CHAPTER 49

Urinary Tract Infections

In healthy people, urine in the bladder is sterile—no bacteria or other infectious organisms are present. The tube 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). Some doctors consider prostate infections (prostatitis) to be lower UTIs (see page 1481). Upper UTIs are infections of the kidneys (pyelonephritis). In paired organs (such as the kidneys), infection can occur in one or both organs. UTIs can occur in children (see page 1767) 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 infection ascends the urethra to the bladder, and sometimes to the kidneys, or both. The other possible route is through the bloodstream, usually 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 among young, sexually active women. Escherichia coli is the most common bacteria to cause a lower UTI. When the person has a kidney stone, Proteus bacteria may be able to grow. Among people between the ages of 20 and 50, bacterial UTIs are about 50 times more common among women than 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 among both men and women, with less difference between the sexes.

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

Fungi: Certain fungi, or yeasts, can infect the urinary tract. This type of infection is often called a yeast infection (yeasts can also cause vaginitis). The fungus Candida is the organism most likely to cause urinary tract yeast infections. 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 may 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 cause a copious greenish yellow, frothy discharge from the vagina in women. Occasionally, the bladder or urethra becomes infected. Trichomoniasis can infect the urethra in men. It usually causes no symptoms, although it can cause inflammation of the prostate (prostatitis).

Factors Contributing to Bacterial Urinary Tract Infections

ASCENDING INFECTIONS

Blockage (for example, by stones) anywhere in the urinary tract

Abnormal bladder function that prevents proper emptying, such as occurs in neurologic diseases

Leaking of the valve-like mechanism between the ureter and the bladder, allowing urine and bacteria to flow backward from the bladder up the ureters, possibly reaching the kidneys (more common among children who have a UTI)

Insertion of a urinary catheter or any instrument by a doctor

Sexual intercourse

Use of a diaphragm with spermicide

Presence of an abnormal connection (fistula) between the vagina and the bladder or the intestine and the bladder

Among men, prostatitis

BLOOD-BORNE INFECTIONS

Infection in the bloodstream (septicemia)

Infection of the heart valves (infective endocarditis)

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 schistosomiasis often causes blood in the urine or blockage of the ureters and may eventually result in bladder cancer.

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.

Urethritis

Urethritis is infection of the urethra, the tube that carries urine from the bladder out of the body.

Bacteria, including those that are sexually transmitted, are the most common cause of urethritis.

Symptoms include pain while urinating, a frequent need to urinate, and sometimes a discharge.

Antibiotics are usually given to treat the infection.

Causes

Urethritis may be caused by bacteria, fungi, or viruses (for example, herpes simplex virus). In most women, the bacteria involved are those that normally live in the lower intestine. These bacteria reach the urethra from the anus. Men are much less likely to develop urethritis because the opening of the male urethra is far removed from the anus, and thus bacteria from the anus less often reach the urethra. Bladder infection (cystitis) develops in most women who have urethritis but not in most men who have urethritis.

Sexually transmitted organisms—such as Neisseria gonorrhoeae, which causes gonorrhea—can spread to the urethra during sexual intercourse with an infected partner (see page 1269). Chlamydia and the herpes simplex virus are also commonly transmitted sexually and can cause urethritis (see page 1266). When men develop urethritis, the gonorrheal organism is a very common cause. Although this organism may infect the urethra in women, the vagina, cervix, uterus, ovaries, and fallopian tubes are more likely to be infected. Trichomonas, a type of microscopic parasite, also causes urethritis in men. Urethritis may also be caused by the bacteria that commonly cause other urinary tract infections, such as Escherichia coli.

Symptoms

In both men and women, there is usually pain with urination and a frequent, urgent need to urinate. Sometimes people have no symptoms. In men, when gonorrhea or chlamydia is the cause, there is usually a discharge from the urethra. The discharge is often yellowish green when the gonococcal organism is involved and may be clear when other organisms are involved. In women, discharge is less common.

Other disorders that cause pain with urination include bladder infection and vaginitis. In vaginitis, urination may cause pain because urine, which is acidic, irritates the inflamed vulva and lining of the vagina.

Complications: Infections of the urethra that are not treated or are inadequately treated eventually can cause a narrowing (stricture) of the urethra. A stricture increases the risk that infections will develop in the bladder or the kidneys. Untreated gonorrhea occasionally leads to an accumulation of pus (abscess) around the urethra. An abscess can cause outpouchings from the urethral wall (urethral diverticula), which can also become infected. If the abscess perforates the skin, the vagina, or the rectum, urine may flow through a newly created abnormal connection (urethral fistula).

