Chapter 45: Urinary Tract Infections

DEFINITIONS AND EPIDEMIOLOGY

Urinary tract infection (UTI) is defined by the presence of a significant number of microorganisms within the urinary tract, which is usually sterile, coupled with pyuria. UTIs are typically divided into lower tract disease, where infection is localized to the bladder and urethra (cystitis and urethritis, respectively), and upper tract disease, where it extends to the ureter and kidney (pyelonephritis). Although either upper or lower tract disease may result in significant morbidity, pyelonephritis has been associated with renal scarring and subsequent hypertension, chronic renal disease, and preeclampsia.^1,2

The lack of a consistent historical consensus definition for UTI poses some challenges in the interpretation of epidemiologic reports on UTI prevalence. Many prior studies define UTI solely on the basis of urine culture results without accounting for the presence of pyuria, meaning that patients with asymptomatic bacteriuria or contaminated specimens are often included in prevalence estimates. Nonetheless, UTI is generally recognized as an important occult cause of fever in young children. Screening studies performed in emergency departments suggest an overall prevalence of UTI of up to 5% in febrile children less than 2 years of age.^3,4 Rates of UTI vary widely with respect to age, gender, race, and circumcision status. Peak incidence of UTI occurs in the first year of life for all children, with a second peak occurring among female adolescents. After infancy, females are far more likely than males to be diagnosed with UTI. A population-based European study reported a cumulative UTI incidence of 7.8% for girls by age 7 years.⁵ One factor influencing the relatively higher rates of UTI in male infants is circumcision status. Uncircumcised males less than 3 months of age are approximately 10 times more likely to develop UTI than their circumcised counterparts (prevalence of 20.1% vs 2.4%, respectively).⁶ In young children, race appears to be an independent risk factor for UTI. In one study, White females less than age 2 years with fever had a UTI prevalence of 16% compared with a 2.7% prevalence among nonwhite girls.⁷

PATHOGENESIS

Bacterial pathogens cause most UTIs, but viruses, fungi, and parasites can also cause infection. UTI occurs when enteric stool pathogens or skin flora ascend through the urethra, infecting the bladder or spreading further into the upper urinary tract. The shorter urethra in females has been implicated in their predisposition to UTI. Similarly, uncircumcised infants harbor increased numbers of uropathogenic bacteria in the periurethral area.⁸ Bacterial invasion results from the interaction between bacteriologic properties such as adhesion, virulence, and motility as well as anatomic and genetic properties that influence host response.⁹ Some racial and genetic differences may be explained by differences in blood group antigens on the surface of uroepithelial cells, which affect bacterial adherence. An association of certain Lewis blood group phenotypes has been found in children with UTIs¹⁰ and in women ...