Top Ten Urinary Consult Questions

Top Ten Urinary Consult Questions

Dianne I. Mawby, Knoxville, Tennessee

Amanda Callens, Knoxville, Tennessee

Joseph W. Bartges, Knoxville, Tennessee

The scope of urologic problems in small animal veterinary practice is wide and encompasses upper and lower urinary tract problems as well as diagnostic, monitoring, and therapeutic challenges. Within this range of problems, 10 of the most common questions posed to specialists are summarized. The brief answers are designed to provide initial guidance for practitioners and often direct the reader to more specific information in this or previous editions of Current Veterinary Therapy.

1 Why Are Urine Culture Results Always Negative When I Send Specimens to the Diagnostic Laboratory?

The diagnostic plan for most patients with lower urinary tract signs includes urinalysis and urine culture. It can be frustrating to receive “false-negative” results when urinary tract infection (UTI) is suspected. False- and true-negatives can be related to procedural and biologic factors.

The timing of sample collection in relation to treatment selection is important. Samples should be collected before initiation of antimicrobial treatment in new cases whenever possible; if this is not possible, the diagnostic laboratory should be notified of current treatments. Antimicrobial administration may inhibit bacterial growth in culture but may not be bactericidal in vivo. Additionally, anaerobic and mycoplasma organisms will not grow with standard aerobic methods.

Handling of the urine sample before transport may have adverse effects. Samples should be refrigerated and transported as soon as possible. Urine samples should not be transported in the same shipment with formalin-preserved biopsy samples. Duration and temperature of transport also may affect culture outcome. Bacteria must be alive at the time of inoculation in the laboratory for growth to occur, and long delays or high temperatures may affect bacterial viability.

Negative urine culture results, of course, often reflect a sterile sample accurately. Urine leukocyte indicators on some urine dipstick tests are not reliable for the detection of inflammatory cells in dog or cat urine, and “bacteria” are overestimated on urine sediment examination. Many artifacts mimic bacteria and even bacterial motion in urine examined microscopically. Sediment examination is enhanced by routine staining (clean Gram stain or new methylene blue) and review by qualified personnel.

Finally, not all lower urinary tract signs are caused by bacteria. Further evaluation for other causes (urolithiasis, neoplasia, idiopathic cystitis) should be pursued in patients with persistent signs and repeated negative culture results.

To overcome the procedural challenges, urine can be cultured initially at the clinic. A small incubator can be used for inoculated blood agar plates. If growth occurs, then these samples may be submitted for species identification and antimicrobial susceptibility testing. It is well worth the time and effort involved in setting up this initial culture in-house to ensure a more reliable outcome.

2 How Do I Treat Urinary Tract Infections That Keep Coming Back?

Sequential urine cultures are invaluable in sorting out recurrent bacterial UTIs. Repeated UTIs can be classified based on the species and pattern of bacteria observed. Recurrent signs caused by the same organism usually are due to a relapse of the original infection. Signs associated with a new infection usually are caused by a different organism. Granted, repeated Escherichia coli infections can be challenging to interpret since E. coli is a common infectious agent as well as a common persistent organism in some patients (see Chapter 194).

“Recurrent” infection also must be evaluated in relation to previous courses of treatment. In a relapse (same bacteria) questions target the effectiveness of the prior treatment plan: Was adequate treatment given to the patient including appropriate drug, dosage, and duration? Were urinalysis and culture results rechecked after treatment completion to ensure clearance of infection? Was it a simple or a complicated infection (especially prostatitis, pyelonephritis, or UTIs with concurrent disorders)?

When reinfections (different organisms) are detected, underlying predisposing causes must be considered. These include host defense problems such as anatomic abnormalities of the lower urinary tract (recessed vulva, vaginal urine pooling, and masses), urethral incompetence, neurologic impairment of micturition, and alteration in urine concentration and volume. Uroliths at any location in the urinary system also can act as a nidus for relapse or reinfection. Other underlying systemic predispositions include immunocompromise such as that associated with endocrine diseases (diabetes mellitus, hyperadrenocorticism) or corticosteroid therapy. Unfortunately, an underlying disorder cannot be detected in many cases.

Even when a distinct reason or underlying cause can be identified, management of recurrent UTIs still can be challenging. Underlying anatomic problems may be surgically corrected to solve the problem, and manageable systemic disorders can be addressed. Other predisposing disorders may or may not be able to be eliminated (e.g., poorly controlled endocrine disorders, irreversible neurologic impairment). These cases should be managed as a complicated UTI with long-term appropriate antimicrobial therapy. Pulse or low-dose daily antibiotic therapy or other prophylaxis can be considered once the initial infection is cleared.

Many of these recurrent infections involve E. coli, which first must adhere to the cell and may actually internalize. Oral therapies that may be used to prevent adhesion include cranberry extract and D-mannose. Cranberry extract contains proanthocyanidins that inhibit E. coli adhesion to epithelial cells. D-Mannose also may be an adhesion deterrent, but clinical studies of its efficacy are lacking.

3 How Do I Treat Highly Resistant Urinary Tract Infections?

Resistant UTIs usually occur in patients with unidentified or unmanageable underlying disease. Typically, many different classes of antimicrobials have been prescribed, and bacteria have become resistant. As with recurrent UTIs, an underlying disease or condition should be investigated and treated if possible.

The diagnostic laboratory can be of great help in maximizing effective antimicrobial choices. Laboratories report antibiotic susceptibility via agar disk diffusion (Kirby-Bauer method) or the antimicrobial dilution technique (minimum inhibitory [bacteriostatic] concentration, or MIC). In the MIC technique, each antibiotic is evaluated with respect to serum levels of the drug, but urinary concentrations may be significantly higher. Renally excreted antimicrobials may be effective even if the laboratory result reports intermediate susceptibility or resistance. Short-term use of injectable antimicrobials may be necessary to eliminate the infection. If aminoglycosides are used, urinalysis should be performed frequently to detect casts that indicate early renal tubular damage. In infections with susceptible organisms, antiseptic therapy with nitrofurantoin can be used, although potential adverse effects must be recognized. As discussed in the previous section, oral cranberry extract and D-mannose may be useful in management of some E. coli infections.

4 What Should I Do about Asymptomatic (Silent) Urinary Tract Infections?

A patient may not be showing clinical signs of lower urinary tract disease even with a culture positive for bacteria. First, urine contamination must be ruled out, and bacterial numbers should be evaluated. If one or two high-growth organisms are isolated from an appropriately handled cystocentesis urine sample, significant infection is likely. Patients with silent infections usually have an uncontrolled underlying disorder. In the course of frequent, deliberate urine cultures performed as part of a monitoring plan, infection may be discovered.

Initially, standard treatment approaches for complicated infections are pursued. Frustration arises when the infections can never be cleared, and resistance may occur. These patients still do not show clinical signs, but the concern is for ascending infection or possible struvite uroliths if bacteria are urease producers. In some cases, no treatment is successful in eliminating the organism. Inhibitory antimicrobial treatment (low-dose, once-daily administration) has been recommended to prevent complications; alternatively, all antimicrobial therapy may be discontinued and, with time, resistant bacteria may be replaced with other bacteria. The urine should be reevaluated and cultured after 1 to 2 months in hopes of finding treatable infection. In other cases, treatment is withheld unless clinical signs of infection or urosepsis occur. See Chapter 194 for more information.

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Tags: Kirks Current Veterinary Therapy XV

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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