Diagnosis

Doctors can usually make a diagnosis of urethritis based on the symptoms and examination. A sample of the discharge, if present, is collected by inserting a soft-tipped swab into the end of the urethra. The urethral swab is then sent to a laboratory for analysis so that the infecting organism can be identified.

Prevention and Treatment

Sexually transmitted diseases that cause urethritis may be prevented by using a condom.

Treatment depends on the cause of the infection. However, identification of the organism causing urethritis can take days. Thus, doctors usually begin treatment with antibiotics that cure the most common causes. For sexually active men, treatment is usually with a ceftriaxone injection for gonorrhea plus oral azithromycin or oral doxycycline for chlamydia. If tests exclude the possibility of gonorrhea and chlamydia, trimethoprim/sulfamethoxazole or a fluoroquinolone antibiotic (such as ciprofloxacin) may be used. Women are treated as if they had cystitis. An antiviral drug, such as acyclovir, may be needed for a herpes simplex infection.

Cystitis

Cystitis is infection of the bladder.

Usually, bacteria are the cause of cystitis.

A frequent need to urinate and pain or burning while urinating are the most common symptoms.

Doctors can base the diagnosis on the symptoms, but they usually examine a urine specimen.

Drugs are needed to treat the infection and often the symptoms.

Interstitial Cystitis: Bladder Inflammation, Not Infection

Interstitial cystitis is painful inflammation of the bladder without evidence of infection. It is usually chronic. The cause is unknown. No infectious organisms are found in the urine. Typically, middle-aged women are affected. It is very unusual for men to be affected.

Symptoms include very frequent, painful urination. The urine may contain pus and blood, which are detected by microscopic examination. Over time, the inflammation may cause the bladder to shrink. An examination of the bladder by cystoscopy may detect small, shallow areas of bleeding and ulceration.

A number of treatments have been tried, but none is routinely satisfactory. Drugs to relieve pain, anticholinergic drugs, or antidepressants sometimes help. Pentosan is a drug taken by mouth that may provide pain relief for some people. Dimethyl sulfoxide, a drug instilled directly into the bladder, can also be of benefit. In extreme cases, when the person has intolerable symptoms that do not respond to treatment, the bladder may need to be surgically removed. In such instances, a new bladder is created from a segment of the intestine.

Causes

Women: Cystitis is common among women, particularly during the reproductive years. Some women have recurring episodes of cystitis. There are a number of reasons why women are susceptible, including the short length of the urethra and the closeness of the urethra to the vagina and anus, where bacteria are commonly found. Sexual intercourse can contribute, too, because the motion can cause a tendency for bacteria to reach the urethra, from which they ascend to the bladder. Pregnant women are especially likely to develop cystitis because the pregnancy itself can interfere with emptying of the bladder.

Use of a diaphragm increases the risk of developing cystitis, possibly because spermicide used with the diaphragm suppresses the normal vaginal bacteria and allows bacteria that cause cystitis to flourish in the vagina.

The decrease in estrogen production that occurs after menopause can thin the vaginal and vulvar tissues around the urethra (atrophic vaginitis and atrophic urethritis), which can predispose a woman to repeated episodes of cystitis. In addition, a drooping (prolapsed) uterus or bladder may cause poor emptying of the bladder and predispose to cystitis. A prolapsed uterus or bladder is more common among women who have had many children.

Rarely, cystitis recurs because of an abnormal connection between the bladder and the vagina (vesicovaginal fistula).

Men: Cystitis is less common among men. In men, cystitis generally starts with an infection in the urethra that moves into the prostate, then into the bladder. The most common cause of recurring cystitis in men is a persistent bacterial infection of the prostate. Although antibiotics quickly clear bacteria from the urine in the bladder, most of these drugs cannot penetrate well enough into the prostate to quickly cure an infection there. Usually antibiotics must be taken for weeks at a time. Consequently, if drug therapy is stopped prematurely, bacteria that remain in the prostate tend to reinfect the bladder.

