Bacterial Urinary Tract Infections

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					                              SMALL ANIMAL – NEPHROLOGY AND UROLOGY
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 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.
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                           The North American Veterinary Conference – 2007
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    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

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                               Small Animal – Nephrology and Urology
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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
existing urinary tract abnormality, the amount of any          1. Lulich JP, Osborne CA, Bartges JW, et al. Canine
additional injury caused by the procedure, and the                 lower urinary tract disorders In: Ettinger SJ,Feldman
duration of the procedure. These risks of infection can            EC, eds. Textbook of veterinary internal medicine.
be minimized by being careful to perform invasive                  5th ed. Philadelphia: WB Saunders, 1999;1747-
procedures only when necessary, by performing the                  1783.
procedure as atraumatically as possible, and by                2. Osborne CA, Kruger JM, Lulich JP, et al. Feline
removing the catheter or endoscope as soon as                      lower urinary tract diseases In: Ettinger SJ,Feldman
possible. Cats with perineal urethrostomies are also at            EC, eds. Textbook of veterinary internal medicine.
higher risk for developing bacterial urinary tract                 5th ed. Philadelphia: WB Saunders, 1999;1710-
infections compared with cats without perineal                     1746.
urethrostomies;12,13 thus, other therapeutic interventions     3. Senior DF. Bacterial urinary tract infections:
should be tried before resorting to this procedure.                Invasion, host defenses, and new approaches to
    Catheter-induced bacterial UTI present a common                prevention. Compen Contin Educ Pract Vet
problem encountered by veterinarians. Bacteria may                 1985;7:334-344.
migrate along the outside of the catheter or through the       4. Lees GE, Osborne CA, Stevens JB. Antibacterial
lumen. Risk of bacterial UTI increases with pre-existing           properties of urine: Studies of feline urine specific
urinary tract disease or urothelial damage. The risk is            gravity, osmolality, and pH. J Am Anim Hosp Assoc
greater in animals with indwelling urethral catheters              1979;15:135-141.
when compared with animals that are intermittently             5. Swenson CL, Boisvert AM, Kruger JM, et al.
catheterized.11 Despite this lower risk, one study                 Evaluation of modified Wright-staining of urine
documented bacterial UTI in 7 of 35 dogs that were                 sediment as a method for accurate detection of
catheterized one time. Bacterial UTI occurs in >50% of             bacteriuria in dogs. J Am Vet Med Assoc
animals after 4 days of an indwelling urethral catheter.           2004;224:1282-1289.
Antibiotic therapy while an indwelling catheter is in place    6. Osborne CA, Lees GE. Bacterial infections of the
decreases the frequency of bacterial UTI; however,                 canine and feline urinary tract In: Osborne CA,Finco
when bacterial UTI occur, the organisms exhibit a                  DR, eds. Canine and feline nephrology and urology.
greater degree of antimicrobial resistance.14 Catheter-            Baltimore: Williams & Wilkins, 1995;759-797.
associated bacterial UTI may be minimized by using             7. Blanco LJ, Bartges JW, New J, et al. Evaluation of
intermittent catheterization when possible, removing               blood agar plates as a transport medium for aerobic
indwelling catheters as soon as possible, using a closed           bacterial urine cultures. J Vet Intern Med
collection    system,      and     avoiding    administering       2001;15:303 (abstract).
antimicrobial agents while urethral catheters are              8. Saunders A, Bartges JW, Bemis DA, et al.
inserted.                                                          Evaluation of blood agar plates and incandescent
    Prophylactic antimicrobial therapy may be indicated            lighting for aerobic bacterial urine cultures. J Vet
in animals with relapses or frequent reinfections. The             Intern Med 2002;16:379 (abstract).
antimicrobial agent should be selected based on urine          9. Barber DL, Finco DR. Radiographic findings in
bacterial culture and susceptibility testing. The agent is         induced bacterial pyelonephritis in dogs. J Am Vet
administered at ½ to 1/3 of the daily therapeutic dose,            Med Assoc 1979;175:1183-1190.
and is usually administered once a day at night. Urine         10. Buffington CA, Chew DJ, Kendall MS, et al. Clinical
should be re-cultured every 4 to 6 weeks to insure                 evaluation of cats with nonobstructive urinary tract
control of the bacterial UTI. Experience has shown that if         diseases. J Am Vet Med Assoc 1997;210:46-50.
animal does not have a “break-through” infection during        11. Barsanti JA, Blue J, Edmunds J. Urinary tract
a 6-month period, then antimicrobial therapy may be                infection due to indwelling bladder catheters in dogs
successfully discontinued. Disadvantages of this                   and cats. J Am Vet Med Assoc 1985;187:384-388.
approach include development of resistant bacteria and         12. Osborne CA, Caywood DD, Johnston GR, et al.
side effects of the antimicrobial agent. An alternative to         Perineal urethrostomy versus dietary management
prophylactic antimicrobial therapy in dogs is                      in prevention of recurrent lower urinary tract
administration of methenamine. Methenamine is a                    disease. J Small Anim Pract 1991;32:296-305.
urinary tract antiseptic that is effective when the urine pH   13. Griffin DW, Gregory CR. Prevalence of bacterial
is less than 6.0. It is used for prophylaxis and requires a        urinary tract infection after perineal urethrostomy in
sterile urine culture prior to use. Because it requires an         cats. J Am Vet Med Assoc 1992;200:681-684.
acidic urine pH, it is contraindicated in dogs with            14. Barsanti JA, Shotts EB, Crowell WA, et al. Effect of
metabolic diseases associated with metabolic acidosis              therapy on susceptibility to urinary tract infection in
(e.g. chronic renal failure and diabetic ketoacidosis); cats       male cats with indwelling urethral catheters. J Vet
do not tolerate methenamine as well as dogs. In                    Intern Med 1992;6:64-70.
addition, methenamine will not be effective with bacterial


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