrasias, polyarthritis) when administered for more than
four weeks. Do urine cultures and urinalysis during antibiotic administration (approximately one-week into treatment) and
one and four weeks after antibiotics are finished.
Dogs with pyelonephritis will usually return to normal health unless the dog also has nephrolithiasis, chronic renal failure
or some other underlying cause for urinary tract infection. Established infection of the renal medulla may be difficult to
resolve because of poor tissue penetration of antibiotics.