Urinary Tract Infection (UTI)
Infection can involve one or more parts of the urinary
tract. A bladder infection is called a lower urinary tract infection. Kidney infections
are called upper urinary tract infections. Lower UTIs can ascend into the kidneys and
conversely, kidney infections can "seed" the bladder with bacteria. The ureters
and urethra are rarely sole sites of infection and are usually infected in conjunction
with the closest anatomic site. The genital tract can be infected from the urinary tract
or vice versa.
Agents of UTI: Most
UTIs are bacterial in nature. It is proposed that the host's defense mechanisms must be
transiently or persistently abnormal for bacterial colonization to occur as continual
instillation of bacteria into intact urinary tracts for 2 weeks to 3 months failed to
establish an infection.
Bacterial isolates
| |
1400 cases |
3,681 cases Bacterial isolate identified by genus only |
| E coli |
37.8% |
44.1% |
| Staph aureus |
14.5% |
11.6% |
| Proteus mirabilis |
12.4% |
9.3% |
| Alpha hemolytic strep
|
10.7% |
5.4% |
| Klebsiella pneumoniae
|
8.1% |
9.1% |
| Pseudomonas aeruginosa
|
3.4% |
3% |
| Enterobacter sp. |
2.6% |
2.3% |
| Other proteus |
2.4% |
|
| Beta hemolytic strep |
1.9% |
|
| Misc. |
6.1% |
|
Another study from the University of Georgia shows 75% were
gram negative of 187 cases. 8-18% of UTI are due to more than one agent. Pyelonephritis is
most often caused by E. coli.
Viruses are a proposed cause of FUS. Fungal/yeast UTI are
uncommon. Cats develop bacterial UTI much less frequently than dogs.
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Routes of infection:
Urinary tract infections most commonly develop by:
- ascension of organisms from the intestinal tract, perineal
flora, or resident flora of urethra, prepuce and vagina
- by hematogenous spread
- by direct extension from adjacent tissues such as the
prostate. Because of the close proximity of the urinary and genital tracts in the male
dog, an intact male dog with a UTI is presumed to also have prostatic infection.
There are many normal host defense mechanisms to prevent UTI. It is
hypothesized that a host defense mechanism must be temporarily or permanently altered to
result in UTI. Normal micturition with complete voiding obliterates mucosal folds and
crevices in which bacteria lodge, mechanically washes out bacteria and decreases the
population of bacteria in the urethra. Examples of abnormal states that interfere with
normal micturition include mechanical obstruction by calculi, strictures, bladder
herniation, prostatic disease, or neoplasia and incomplete bladder emptying from abnormal
innervation or retension of urine in a urachal diverticulum.
A functional high pressure zone in the mid urethra of
the female inhibits urethrovesicular reflux although the shorter urethra in females
is less a barrier to ascending bacteria than the male urethra. Ureteral and urethral
peristalsis may inhibit ascending migration of organisms. Ureters enter the bladder at an
oblique angle to prevent vesicoureteral reflux.
Bacteria must first attach (adhere) to the epithelial
surface of the urinary tract before they can proliferate (colonization). The epithelium of
the urinary tract produces a mucoprotein which coats the surface of the urinary tract and
reduces the adherence of bacteria. Macrophages in the bladder wall and local IgA
production assist in elimination of bacteria. Anything which damages the epithelial lining
of the urinary tract can predispose to infection including: urinary catheterization,
calculi, neoplasia, or cytoxic drugs in urine (cyclophosphamide). Impaired
immunocompetence from immunosuppressive drugs or cortisol can affect cellular or humeral
immunity and predispose to infection.
Urine itself has antimicrobial properties which are
enhanced when the urine is concentrated. Dilute urine is more likely to support bacterial
growth.
The renal cortex is more resistant to infection than the
renal medulla because there is less blood flow to medulla which leads to less antibody,
less complement, and fewer leukocytes. High osmolality inhibits migration and phagocytosis
by leukocytes. High ammonia concentration inhibits complement. The medulla is called an
"immunologic desert". Renal infections develop in the medulla.
Catheterization can cause UTI by causing physical damage to
the mucosa or by allowing bacterial migration through, around or introduced by the
catheter. In one study of 70 normal dogs, catheterization induced a positive urine
culture in 0/35 males and 7/35 (30%) of females. Use the smallest and most flexible
catheter possible and atraumatic/aseptic
technique to avoid damage.
Repeated intermittent catheterization is associated with
less risk of infection than indwelling catheters. If an indwelling catheter is
necessary, the catheter and tubing should be a closed system.
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Diagnosis of UTI: Historical
complaints which suggest lower urinary tract disease include:
- dysuria, stranguria
- pollakiuria
- nocturia (reflecting pollakiuria)
- hematuria
Historical complaints which suggest upper urinary tract
disease include:
- signs of renal failure
- systemic signs of anorexia, depression, fever +/-, or renal
pain +/-.
