Because of the close proximity of the urinary and genital tracts in the male dog, an intact male dog with a UTI is presumed to also have prostatic infection. Normal micturition with complete voiding obliterates mucosal folds and crevices in which bacteria lodge, mechanically washes out bacteria and decreases the population of bacteria in the urethra. Examples of abnormal states that interfere with normal micturition include mechanical obstruction by calculi, strictures, bladder herniation, prostatic disease, or neoplasia and incomplete bladder emptying from abnormal innervation or retension of urine in a urachal diverticulum. A functional high pressure zone in the mid urethra of the female inhibits urethrovesicular reflux although the shorter urethra in females is less a barrier to ascending bacteria than the male urethra. Ureteral and urethral peristalsis may inhibit ascending migration of organisims.
Ureters enter the bladder at an oblique angle to prevent vesicoureteral reflux.
The epithelium of the urinary tract produces a mucoprotein which coats the surface of the urinary tract and reduces the adherence of bacteria. Macrophages in the bladder wall and local IgA production assist in elimination of bacteria. Impaired immunocompetence from immunosuppressive drugs or cortisol can affect cellular or humeral immunity and predispose to infection. Urine itself has antimicrobial properties which are enhanced when the urine is concentrated. Dilute urine is more likely to support bacterial growth.