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

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.

top of page

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.

top of page

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.

top of page

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.

Contaminants are frequently gram positive, originating from the urethral flora.

top of page

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.

top of page

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

top of page

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.

top of page

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