Introduction

In healthy people, urine in the bladder is sterile--no bacteria or other infectious organisms are present. The channel that carries urine from the bladder out of the body (urethra) contains no bacteria or too few to cause an infection. However, any part of the urinary tract can become infected; an infection anywhere along the urinary tract is called a urinary tract infection (UTI).

UTIs are usually classified as upper or lower according to where they occur along the urinary tract. Lower UTIs are infections of the urethra (urethritis) or bladder (cystitis); upper UTIs are infections of the kidneys (pyelonephritis) or ureters (ureteritis). UTIs can occur in children (see Section 23, Chapter 272) as well as in adults.

Causes

The organisms that cause infection usually enter the urinary tract by one of two routes. The most common route by far is through the lower end of the urinary tract--the opening of a man's urethra at the tip of the penis or the opening of a woman's urethra at the vulva. The result is an ascending infection that spreads up the urethra. The other possible route is through the bloodstream, usually directly to the kidneys.

UTIs are almost always caused by bacteria, although some viruses, fungi, and parasites can infect the urinary tract as well. More than 85% of UTIs are caused by bacteria from the intestine or vagina. Ordinarily, however, bacteria that enter the urinary tract are washed out by the flushing action of the bladder as it empties.

Bacteria: Bacterial infections of the lower urinary tract--the bladder and urethra--are very common, especially in adolescent girls. Escherichia coli is the most common bacteria. When the person has a kidney stone, Proteus bacteria may be able to grow. Among people between the ages of 20 and 50, UTIs caused by bacteria are about 50 times more common in women than in men. In men, the urethra is longer, so it is more difficult for bacteria to ascend far enough to cause an infection. In people older than 50, UTIs become more common in both men and women, with less difference between the sexes.

See the sidebar Factors Contributing to Bacterial Urinary Tract Infections.

Viruses: The herpes simplex virus type 2 (HSV-2) may infect the urethra to cause a UTI, making urination painful and emptying of the bladder difficult.

Fungi: Certain fungi or yeasts can infect the urinary tract to cause a UTI. The most common type is Candida, which causes candidiasis. Candida frequently infects people who have an impaired immune system or a bladder catheter in place. Rarely, other types of fungi, including those that cause blastomycosis (Blastomyces) or coccidiomycosis (Coccidioides), infect the urinary tract. Fungi and bacteria often infect the kidneys at the same time.

Parasites: A number of parasites, including certain types of worms, can infect the urinary tract.

Trichomoniasis, caused by a type of microscopic parasite, is a sexually transmitted disease that can produce a copious greenish yellow, frothy discharge from the vagina in women. Rarely, the bladder or urethra becomes infected. Trichomoniasis in men usually produces no symptoms, although it can cause inflammation of the prostate gland (prostatitis).

Schistosomiasis, an infection caused by a type of worm called a fluke, can affect the kidneys, ureters, and bladder. This infection is a common cause of severe kidney failure among people who live in Africa, South America, and Asia. Persistent bladder infections with schistosomiasis may eventually result in bladder cancer.

Malaria, a disease caused by a type of microscopic parasite carried by mosquitoes, can destroy the small blood vessels (glomeruli) of the kidneys or can rapidly damage red blood cells, causing acute kidney failure.

Filariasis, a threadworm infection, obstructs lymphatic vessels, causing lymph fluid to enter the urine (chyluria). Filariasis can cause enormous swelling of tissues (elephantiasis), which, in men, may involve the scrotum.