Bacterial urinary tract infection (UTI) occurs in approximately 14% of dogs in their lifetime, with a variable age of onset. Animals with a UTI can present with stranguria, pollakiuria, dysuria and urinary incontinence, although some animals may have no clinical signs. Spayed female dogs are at increased risk for a UTI, which is likely to be due to anatomical differences as well as possible protective secretions from the prostate in sexually intact males. Urinary tract infection occurs when bacteria colonise portions of the urinary tract that are normally sterile (i.e., kidney, ureter, bladder and proximal urethra). Escherichia coli, Staphylococcus spp. and Proteus spp. account for most cases of UTI in primary care practices. Some strains of E. coli possess virulence factors that enhance attachment to the uroepithelium. Enterococcus spp. are isolated in dogs and cats with UTI with increasing frequency and have limited treatment options at times because of resistance patterns of this organism. A urinalysis and aerobic quantitative urine culture should be performed in all pets suspected of having a UTI. The presence of bacteriuria and pyuria are highly suggestive for a UTI. By performing a quantitative urine culture and susceptibility the clinician can confirm the existence of bacterial UTI, and detect the presence of resistance patterns that may predict treatment failure.

Antimicrobial therapy is the mainstay of treatment for UTI. The urinary concentration of the antimicrobial agent that is chosen based on minimum inhibitory concentration (MIC) testing is the most important factor in determining the likelihood of bacteriological cure in uncomplicated UTI. Major anatomical, metabolic or functional abnormalities within the urinary system can make it impossible to effect or maintain long-term sterility with the urinary tract regardless of the antimicrobial agent that is chosen. A drug with a high likelihood toward the organism's susceptibility should be chosen to start therapy; the drug may need to be changed following return of susceptibility testing from the microbiology laboratory. In UTI seated deeply within tissues (kidney, prostate, very thickened bladder wall), the concentration of the antibacterial agent that can be achieved in the plasma and tissues is more important than that achieved in the urine. If the average urine concentration is greater than or equal to that of the MIC value × 4, it is likely that this drug will be at least 90% effective in eradication of the offending organism in an otherwise healthy patient. Attainable urine concentrations of some antimicrobials can achieve up to 100 times that attained in serum. Consequently, sterility within the urinary tract can be achieved at times when it would otherwise appear that these drugs would fail to do so at concentrations achieved in the plasma.

While it is preferable to choose an antimicrobial based on MIC testing, empirical use of an antimicrobial is often warranted for patients that have overt clinical signs. Pain relief with drugs such as buprenorphine or tramadol can be considered while awaiting results of a urine culture when the diagnosis of UTI is questionable. When the bacteria are identified as rod-shaped organisms, it is reasonable to choose some antimicrobial agent with gram-negative coverage such as a fluoroquinolone, amoxicillin-clavulanic acid, or trimethoprim (ormetoprim)-sulfa product. If sediment examination reveals cocci, then it is likely that the infection is due to a gram-positive organism (Enterococcus spp. or Staphylococcus spp.). In that case, a penicillin can be considered. If there are cocci present and the urinalysis reveals alkaline pH, it is likely that the infection is caused by a Staphylococcus spp. (because of urease production). If Staphylococcus spp. are suspected, it is more prudent to use amoxicillin-clavulanate because of the common production of beta-lactamase by staphylococci. For empirical therapy the Working Group of the International Society for Companion Animal Infectious Diseases recommends urinary antibacterial drugs likely to be effective against greater than 90% of the urinary isolates when this information is available. In general, ISCAID recommends initial therapy for uncomplicated UTI with amoxicillin (11–15 mg/ kg orally q8h) or trimethoprim-sulphonamide (15 mg/kg orally q12h); amoxicillin-clavulanate was not recommended for initial treatment in these cases due to lack of evidence for the need for clavulanic acid in addition to amoxicillin. Empirical therapy should not be prescribed for patients with a complicated UTI (three or more per year), if the infection was acquired in a hospital setting, if the animal had no clinical signs or in animals with a history of extensive antimicrobial use.