Both Sexes: If the flow of urine becomes partly obstructed because of a stone in the bladder or urethra, an enlarged prostate (in men), or a stricture in the urethra, bacteria that enter the urinary tract are less likely to be flushed out with urine. Bacteria that are left in the bladder after voiding can multiply rapidly. The more bacteria in the bladder, the more likely is infection. People with longstanding or repeated obstruction of urine flow may develop a bladder out-pouching (diverticulum). This pocket retains urine after voiding, further increasing the risk of infection.

Cystitis can also be caused by a catheter or any instrument inserted into the urinary tract that introduces bacteria into the bladder. In men and women, an abnormal connection between the bladder and the intestine (vesicoenteric fistula) can develop, allowing fecal material to pass from the intestine into the bladder, causing bladder infection.

Sometimes the bladder can become inflamed without an infection being present (interstitial cystitis).

Symptoms

Cystitis usually causes a frequent, urgent need to urinate and a burning or painful sensation while urinating. These symptoms usually develop over several hours or a day. The urgent need to urinate may cause an uncontrollable loss of urine (urge incontinence), especially in older people. Fever is rarely present. Pain is usually felt above the pubic bone and often in the lower back as well. Frequent urination during the night (nocturia) may be another symptom. The urine is often cloudy and contains visible blood in about 30% of people. Air can be passed in the urine (pneumaturia) when infection results from an abnormal connection between the bladder and the intestine or the vagina (fistula).

Symptoms of cystitis may disappear without treatment. Sometimes cystitis causes no symptoms, particularly in older people, and is discovered when urine tests are performed for other reasons. A person whose bladder is malfunctioning because of nerve damage (neurogenic bladder—see page 293) or a person who has a permanently placed catheter may have cystitis with no symptoms until a kidney infection or an unexplained fever develops.

Diagnosis

Doctors can usually diagnose cystitis based on its typical symptoms. A midstream (clean-catch) urine specimen (see box on page 258) is collected so that the urine is not contaminated with bacteria from the vagina or the tip of the penis. A strip of test paper is sometimes dipped into the urine to perform two quick and simple tests for substances that are normally not found in the urine. The testing strip can detect nitrites that are released by bacteria. The testing strip can also detect leukocyte esterase (an enzyme found in certain white blood cells), which may indicate that the body is trying to clear the urine of bacteria. In adult women, these may be the only tests necessary.

In addition, the urine specimen can be examined under a microscope to see whether it contains red or white blood cells or other substances. Bacteria are counted, and the sample can be cultured to identify the numbers and type of bacteria. If the person has an infection, one type of bacteria is usually present in large numbers.

Did You Know…

It is not clear whether wiping from front to back or avoiding the use of tight, nonporous underwear helps women prevent bladder infections.

In contrast, cranberry juice does seem to help prevent infections.

In men, a midstream urine specimen is usually sufficient for a urine culture. In women, a specimen is more likely to be contaminated with bacteria from the vagina or vulva. When the urine contains only small numbers of bacteria, or several different types of bacteria simultaneously, the urine has likely been contaminated during the collection process. To ensure that the urine is not contaminated, doctors sometimes must obtain a specimen directly from the bladder with a catheter.

It is important for doctors to find the cause of cystitis in several different groups. The cause should be found in children, in men of any age, and in some women with frequently recurring infections (3 or more per year), especially when accompanied by symptoms of obstruction, an upper urinary tract infection, or infection with the Proteus bacteria. In these types of people, there is a greater likelihood of finding a cause that requires treatment other than simply giving drugs to treat the infection (for example, a large kidney stone). Doctors may perform an x-ray study in which a radiopaque dye, visible on x-rays, is injected into a vein, then excreted into the urine by the kidneys (intravenous urogram, or IVU). The x-rays then provide images of the kidneys, ureters, and bladder. Ultrasonography or computed tomography (CT) may be done instead of IVU. Performing voiding cystourethrography, which involves injecting a radiopaque dye into the bladder and filming its exit, is a good way to investigate the backflow (reflux) of urine from the bladder, up the ureters, particularly in children, and may also identify any narrowing (stricture) of the urethra. Retrograde urethrography, in which the radio-paque dye is injected directly into the urethra, is useful for detecting stricture, outpouching, or an abnormal connection (fistula) of the urethra in both men and women. Looking directly into the bladder with a flexible viewing tube (cystoscopy) may help diagnose the problem when cystitis does not resolve with treatment.