- polyuria/polydipsia
UTI in both dogs and cats can be subclinical.
Physical examination should include palpation of the
bladder for wall thickness, degree of distension, masses, calculi, and ease of expression.
The kidneys should be palpated for size, contour, and pain. The prostate should be
palpated for size, position, pain, and symmetry. Examine the external genitalia for any
discharge, swelling, or odor.
The hemogram is usually normal in patients with lower UTI.
Neutrophilia may be observed in patients with acute pyelonephritis or acute
prostatitis. Nonregenerative anemia can be present if infection is chronic or if decreased
renal function has developed as a consequence of pyelonephritis. A regenerative anemia can
be present if hematuria is a prominent feature of UTI. There may be serum chemistry
abnormalities associated with uremia if pyelonephritis damages >3/4 of nephrons or with
obstruction leading to postrenal uremia (azotemia, increased phosphorus).
The method of urine collection influences results of the
urinalysis. Midstream voided urine evaluates all of urogenital tract but may contain
bacteria from the urethra. Catheterization evaluates the bladder, ureters, and
kidneys but may result in iatrogenic trauma or iatrogenic infection. Manual expression can
result in iatrogenic hematuria. Cystocentesis evaluates the bladder and kidneys and there
is little chance of contamination of sample. Prostatic fluid may reflux into the bladder
so evidence of prostatic disease may be detected in cystocentesis samples.
Isosthenuria suggests renal impairment due to
pyelonephritis or a polyuric disorder may be present and predisposing to infection. White
blood cells in the urine is pyuria. Normal urine contains fewer than 3-7 WBC/HPF. Patients
under the influence of glucocorticoids (Cushing's disease or exogenous steroids) may have a
UTI without increased numbers of WBC in the urine as steroids suppress the migration of
neutrophils to the urinary tract. Urease producing bacteria such as Staphylococcus
aureus and Proteus produce urease which split urea to ammonia resulting in
increased pH.
Gross hematuria is not a
prominent feature of uncomplicated UTI. For example, in one study of dogs
with UTI secondary to endocrine disorders (diabetes or Cushing's) 2 of
42 dogs (<5%) showed gross hematuria but 50% showed microscopic
hematuria. Normal urine contains fewer than 3-7 RBC/HPF.
Proteinuria may reflect inflammation and/or
hemorrhage. Bacteria in the urine may be pathogens or contaminants from the urethra. There
is a rapid increase in bacterial numbers in urine in vitro. Consider the method of urine
collection when interpreting bacteriuria. A few granular casts are normal due to turnover
of renal tubular cells, but an increased number suggests renal tubular damage especially
if repeatable on several samples.
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Urine culture:
The method of urine collection influences the culture results. Cystocentesis is the
preferred method by which to collect urine for bacterial culture. Animals with lower UTI
will void often making it difficult to collect a cystocentesis sample. If urine is
collected by voiding or catheterization, a quantitative culture should be obtained to
differentiate infection from contamination. The urine should be cultured soon after
collection as bacteria will proliferate in vitro. If antibiotics have already
been started they should be discontinued for 3 to 5 days before obtaining a
culture. If laboratory processing of the urine sample is delayed by more than 30
minutes the sample should be refrigerated. At room temperature bacterial counts
double about every 20 minutes. Commercially available urine culture collection
tubes combined with refrigeration can be used to preserve urine samples up to 72
hours before processing. In house urine culture kits are available.
| Collection Method |
Significant number of organisms (numbers consistent with
infection) |
| voided
or expressed |
> 100,000 organisms/ml |
| catheterization |
> 10,000 organisms/ml |
| cystocentesis |
> 1,000 organisms/ml |
Contaminants are frequently gram positive, originating from
the urethral flora.
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Radiographic studies: Survey radiographs
may disclose calculi and delineate kidney size and contour. Contrast studies or
ultrasound are needed to demonstrate masses within the bladder and the structure of the
renal pelvis.
Make sure urine has been collected for diagnostic purposes
before contrast agents are introduced into the urinary tract as iodine based contrast
agents can alter urine specific gravity and inhibit bacterial growth in vitro. A cystogram
can show wall thickness, radiolucent calculi, and filling defects. An intravenous
pyelogram (IVP) can illustrate renal size, pelvic dilation, calculi, and mass lesions in
kidney.
Ultrasound examination of the urinary and genital tracts
can provide useful information but interpretation is dependant upon the skill and
experience of the ultrasonographer. Information that can be obtained from
ultrasound
examination includes:
- kidney size and shape, renal calculi, size of the renal
pelvis, cysts or masses within the kidneys
- bladder wall integrity and thickness, intraluminal masses,
calculi, cellular debris in bladder
- urethral calculi
- prostatic size, shape and architecture
- uterine size and contents including diagnosis of pregnancy
and pyometra
Ultrasound can be used to guide aspiration or biopsy of
organs and structures.