Seven to 14 days of an appropriate antimicrobial for treatment of an uncomplicated lower UTI is often recommended. At least 30–60 days of antimicrobial therapy is usually needed to sterilise chronic upper UTI (kidneys and ureters). Sometimes long-term bacteriological cure is not possible. In intact males with chronic prostatitis and UTI, antibacterial treatment usually needs to be given for a longer period of time. These guidelines for duration of treatment are based on conventional experience over the years, but surprisingly little data exist to support or refute these protocols. Ultimately, antimicrobials should be given for as long as is necessary to effect a bacteriologically sterile urine during administration of the medication and for a protracted time following discontinuation of the treatment.

Cefovecin is a third-generation cephalosporin that exerts good in vitro activity against many bacterial pathogens associated with UTI in dogs, but not against Pseudomonas spp. or Enterococcus spp. Sixty-one clinical dogs with uncomplicated UTI were injected with 8 mg/kg of a long-acting cefovecin formulation given as a single subcutaneous dose. Bacteriological cure was found in 88% of the treated dogs 21 days after the cefovecin was administered. In the same study dogs were treated with oral cephalexin at 15 mg/kg twice daily for 14 days; this achieved a bacteriological cure in 57.4% of the dogs. It should be noted that 75% of dogs treated with cephalexin had urinary pathogens susceptible to cephalexin; 90.0% of dogs treated with cefovecin had urinary pathogens susceptible to cefovecin based on MIC analysis.

In a recent prospective clinical study, treatment of uncomplicated bacterial UTI in dogs was compared using either a high-dose short-duration (HDSD) course of enrofloxacin or a standard duration regimen of amoxicillin-clavulanate in an interim analysis. Dogs were excluded from the study if they had a history of persistent or recurrent UTI, defined as more than three UTIs in 1 year with or without a period of sterility; uncontrolled comorbid diseases or concurrent urinary problems such as calculi or neoplasia; furthermore those dogs recently receiving antimicrobials or glucocorticosteroids were not eligible. Dogs were randomised into one of two groups. Dogs in group 1 received enrofloxacin at 18–20 mg/kg orally once daily for 3 consecutive days and those in group 2 received amoxicillin-clavulanate at 13.75–25 mg/kg orally twice daily for 14 days. Both treatment groups had urinalyses and urine cultures submitted on day 0, 10 and 21. The microbiological and clinical cure rates were compared between groups 7 days after completing the antimicrobial regimen (day 10 for group 1 and day 21 for group 2). Thirty-six dogs were analysed in this interim report. Bacteriological cure was achieved in 15 dogs (83%) treated with enrofloxacin and 14 dogs (78%) treated with amoxicillin-clavulanate, respectively. These data suggest that the HDSD enrofloxacin protocol was as effective as the standard protocol of 14 days of treatment with amoxicillin-clavulanate in treating uncomplicated canine UTI in this sample patient population and may represent a viable alternative therapeutic regimen for similar patients.

Successful treatment is defined as sterile urine during and after medication administration. Resolution of clinical signs, such as haematuria, proteinuria and microscopic bacteriuria, can be misleading as these may resolve transiently because of reduced activity of the UTI without eradication. For dogs with recurrent UTI, quantitative urine cultures are recommended at 5–7 days, 1 month and 3 months after medication has been discontinued to ensure sterility of the urinary tract.

References

1.  Irom S, Westropp J, et al. Interim evaluation of the efficacy and safety of a high dose short duration enrofloxacin treatment regimen for urinary tract infection in dogs. Journal of Veterinary Internal Medicine 2011;25:723.

2.  Ling GV, Norris CR, et al. Interrelations of organism prevalence, specimen collection method, and host age, sex, and breed among 8,354 canine urinary tract infections (1969–1995). Journal of Veterinary Internal Medicine 2001;15:341–347.

3.  Passmore CA, Sherington J, et al. Efficacy and safety of cefovecin (Convenia) for the treatment of urinary tract infections in dogs. Journal of Small Animal Practice 2007;48:139–144.

4.  Seguin MA, Vaden SL, et al. Persistent urinary tract infections and reinfections in 100 dogs (1989–1999). Journal of Veterinary Internal Medicine 2003;17:622–631.

5.  Weese JS, Blondeau JM, et al. Antimicrobial use guidelines for treatment of urinary tract disease in dogs and cats: antimicrobial guidelines Working Group of the International Society for Companion Animal Infectious Diseases. Veterinary Medicine International 2011. doi: 10.4061/2011/263768.