Prevention

People who have frequent bladder infections may continuously take low doses of antibiotics. The antibiotic can be taken daily, 3 times a week, or immediately after sexual intercourse. Postmenopausal women with frequent bladder infections and atrophic vaginitis or atrophic urethritis may benefit from estrogen creams applied to the vulva or estrogen suppositories inserted into the vagina.

Preventing Bladder Infections in Women

In women who experience three or more bladder infections in a year, these measures may help:

Drinking cranberry juice (about 10 ounces [about 300 milliliters] of juice or 2 ounces [about 50 milliliters] of concentrate per day) or taking cranberry pills, because cranberry fruit contains a substance that directly inhibits bacterial attachment to the bladder and because it acidifies the urine (making it a less hospitable environment for bacterial growth)

Increasing the intake of fluids

Urinating often

Urinating within a short time after sexual intercourse

Avoiding the use of spermicides (used with a diaphragm for birth control)

Taking antibiotics continually in low doses. Typically, the antibiotic is taken daily, 3 times a week, or immediately after sexual intercourse

For postmenopausal women who have atrophic vaginitis or atrophic urethritis, applying estrogen cream to the vulva or inserting estrogen suppositories into the vagina

Drinking plenty of fluids may help to prevent cystitis. The flushing action of the urine washes many bacteria out of the bladder. The body’s natural defenses eliminate the remainder of the bacteria. It is commonly believed that wiping from front to back, urinating soon after sexual intercourse, and avoiding the use of tight, nonporous underwear helps women prevent bladder infections. However, it is not clear whether any of these strategies is effective.

Treatment

Cystitis is usually treated with antibiotics. Before prescribing antibiotics, the doctor determines whether the person has a condition that would make cystitis more severe, such as diabetes or a weakened immune system (which reduces the person’s ability to fight infection), or more difficult to eliminate, such as a structural abnormality. Such conditions may require more potent antibiotics taken for a longer period of time, particularly because the infection is likely to return as soon as the person stops taking antibiotics.

For women, taking an antibiotic by mouth for 3 days is usually effective if the infection has not led to any complications, although some doctors prefer to give a single dose. For more stubborn infections, an antibiotic is usually taken for 7 to 10 days. For men, cystitis usually is caused by prostatitis, and antibiotic treatment is usually required for weeks.

A variety of drugs can relieve symptoms, especially the frequent, insistent urge to urinate and painful urination. Drugs that have anticholinergic effects (such as oxybutynin and tolterodine) may relieve bladder spasms that cause the sense of urgency. These drugs should be used with caution in men with a large prostate gland because the drugs may cause urinary retention. Other drugs, such as phenazopyridine, reduce the pain by soothing the inflamed tissues.

Surgery may be necessary to relieve any physical obstruction to the flow of urine or to correct a structural abnormality that makes infection more likely, such as a drooping uterus or bladder. Until surgery can occur, draining urine from an obstructed area through a catheter helps control the infection. Usually, an antibiotic is given before surgery to reduce the risk of the infection spreading throughout the body.

Pyelonephritis

Pyelonephritis is a bacterial infection of one or both kidneys.

Infection can spread up the urinary tract to the kidneys, or the kidneys may become infected through bacteria in the bloodstream.

Chills, fever, back pain, nausea, and vomiting can occur.

Urine and sometimes blood tests are done to diagnose pyelonephritis.

Antibiotics are given to treat the infection.

Causes

Pyelonephritis is more common among women than men. Escherichia coli, a type of bacteria normally in the large intestine, causes about 90% of cases of pyelonephritis among people who are not hospitalized or living in a nursing home. Infections usually ascend from the genital area through the urethra to the bladder, up the ureters, into the kidneys. In a person with a healthy urinary tract, an infection is usually prevented from moving up the ureters into the kidneys by the flow of urine washing organisms out and by closure of the ureters at their entrance to the bladder. However, any physical obstruction to the flow of urine, such as a structural abnormality, kidney stone, or an enlarged prostate, or the backflow (reflux) of urine from the bladder into the ureters increases the likelihood of pyelonephritis. The risk of pyelonephritis is increased during pregnancy. During pregnancy, the enlarging uterus puts pressure on the ureters, which partially obstructs the normal downward flow of urine. Pregnancy also increases the risk of reflux of urine up the ureters by causing the ureters to dilate and reducing the muscle contractions that propel urine down the ureters into the bladder.

Infections can also be carried to the kidneys from another part of the body through the bloodstream. For instance, a staphylococcal skin infection can spread to the kidneys through the bloodstream.

The risk and severity of pyelonephritis are increased in people with diabetes or a weakened immune system (which reduces the body’s ability to fight infection). Pyelonephritis is usually caused by bacteria, but rarely it is caused by tuberculosis, fungal infections, and viruses.

Some people develop long-standing infection (chronic pyelonephritis). Almost all of them have major underlying abnormalities, such as a urinary tract obstruction, large kidney stones that persist, or, more commonly, reflux of urine from the bladder into the ureters (which occurs mostly in young children). Chronic pyelonephritis can cause bacteria to be released into the bloodstream, sometimes resulting in infections in the opposite kidney or elsewhere in the body. Rarely, chronic pyelonephritis can eventually severely damage the kidneys.

Some people develop xanthogranulomatous pyelonephritis, an unusual type of chronic pyelonephritis that is usually caused by kidney stones. Severe kidney scarring (causing permanent kidney damage) and kidney abscesses (causing fever and severe pain) often develop.

Symptoms

Symptoms of pyelonephritis often begin suddenly with chills, fever, pain in the lower part of the back on either side, nausea, and vomiting.

About one third of people with pyelonephritis also have symptoms of cystitis, including frequent, painful urination. One or both kidneys may be enlarged and painful, and doctors may find tenderness in the small of the back on the affected side. Sometimes the muscles of the abdomen are tightly contracted. Irritation from the infection or the passing of a kidney stone (if one is present) can cause spasms of the ureters. If the ureters go into spasms, people may experience episodes of intense pain (renal colic). In children, symptoms of a kidney infection often are slight and more difficult to recognize (see page 1767). In older people, pyelonephritis may not cause any symptoms that seem to indicate a problem in the urinary tract. Instead, older people may have delirium or an infection of the bloodstream (sepsis).

In chronic pyelonephritis, the pain may be vague, and fever may come and go or not occur at all.

Diagnosis

The typical symptoms of pyelonephritis lead doctors to perform two common laboratory tests to determine whether the kidneys are infected: examining a urine specimen under a microscope and culturing bacteria in a urine specimen to determine which bacteria are present. Blood tests may be performed to check for elevated white blood cells or bacteria in the blood.

Additional tests are performed in people who have intense back pain typical of renal colic, in those who do not respond to antibiotic treatment within 48 hours, in those whose symptoms return shortly after antibiotic treatment is finished, in those with long-standing or recurrent pyelonephritis, and in men (because they so rarely develop pyelonephritis). Ultrasonography or helical (spiral) computed tomography (CT) studies performed in these situations may reveal kidney stones, structural abnormalities, or other causes of urinary obstruction.

Prognosis

Most people recover fully. Delayed recovery and the chance of complications are more likely if the person needs hospitalization, the infecting organism is resistant to commonly used antibiotics, or the person has a disorder that weakens the immune system (such as certain cancers, diabetes mellitus, or AIDS) or a kidney stone.

Prevention and Treatment

Antibiotics are started as soon as the doctor suspects pyelonephritis and samples have been taken for laboratory tests. The choice of drug or its dosage may be modified based on the laboratory test results (including culture results), how sick the person is, and whether the infection started in the hospital, where bacteria tend to be more resistant to antibiotics.

Outpatient treatment with antibiotics given by mouth is usually successful if the person has:

No nausea or vomiting

No signs of dehydration

No other disorders that weaken the immune system, such as certain cancers, diabetes mellitus, or AIDS

No signs of very severe infection, such as low blood pressure or confusion

Pain that is controlled with drugs taken by mouth

Otherwise, the person is usually treated initially in the hospital. If hospitalization is needed and the person needs antibiotics, the antibiotics are given intravenously for 1 or 2 days, then they can usually be given by mouth.

Antibiotic treatment of pyelonephritis is given for 14 days so that infection will not recur. However, antibiotic therapy may continue for up to 6 weeks for men in whom the infection is due to prostatitis, which is more difficult to eradicate. A final urine sample is usually taken shortly after the antibiotic treatment is finished to make sure the infection has been eradicated.

Surgery may be needed if tests reveal a predisposing condition, such as an obstruction, a structural abnormality, or a stone. Surgical removal of the infected kidney is usually necessary for people with xantho-granulomatous pyelonephritis because repeated infections are likely. Removal of the infected kidney may also be necessary for people with chronic pyelonephritis who are about to undergo a kidney transplant. