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Characterization of subclinical bacteriuria, bacterial cystitis, and pyelonephritis in dogs with chronic kidney disease | VETgirl Veterinary Continuing Education Podcasts

In this VETgirl online veterinary continuing education podcast, we review the prevalence of subclinical bacteriuria, bacterial cystitis and pyelonephritis in dogs with chronic kidney disease (CKD).

Part of the typical workup for small animals with chronic kidney disease is to perform a urinalysis and urine culture. (As Dr. Jody Lulich, DVM, PhD, DACVIM from University of Minnesota says, “You’ve got to grow it to know it!”) Human medical standards have driven the wave to create standards in canine and feline medical diagnostic and treatment approaches across the board. Many of us feel that chronic kidney disease is a predisposing factor to development of bacterial urinary tract disease including pyelonephritis. However, in humans, presence of bacteria in the urine does not often warrant antimicrobial therapy. So, Foster et al out of University of Pennsylvania wanted to evaluate this in a study called Characterization of subclinical bacteriuria, bacterial cystitis, and pyelonephritis in dogs with chronic kidney disease.1 The goal of their study was to retrospectively evaluate medical records from dogs diagnosed with CKD to determine the prevalence of bacteriuria and to determine the breakdown of these cases as having subclinical bacteriuria, bacterial cystitis, or pyelonephritis. After all, how prevalent is bacteriuria in dogs with CKD and are we diagnosing and treating it appropriately?

In this study, records from 195 dogs described as having CKD based on IRIS staging were included in this study. These dogs had to be screened to exclude those with systemic illnesses that would predispose to bacteriuria such as diabetes mellitus or urinary incontinence. All dogs had urine cultures submitted that were obtained via cystocentesis (In VETgirl’s opinion, the best way to obtain urine for culture). Baseline blood work, urinalyses, urinary tract ultrasonography, and urine cultures were evaluated for all dogs. Presence of > 1,000 colony-forming units of bacteria/mL were indicative of bacteriuria. Records from dogs included in this study were then categorized into one of three groups: subclinical bacteriuria (SBU), pyelonephritis, bacterial cystitis. The authors described subclinical bacterial urinary tract infections as those that had presence of bacteria in the urine without exhibiting dysuria, and without leukocytosis or evidence of toxic neutrophilic changes in peripheral blood. Patients with subclinical bacteriuria also had no suggestive findings on urinary tract ultrasound consistent with pyelonephritis, and had either a static azotemia, or had azotemia that responded to antimicrobial therapy with a negative urine culture after antimicrobial therapy. To be categorized in the bacterial cystitis group, the dogs fit all the criteria for subclinical bacteriuria, but had clinical signs present such as stranguria or pollakiuria. Dogs in the pyelonephritis group had elevated body temperatures, pain on renal palpation, a leukocytosis with either a neutrophilia or neutropenia and evidence of toxic neutrophils or bands. These dogs also had urinary casts, glucosuria without peripheral hyperglycemia (indicates early renal tubular glucosuria and renal tubular injury), progressive azotemia that responded to antimicrobial therapy, a negative urine culture after appropriate antimicrobial therapy, and ultrasound findings suggestive of pyelonephritis.

Of the 182 dogs with CKD evaluated in this study, the authors found a positive urine culture prevalence of 18.1% in this group, which is similar to a prior feline study (17%).2 Interestingly, the authors found the prevalence of positive urine culture in CKD dogs to be less than dogs with other predisposing clinical conditions such as diabetes mellitus and hyperadrenocorticism. In the CKD patients, subclinical bacteriuria was the most commonly associated diagnosis in the dogs (45% of all cultures). Pyelonephritis was a close second diagnosis (40% of all urine cultures), leaving bacterial cystitis a distant third (15%). The patient breakdown for diagnosis of the 33 dogs with positive urine cultures is as follows: 17 dogs had subclinical bacteriuria, 13 dogs had pyelonephritis, 6 dogs had cystitis. The most common bacterial isolate found in urine cultures from all three diagnoses groups was Escherichia coli and is in agreement with feline and human studies reported by these authors. Patients with positive urine cultures received systemic antimicrobial medications based on results of susceptibility testing with a duration of treatment ranging from 2 to 6 weeks at the discretion of the primary clinician. It took ≤72 hours after initiation of appropriate antimicrobial therapy for resolution of clinical signs related to bacterial cystitis. According to this study, the severity of chronic kidney disease (attributed by the patient’s IRIS staging) had no association with the prevalence of subclinical bacteriuria, pyelonephritis, or bacterial cystitis.

So, what do we take away from this VETgirl podcast? With the finding of subclinical bacteriuria in our canine CKD patients, should we be advocating for a standard medical practice of prescribing antimicrobials to these patients? Let’s look at what other fields are doing. These authors summarize and draw comparisons between the human and dogs criteria for diagnosis of subclinical bacteriuria. In humans, more than one positive urine culture in women and only one positive urine growth in men are needed to diagnose an individual with subclinical bacteriuria. In dogs, only one positive growth is needed. When using only one positive culture as the basis for diagnosis of subclinical bacteriuria when clinical signs are absent, this raises questions as to the validity of the diagnosis. Could contamination from the skin/hair be the cause for the positive culture instead of a true subclinical bacteriuria? If this condition had been left untreated, would the subclinical bacteriuria then have progressed into a true bacterial cystitis? We don’t know whether the subclinical bacteriuria dogs in this study would have gone on to develop bacterial cystitis or whether they could have cleared the infection on their own, so the utility of a diagnosis of subclinical bacteriuria is questionable. Perhaps subclinical bacteriuria is only a transitional and temporary condition. The authors note that in humans, subclinical bacteriuria is not treated with antimicrobials unless the patient is pregnant or undergoing invasive urologic procedures. This opens the question as to whether we should be treating dogs subclinical bacteriuria with antimicrobials, or waiting until clinical signs develop. These authors note that further studies are needed to determine whether antimicrobial therapy is warranted in CKD dogs with subclinical bacteriuria. However, future studies in this field may prove difficult. Since clinical symptoms, such as dysuria, are needed to label the condition as bacterial cystitis (thus warranting antimicrobial treatment), secondary observers would be relied upon to document such behaviors in the patients, and such a subjective finding would be a confounding factor in any future study. At this time, the clinical significance of subclinical bacteriuria in dogs with CKD remains unknown, and the authors provide no recommendations on whether to treat subclinical bacteriuria in dogs with CKD.

Abbrevations:
CKD Chronic kidney disease
IRIS International Renal Interest Society
SBU Subclinical bacteriuria
UTI Urinary tract infection

References:
1. Foster JD, Krishnan H, Cole S. Characterization of subclinical bacteriuria, bacterial cystitis, and pyelonephritis in dogs with chronic kidney disease. J Am Vet Med Assoc 2018;252(10):1257-1262.
2. White JD, Stevenson M, Malik R, et al. Urinary tract infections in cats with chronic kidney disease. J Feline Med Surg 2013;15:459–465.

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