Cystocopy is an emerging diagnostic
technique which can be used to evaluate the lower urinary tract. Rigid cystoscopes can
only be used in female dogs and cats. The
Storz
Veterinary Endoscopy company of Goleta, California markets a rigid cystoscope with
excellent optics. Flexible endoscopes used for airway examinations can be passed into the
bladder of large female dogs but usually are too large in diameter to pass through the
urethra of male dogs. With practice, the ureteral openings into the trigone of the bladder
can be visualized. Cystoscopy may aid in the diagnosis of calculi, neoplasia, ectopic
uretors and interstitial cystitis in
cats.
 |
This is the normal cystoscopic appearance of
the bladder. Urine has not been removed so hence the yellow color. The normal bladder is
thin walled and distensible. The blood vessels are numerous and prominent. |
 |
This is an abnormal bladder filled with
amorphous debris. The urine has been removed and the bladder is filled with air. The wall
appears thick and there is a large area of hemorrhage in the foreground of the photo. |
Prostatic disease may
result in recurrent UTI. "Bacterial urinary tract infections that occur in
noncastrated adult male dogs should be presumed to involve the prostate gland,"
Rogers-Lees, CVT 10, 1989.
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Localization of
the UTI is necessary to prognosticate and to treat. Dysuria, pollakiuria,
and/or urethral discharge indicate lower urinary or genital tract disease. Renal
pain indicates the kidneys are involved. Fever and other systemic signs indicate the
kidney or prostate is the site of infection.
Treatment should
be based on the results of urine culture. Any predisposing cause of infection should be
identified and corrected. Urine concentrations of antimicrobial agents are more important
than blood concentrations in the successful treatment of UTI. Urine concentration may be
up to 100 times greater than serum levels. Antibiotic selection should take into
consideration the ease of use, frequency of side effects and expense. Treatment of
uncomplicated UTI should continue ~14 days. Bacterial urinary tract infections in intact
adult male dogs should be treated as if the prostate gland is infected for a minimum of
4 weeks.
If culture results are not available and the animal has not
received recent antibiotic treatment, the following antibiotics are likely to be effective
the percentage of time listed:
- Staphylococcus sp. -
amoxicillin
- Streptococcus sp. -
amoxicillin
- Escherichia coli - trimethoprim sulfa
or enrofloxacin
- Proteus mirabilis -
amoxicillin
- Pseudomonas aeruginosa - tetracycline
- Klebsiella pneumoniae -
first generation cephalosporin or enrofloxacin
- Enterobacter sp. - trimethoprim sulfa
or enrofloxacin
If only a gram stain is
obtained:
gram +
ampicillin, amoxicillin or amoxicillin clavulanic acid
gram - trimethoprim sulfa
or enrofloxacin
Antimicrobial susceptibility tests must be conducted when
treatment fails or with recurrence of infection.
Culture the urine 7-14 days following a course of therapy
to confirm successful elimination of the organism.
Multiple episodes of UTI are either due to bacterial
reinfection (different strain or species) or bacterial relapse (same strain or species).
Both reinfection and relapse may indicate the presence of a predisposing cause or can be
iatrogenic and introduced at time of follow up urine collections. Bacterial relapse
can be due to administration of the wrong antibiotic, too low a dose or too short a
duration.
Animals who develop repeat infections that cannot be cured
should be managed as for single episodes, followed by the administration of ~ 1/3 of
normal daily dose of antibiotics daily for six months. Gram negative or mixed infections
can be treated with trimethoprim (sulfa), cephalexin, or nitrofurantoin. Gram positive
infections can be treated with amoxicillin or ampicillin. A monthly culture should be
obtained by cystocentesis. If the culture is negative, continue unchanged. If it is
positive, treat with full dose of appropriate antibiotic for 2 weeks then resume once
daily regime for six months with monthly follow-ups. Give once daily antibiotics just
prior to a period of confinement i.e., bedtime to increase contact time in the urine
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Other treatment considerations:
- Antiseptics such as methenamine have antibacterial
properties in acid urine.
- Antibiotic lavage is not effective in treating UTI and may
introduce bacteria.
- Analgesics and antispasmodics are of questionable value in
treating UTI in dogs and cats. Phenazopyridine is an analgesic used in people but its
clinical value in veterinary patients is not documented. It causes hemolytic heinz body
anemia in the cat.
- Frequent opportunities to void results in mechanical
flushing of bacteria from the urinary tract.
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Special considerations in the treatment of
pyelonephritis: Antibiotics must reach good
levels in the medulla so antibiotic choices should be based on blood levels achieved by
the antibiotic rather than on urine levels. Nephrotoxic drugs should be avoided. Treatment
should continue for 6-8 weeks or longer.
The potential consequences
of UTI include
- pyelonephritis
- infertility by spread of infection to the genital tract
- septicemia (urosepsis)
- diskospondylitis
- struvite urolithiasis
Last Edited: Jul 26, 2007 1:51 PM
