SMALL ANIMAL – NEPHROLOGY AND UROLOGY
BACTERIAL URINARY TRACT INFECTIONS these pads are not reliable for ruling-in or ruling-out
bacterial UTI in dogs and cats. A urine sediment
Joseph W. Bartges, DVM, PhD examination should always be performed as part of a
Dipl. ACVIM (Small Animal Internal Medicine) & ACVN complete urinalysis. Presence of pyuria (>5 white blood
The University of Tennessee, Knoxville, TN cells/high-powered field) is important because other
causes of lower urinary tract disease are associated with
INTRODUCTION AND INCIDENCE hematuria and proteinuria but minimal pyuria.
A bacterial urinary tract infection (UTI) exists when Identification of bacteria on urine sediment examination
bacteria adhere, multiply, and persist in a portion of the is helpful; however, it should not be relied upon to rule-in
urinary tract. The infection may or may not produce or rule-out bacterial UTI. Evaluating a modified Wright’s
clinical signs. Bacterial urinary tract infections are stain urine sediment preparation is a cost-effective and
reported to occur in 2 to 3% of dogs and in less than 1% easy method that provides more reproducible and
of cats.1,2 Cats appear to be more innately resistant to accurate results than evaluating an unstained urine
bacterial UTI than dogs. The urinary tract is in contact sediment preparation.5 Urine specific gravity may be
with the external environment and has many defense normal; however, dilute urine may be a risk factor for
mechanisms to prevent bacterial UTI.3 Anatomically, the development of bacterial UTI or a consequence of an
length of the urethra, presence of high pressure zones upper urinary tract infection.
within the urethra, urethral and ureteral peristalsis, A urine culture is the most definitive means of
vesicoureteral flaps to prevent reflux of urine from the diagnosing bacterial UTI.6 Care must be taken to collect,
bladder into the ureters, and extensive renal blood preserve, and transport the urine sample to avoid
supply and flow are protective. Mucosal defense barriers contamination, or proliferation or death of bacteria. Urine
that prevent migration of bacteria and subsequent specimens for aerobic bacterial culture should be
colonization include the presence of a transported and stored in sealed, sterilized containers,
glycosaminoglycan layer, antibodies, intrinsic mucosal and processing should begin as soon as possible. If
antimicrobial properties, exfoliation of cells, and bacterial laboratory processing is delayed by more than
interference by commensal microbes of the distal urethra 30 minutes, the specimen should be refrigerated (4ºC).
and distal genital tract. The composition of the urine also Blood agar plates may be inoculated and incubated for
aids in prevention of development of bacterial urinary 24 hours. If bacteria are present on the plate after 24
tract infection. Urine produced by cats is normally hours, the plate may be submitted for identification and
concentrated, oftentimes having a specific gravity determination of antibiotic sensitivities.7,8
>1.045 with an associated high osmolality.4 Urine also The most common technique used for determining
contains substances that are inhibitory to bacterial antimicrobial susceptibility is the Kirby-Bauer agar
colonization including high urea concentration, and diffusion test. After an organism is isolated, it is streaked
presence of organic acids, low-molecular weight on an agar plate and antimicrobial discs are placed on
carbohydrates, and Tamm-Horsfall mucoprotein. Cell- the plate. This test is based on attainable antimicrobial
mediated and humoral-mediated immunity present within concentrations in human plasma and the assumption is
the urine or urinary tract also impart protection. Frequent that plasma concentrations are similar between species.
and complete voiding also helps to wash out bacteria However, many antibiotics attain higher concentrations
that have migrated into the bladder, ureters, and in urine than in plasma; therefore, this technique may
kidneys. underestimate a microbe’s susceptibility to
antimicrobials. A more sensitive and specific technique
PHYSICAL EXAMINATION FINDINGS is to determine the minimum inhibitory concentration
AND CLINICAL SIGNS (MIC). The MIC is the lowest concentration of
Clinically, bacterial urinary tract infections may be antimicrobial required to inhibit bacterial growth. It is
symptomatic or asymptomatic. Bacterial infection of the performed using a series of dilutions of each antibiotic in
lower urinary tract is usually associated with clinical a multi-well plate to which a standard number of bacteria
signs that are similar to other diseases of the lower are added. Although this is a better test, it is more
urinary tract. These signs include, but are not limited to, expensive and for most bacterial isolates, the Kirby-
pollakiuria, dysuria, stranguria, hematuria, and Bauer method is acceptable.
inappropriate urination. Bacterial urinary tract infection of Bacteria that commonly cause UTI are the same in
the kidneys may be associated with hematuria, or if dogs and cats. Infections caused by Escherichia coli are
septicemia develops, the animal may be systemically ill. the most common, accounting for one-third to one-half of
In addition, upper urinary tract infections may cause all organisms isolated from the urine of infected animals.
recurrent lower urinary tract infections. Gram-positive cocci are the second major group of
organisms. Staphylococci and streptococci account for
DIAGNOSIS one fourth to one third of the isolates recovered. Bacteria
Urinalysis and Urine Culture that cause the remaining one-fourth to one-third of
Evaluating results of a complete urinalysis of a urinary tract infections include Proteus spp., Klebsiella
sample collected by cystocentesis is the best way to spp., Pasteurella spp., Enterobacter spp., Pseudomonas
screen for bacterial UTI. Some dipsticks contain reagent spp., Corynebacterium spp., and Mycoplasma spp.;
pads for nitrate or leukocyte esterase activity; however, however, these are uncommon.
