Larry G. Adams, DVM, PhD, Diplomate ACVIM (Small Animal Internal Medicine), Purdue University

When a patient presents with a second incident of urinary tract infection (UTI), it may be a relapse or a reinfection. Relapses involve recurrence of the same species and serologic strain of microorganisms within several weeks of withdrawal of therapy. Common causes include inappropriate antibiotic use (incorrect dose or duration, poor owner compliance), failure to eliminate predisposing causes (uroliths, neoplasia), deep-seated infections inaccessible to the antibiotic (pyelonephritis, prostatitis), and emergence of drug-resistant pathogens. Reinfections are caused by a different pathogen from that of the previous UTI. Common causes of reinfection include failure to eliminate predisposing causes for UTI, presence of multiple pathogens in which only sensitive pathogens were eliminated by therapy, iatrogenic infection during follow-up procedures (catheterization), and spontaneous reinfection. The standard diagnostic evaluation for dogs with recalcitrant UTI should include CBC, serum biochemistry profile, urinalysis, urine culture, abdominal radiographs, and ultrasonography. The history should be reviewed for diseases or drugs that could contribute to immunosuppression. Subtle abnormalities of the perivulvar region are easily overlooked during routine physical examination and should be carefully evaluated. The patient should be tested for hyperadrenocorticism if there are any supportive data. Cystoscopy helps rule out anatomic abnormalities, neoplasia, or uroliths and permits mucosal biopsy for culture, cytology, and histopathology. Ureteral catheterization can be performed to obtain cultures of the upper urinary tract. Treatment should be based on culture and sensitivity of urine obtained by cystocentesis. A follow-up urine culture should be obtained 7 to 10 days into therapy to prove efficacy of the drug selected.

COMMENTARY: An extensive search for an underlying cause should be at the forefront of evaluating patients with repeat UTIs. Differentiating between relapse and reinfection is critical to long-term medical management. Follow-up cultures should be conducted to ensure that the drug selected for treatment is indeed the appropriate choice.