Urinary-Tract Infection

  • Basics

    A urinary tract infection (UTI) is a bacterial infection of the urinary system, which consists of the kidneys, ureter, bladder, and urethra. Urinary tract infections are one of the most common causes of doctor visits, especially for women.

    There are several types of UTI, the most common being a bladder infection (cystitis). Urinary tract infections can also involve the ureters and kidneys; these infections are more difficult to treat. The most commonly seen UTIs are cystitis and kidney infections (pyelonephritis), with cystitis being 18 times more common.

    Most UTIs occur in women Table 01. The design of the female urinary tract makes it easy for bacteria to enter and cause an infection. It's estimated that 25% to 35% of women between the ages of 20 and 40 have had at least one UTI. Men are more likely to develop a UTI during infancy or after age 50. However, UTIs can affect anyone at any time of life.

    Table 1.  Development of UTIs by Gender and Age

    Age group Overall incidence Males vs. females
    Newborns/Infants 1% 1.5 to 1
    Preschoolers 1.5% to 3% 1 to 10
    School age 1.2% 1 to 30
    Reproductive years 3% to 5% 1 to 50
    Seniors (50+) 10% to 30% 1 to 1.5
  • Causes

    Bacteria are the most common cause of UTIs. In 85% of cases, the bacteria responsible for UTIs originate in one's own intestine or vagina. Vaginal bacteria, for example, may be introduced from condoms, or from the act of intercourse, which can push bacteria from outside the urinary system up into the urethra.

    While the bladder flushes out the majority of these bacteria during urination, sometimes bacteria can remain, leading to an infection. The most common bacterium to cause UTI is Escherichia coli.Other bacteria that can cause this infection include Proteus, Klebsiella, and Staphylococcus saprophyticus.

    Viruses, fungi, and parasites can also cause UTIs. The herpes simplex virus type 2 and fungal infections (i.e., yeast infections) can also cause UTIs. For example, the parasites that cause malaria can block the kidney's blood vessels and lead to UTIs. Similarly, a worm infection calledschistosomiasis can cause urinary infections. Infections caused by worms or parasites, however, tend to be rare in the U.S.

    Anatomical factors in both sexes can cause urinary infections. Any anatomical problem that obstructs the flow of urine can make it easier for bacteria to become trapped, leading to a UTI. An enlarged prostate, for example, is the most common reason why men over the age of 50 get UTIs. An uncircumcised penis can also trap bacteria, leading to a UTI. Similarly, an abnormality in women called a vesicovaginal fistula that links the bladder directly to the vagina can provide an easy route for bacterial infections.

    Physical changes during pregnancy can also predispose women to urinary tract infections. While the reasons for this are still unclear, one theory is that the hormonal changes of pregnancy dilate the urinary tract, impairing urine flow.

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