Escherichia coli, Staphylococcus
spp., and Enterococcus
spp., have been reported as the most common canine uropathogens, composing 44.1%, 11.6%, 9.3%, 9.1%, and 8.0% of microbial isolates, respectively (14
). Other studies have had similar results for E. coli
spp. prevalence, but for Proteus
spp. the prevalence fluctuates between 18% and 35.2% (15
). The differences in prevalence between our study and previous reports are likely attributable to the WCVM-VTH being a predominantly primary care hospital. Compared to referral cases, primary care cases are less likely to have been treated on multiple occasions, producing different selection pressures for uropathogens. A primary care caseload likely includes a smaller proportion of cases with secondary UTIs such as catheter-associated infections compared with a referral caseload. The nature of the caseload may also partially account for the lower frequency of mixed infections in our study compared with previous reports (14
). Additionally, geographical factors may contribute to the differences in the prevalence of bacterial uropathogens (10
). Unlike findings in a previous study where mixed infections were more common in females, there was no significant association between sex and prevalence of mixed infections in our study population (14
Isolation of the same bacterial species on more than 1 occasion can result from re-infection (2 infections separated by a period where the pathogen is eradicated from the urinary tract) or persistent infection (where the pathogen is not eradicated). Although these processes were indistinguishable from each other in this study, E. coli is over-represented in cases where multiple positive cultures revealed the same organism. Genetic analysis would allow distinction to be made between persistence and re-infection.
Uropathogenic E. coli
possess multiple adaptations for survival and persistence in the urinary tract. These adaptations facilitate their invasion into transitional epithelial cells, where they can either enter a latent state within membrane-bound vesicles or reproduce and establish biofilm-like intracellular bacterial communities free within the cytoplasm (19
). Bacteria later emerge from the intracellular communities and, by assuming a filamentous form, can bridge between epithelial cells without entering the urine (21
). Some of the intracellular bacteria also emerge from the epithelial cells into the urine (21
). The normal host defense of exfoliating infected epithelial cells disseminates E. coli
in the environment and exposes deeper layers of tissue for invasion by bacteria in the urine (23
). Neutrophil recruitment disrupts the integrity of the epithelium, which may also contribute to deeper penetration of the infection (23
). While these mechanisms for survival in the urinary tract have been studied extensively in mouse models of human disease, their role in canine E. coli
UTIs remains unclear. However, there is remarkable genetic similarity between uropathogenic E. coli
isolated from humans and dogs (3
). Because uropathogenic E. coli
can survive and multiply within epithelial cells, urine samples can produce negative culture results despite ongoing infection, and infection can persist despite the presence of bactericidal concentrations of antimicrobial drugs in urine (21
). Therefore, therapeutic efficacy likely depends on achieving appropriate antimicrobial concentrations within the uroepithelium.
With the exception of decreased resistance to group 1 and group 2 (first generation) cephalosporins among nonrecurrent E. coli,
antimicrobial resistance increased over the study period. While fluoroquinolone resistance increased among E. coli
isolates in a previous study, no change was observed in the present study (based on resistance to enrofloxacin) (12
). Gentamicin and chloramphenicol are not commonly used to treat bacterial UTIs in dogs so the increased resistance of recurrent E. coli
to these antimicrobials is likely attributable to co-selection, as these isolates were resistant to multiple antimicrobials (data not shown). Co-selection for antimicrobial resistance may be facilitated by the presence of class 1 integrons. These genetic elements facilitate the uptake and maintenance of gene cassettes coding for antimicrobial resistance (25
). Co-selection of antimicrobial resistance occurs when multiple gene cassettes are included in the integron, such that exposure to any drug to which the bacterium is resistant selects for all resistance genes present (25
). Class 1 integrons containing chloramphenicol and gentamicin resistance genes have been reported in uropathogenic and other E. coli
strains isolated from humans and animals (27
). Further study is needed to determine the role of class 1 integrons in disseminating antimicrobial resistance in canine uropathogenic E. coli
Laboratory culture and susceptibility results are often used for monitoring antimicrobial resistance, but there is inherent bias to this approach (31
). Samples tend to be submitted from animals with more severe clinical signs. In some cases, urine may not be submitted for culture and susceptibility testing until initial antimicrobial therapy (based on urinalysis results and Gram staining) has failed. This case selection bias may overestimate antimicrobial resistance. However, case selection bias is counterbalanced by the limited sensitivity of disk diffusion testing to changes in resistance. Because the results are reported as categorical data, changes are observed at only one breakpoint representing the transition between “intermediate” and “resistant” (permitting classification as “not resistant” and “resistant”). Consequently, shifts in minimum inhibitory concentrations are not apparent unless they occur around the breakpoint.
Prudent use of antimicrobials is an important step in reducing the emergence of antimicrobial resistance. In the context of canine UTIs, prudent use includes considering likely pathogens and their susceptibility patterns when choosing empirical treatment. Antimicrobial impact factors calculated using FRAT reflect the probability that a pathogen randomly selected from the study population is susceptible to a particular antimicrobial on disk diffusion testing. Based on these factors, canine UTIs are likely to be susceptible to a number of antimicrobials. However, the antimicrobial impact factors should not be used alone to select empirical therapy. The impact factors are based on in vitro susceptibility data, which does not necessarily reflect clinical efficacy for antimicrobials that achieve substantially higher concentrations in urine than what is evaluated in vitro or in cases involving intracellular infection. In the present study, gentamicin, enrofloxacin and amoxicillin-clavulanic acid had cumulative impact factors between 90 and 94. Small differences in impact factors are likely of little clinical significance, so other factors including pharmacokinetics, antimicrobial use strategies to reduce the emergence of resistance, drug safety profile, cost and convenience of administration must be considered when choosing empirical therapy. However, impact factors decreased between 2002 and 2007 for all antimicrobials except tetracycline and trimethoprim-sulfamethoxazole, consistent with increasing prevalence of antimicrobial resistance. Culture and susceptibility testing for individual cases remains the best instrument for guiding treatment decisions, especially for recurrent infections.
Increasing antimicrobial resistance is a growing concern in both human and veterinary medicine. Because pathogens isolated from recurrent infections are more resistant and resistance is increasing over time, appropriate management of recurrent infections is critical to control antimicrobial resistance. Underlying anatomic or metabolic problems should be identified and addressed whenever possible. Novel treatment strategies should address the virulence mechanisms of uropathogenic E. coli which facilitate persistent infection.