Center for Clinical Veterinary Medicine, Ludwig Maximilian University, Munich, Germany
Bacterial urinary tract infection (UTI) is an uncommon cause of feline lower urinary tract (LUTD) signs in young to middle-aged cats, but the incidence increases significantly in geriatric cats. Increased age and female gender have been identified as risk factors in several studies. In dogs, the majority of UTIs are classified as uncomplicated UTI, meaning that there is a sporadic infection of the bladder in an otherwise healthy individual with normal urinary tract anatomy and function. In cats, however, a high proportion of UTIs need to be classified as complicated UTIs because 80–83% of cats suffer from concurrent diseases, such as chronic kidney disease, hyperthyroidism, diabetes mellitus, or neurologic disease. An increased risk of UTI is also associated with urethrostomy, urolithiasis, and gastrointestinal disease such as constipation and megacolon.1 These diseases compromise the natural local or systemic defense mechanisms in a more profound or permanent way and predispose cats to bacterial UTIs. Apart from cats with typical clinical signs of LUTD, a considerable proportion of cats have asymptomatic or subclinical bacteriuria. Subclinical bacteriuria is defined as the presence of bacteria in the urine, as determined by positive bacterial cultures in the absence of clinical and cytological evidence of UTI.2
The diagnosis of UTI is based on clinical signs, urinalysis findings (hematuria, pyuria, bacteriuria) and a quantitative bacterial culture. Whenever possible, urine samples should be obtained via cystocentesis. In cats it is important to perform a stained urine sediment examination for a preemptive diagnosis of bacterial UTI. It has been shown that there is a very poor correlation between bacteriuria identified on an unstained wet urine sediment and bacterial culture results.3 In that study, the specificity of the wet-unstained sediment for true bacteriuria was only 56%, whereas the dry-stained sediment had a specificity of 99%. The gold standard for diagnosing bacterial UTI is a quantitative urine culture from a cystocentesis-derived urine sample. Urine samples for culture/susceptibility testing should be refrigerated as soon as possible and processed in a microbiology laboratory within 24 hours. Complete blood count, serum chemistry, T4, FIV-antibody, and FeLV-antigen testing, and abdominal ultrasound and/or radiographs should be performed to identify a possible predisposing disorder.
In the majority of feline UTIs, growth of a single bacterial isolate is identified. Only 12 to 22% of the infections involve two or more isolates.4-6 The most commonly cultured bacterial species (spp.) in feline UTI are Escherichia coli (E. coli), Enterococcus spp., Staphylococcus spp., and Streptococcus spp. In cats with diabetes mellitus, hyperthyroidism, or CKD, E. coli seems to be predominant,6,7 while there is a higher proportion of Staphylococcus and Enterococcus spp. in cats with LUT signs8. Great caution is required when interpreting results of in vitro susceptibility testing of Enterococcus faecalis because of their inherent resistance to cephalosporins, clindamycin, and TSO. Despite in vitro susceptibility they are not effective in vivo.
Current guidelines for antimicrobial use for bacterial UTI in cats recommend the use of amoxicillin clavulanic acid for empirical treatment of uncomplicated UTI, or the use of amoxicillin without clavulanic acid for infections with gram-positive bacteria and amoxicillin clavulanic acid for infections with gram-negative organisms.
However, Gram staining will not be feasible in every veterinary practice. Two studies from Germany and Norway have shown that the susceptibility of bacterial urinary tract pathogens to ampicillin, the representative of the penicillins without clavulanic acid and assumed cross-resistance with amoxicillin, is low (58%).9,10 On the other hand, there is concern in human and veterinary medicine that the use of broad-spectrum antimicrobials, such as amoxicillin with clavulanic acid, increases the prevalence of B-lactamase-producing and methicillin-resistant bacteria. It is, therefore, advantageous to base the use of antimicrobials on the results of culture/susceptibility whenever possible. Treatment with analgetic drugs (buprenorphine, NSAID in cases with normal hydration status and renal function) for alleviating clinical signs can be used while results of culture/susceptibility are pending. Postponing treatment is of course not possible if the patient has signs of pyelonephritis, such as fever and/or inflammatory leukogram. The recommended duration of treatment is 7 days for uncomplicated infection and 28 days for complicated UTI. This recommendation is based on expert opinions. There are no prospective studies evaluating the duration of antimicrobial treatment of feline UTI. In cats with indwelling urinary catheters, prophylactic antimicrobial therapy for prevention of UTI is never indicated. A urine sample (ideally collected by cystocentesis) should be submitted for culture at the time of catheter removal and possible infection treated based on results of culture/susceptibility testing. So far, the significance of subclinical bacteriuria in dogs and cats is not fully understood, and it is not known whether treatment of cats with asymptomatic bacteriuria is always beneficial. In a recent study, 67 nonazotemic cats >7 years had urine cultures performed on up to 5 occasions over a three-year period.11 The incidence of subclinical bacteriuria varied between 10 and 13%. Cats with positive urine culture were not treated with antimicrobials. An adverse association between subclinical bacteruria and survival was not identified in this study population. Asymptomatic bacteriuria is a common finding in healthy women and in women and men with comorbidities. Several randomized clinical trials have identified no benefits of antimicrobial treatment in asymptomatic individuals. It is, however, not possible to extrapolate these results to cats and dogs. So far, there are no prospective studies in cats with subclinical bacteriuria. Current guidelines recommend considering treatment for asymptomatic bacteriuria in patients at high risk of ascending or systemic infections, such as chronic kidney disease, systemic immunosuppression or diabetes mellitus.
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