Copyright © 2011 by Saunders, an imprint of Elsevier Inc. All rights reserved.
Cathy E. Langston, DVM, DACVIM (Small Animal)
Pyelonephritis (Kidney Infection)
Bacterial infection of the kidney is termed pyelonephritis . Infection
may occur within kidney tissue or in the renal pelvis, the area of
the kidney where urine collects before being transported to the
In most cases, a urinary tract infection starts in the bladder and
the bacteria travel upstream to the kidney. Anything that decreases
the free flow of urine, such as obstruction of the urethra (tube
that carries urine from the bladder to the outside), bladder, ureter
(tube that carries urine from the kidney to the bladder), or kid-
ney, increases the risk that the infection will spread to the kid-
ney. The presence of stones and growths in the bladder and kidney
also increases the risk. Other contributing factors include chronic
kidney disease, diabetes mellitus, and conditions that impair the
immune system or cause dilute (watery) urine.
The most common bacterial infection in the kidney is with
Escherichia coli . Other bacteria may also be involved, and fungal
infections occur rarely.
If only one kidney is infected, no clinical signs may be noted,
and blood and urine tests may also be normal. If both kidneys are
infected, signs may be those of kidney failure, such as increased
volume of urine, increased thirst, poor appetite, vomiting, nausea,
and lethargy. Signs of a bladder infection, such as frequent urina-
tion of small volumes of urine, pain on urination, and straining,
may also be present. Blood in the urine can arise with infection in
the kidneys or the bladder.
If the infection causes acute kidney failure, decreased or no
urine production may occur, and the kidneys may become pain-
ful and swollen. Fever and a high white blood cell count may be
present, but the absence of either of these does not exclude kid-
ney infection. With chronic kidney infections, clinical signs may
be minimal or absent. Chronic infection causes damage to the
kidney; however, that results in scar tissue and shrinkage of the
Initial diagnostic tests typically include blood tests to evaluate kidney
function and urine tests to evaluate urine concentrating ability.
A urine culture is performed, but it is not uncommon for culture of
urine collected from the bladder to be negative despite infection in the
kidney. Abdominal x-rays and an ultrasound may be recommended.
Although culture of a piece of kidney tissue obtained by biopsy
increases the chance of finding the infection, the invasiveness of
the procedure makes it too risky for general use (the biopsy would
need to be taken from deeper within the kidney than the average
kidney biopsy). Contrast x-ray studies, such as an excretory uro-
gram (intravenous pyelogram), are sometimes helpful. An excre-
tory urogram involves taking a series of x-rays after a dye (that
shows up white on x-rays) is given intravenously. Other tests may
be recommended to rule out diseases that cause similar clinical
signs and other causes of kidney disease.
TREATMENT AND FOLLOW-UP
Antibiotics are the mainstay of treatment of bacterial pyelonephritis
and are often chosen based on a urine culture. If the urine culture is
negative, a good antibiotic choice is one that is effective against
E. coli . Kidney infections take much longer to cure than simple blad-
der infections, so antibiotics are usually continued for 4-6 weeks. If
infection is related to a complete or partial blockage, removal of the
obstruction may be necessary to achieve resolution of the infection.
With acute infections that cause complete kidney shutdown,
improvement in kidney function may be seen within 4-7 days of
starting antibiotics. With chronic kidney infections, no improve-
ment in kidney function may occur, but decreasing the amount of
ongoing damage in the kidney is still beneficial.
Repeating a urine culture 5-7 days after starting antibiotics is
sometimes recommended to make sure the infection is resolving.
Culture is often recommended about a week after completing the
antibiotics, and again a month later. Blood tests for kidney function
are usually performed at the same time. The need for repeated abdom-
inal ultrasounds to monitor the course of the disease is variable.
Kidney infections can be serious, life-threatening conditions, but of
all the causes of complete, acute kidney shutdown, they are one of the
most treatable. About 75% of affected animals recover from an acute
infection. The success rate for chronic infections is much lower. The
rate of progression of kidney failure with chronic infections is vari-
able; however, many animals can live for years after an infection.
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