The North American Veterinary Conference – 2007
Unless septicemia is present, results of a CBC appropriate dosage and frequency, clinical signs should
should be normal. If septicemia is present, leukocytosis resolve within 48 hours. Additionally, results of a
and a left shift may be present. Bacterial infection of the complete urinalysis should improve within this same time
lower urinary tract does not cause changes in serum frame. If possible, a urine culture should be performed 5
biochemical analysis. In animals with pyelonephritis, to 7 days after cessation of antimicrobial therapy in order
serum biochemical analysis may be normal if only one to ensure eradication of the UTI.
kidney is infected or if minimal damage has occurred, or
it may reveal biochemical changes consistent with renal Complicated Bacterial UTI
failure. Hyperthyroidism has also been associated with Reproductively intact dogs, all cats, and animals with
inducing a diuresis and bacterial urinary tract infections identifiable predisposing causes for bacterial UTI
in cats. Additional laboratory evaluation of cats may (eg, renal failure, hyperadrenocorticism, diabetes
include testing for Feline Leukemia Virus and Feline mellitus) should be considered to have a complicated
Immunodeficiency Virus, which may compromise the bacterial UTI. Pyelonephritis and prostatitis are
immune system. examples of complicated bacterial UTI. When a cat is
confirmed with a bacterial UTI, they should be treated as
Radiography, Ultrasonography, and Endoscopy a complicated UTI because cats are inherently resistant
In many animals with bacterial urinary tract to development of an infection. Treatment with
infections, radiography will be normal. However, survey antibiotics for longer than the routine 10 to 14 days may
radiography may reveal uroliths, renomegaly, or other be indicated, and are usually administered for 4 to 6
defects that may predispose to development of bacterial weeks. Urine should be evaluated in the first week of
urinary tract infection. If no abnormalities are found by treatment for response to therapy and prior to
survey abdominal radiography, ultrasonography or discontinuing therapy. After antimicrobial therapy is
contrast radiography should be performed. The upper discontinued, urine should be cultured 5 to 7 days later.
urinary tract may be evaluated by use of excretory Use of once a day antibiotic treatment may be necessary
urography;9 whereas, the lower urinary tract may be in order to control bacterial urinary tract infections that
evaluated by use of contrast cystography and are difficult to eradicate.
urethrography, and double contrast cystography. A
disadvantage of performing contrast radiography of the Recurrence of Bacterial UTI
lower urinary tract is risk of inducing bacterial urinary Relapse – A relapse is defined as recurrence of a
tract infections during catheterization. Ultrasonography is bacterial UTI due to the same organism. Relapses
a non-invasive technique and can evaluate the kidneys usually occur within days to weeks of discontinuing
and bladder; however, its use is limited for evaluating the antimicrobial therapy. Possible causes of relapse include
ureters and majority of the urethra. use of an inappropriate antimicrobial agent,
Endoscopy of the lower urinary tract may be useful in administering an appropriate antimicrobial agent at the
identifying mucosal and intra-luminal lesions of the inappropriate dosage, frequency, or duration, or
urinary tract, which may predispose to bacterial infection. complicating factors. A urine culture should be evaluated
In one study, a urolith not visible by survey radiography prior to re-instituting antimicrobial therapy. Additionally,
was visualized during cystoscopy.10 Disadvantages of further diagnostic evaluation may be warranted.
cystourethroscopy include requiring anesthesia to Reinfection – A reinfection is defined as an infection
perform the procedure, invasion of the lower urinary tract with a different organism than what was initially present.
which may compromise host defense mechanisms, and Reinfections usually occur weeks to years after
difficulty of performing the procedure in male cats cessation of antimicrobial therapy. Although
without perineal urethrostomies. predisposing risk factors may be present, many animals
that become reinfected often do not have identifiable risk
TREATMENT factors. If reinfections are infrequent, each episode may
Treatment of bacterial UTI is dependent upon be treated as an uncomplicated bacterial UTI. However,
whether the infection occurs due to a temporary breech if reinfections occur at a frequency of >3 per year, then
in the body’s defense mechanisms (uncomplicated) or animals should be treated as having a complicated
whether there is an irreversible breech in the defense bacterial UTI. Additionally, prophylactic antimicrobial
mechanisms (complicated). Eradication of bacterial therapy may be warranted.
urinary tract infection is dependent on selection of the Superinfections – A superinfection occurs when a
appropriate antibiotic, administering it at the proper second bacterial organism is isolated while an animal is
dosage and duration, and appropriate follow-up. receiving antimicrobial therapy. Oftentimes, this
organism displays a high degree of antibiotic resistance.
Uncomplicated Bacterial UTI A bacterial UTI that occurs in animals receiving
Uncomplicated bacterial UTI are those where no antimicrobial therapy that also have an indwelling
underlying structural, neurologic, or functional urethral catheter is an example of a superinfection.11
abnormality is identified. Uncomplicated bacterial UTI
are usually successfully treated with a 10- to 14-day PREVENTION
course of an appropriate antimicrobial agent. If the Bacterial UTI can be prevented by minimizing
proper antibiotic is chosen and administered at the bacterial contamination of the urinary tract and by
Small Animal – Nephrology and Urology
avoiding or minimizing conditions that impair host UTI that involve urease-producing microbes that are
defenses. Catheterization and endoscopy of the urinary associated with alkaluria.
tract always carry a risk of inducing an infection. The
magnitude of the risk increases with the degree of pre- References
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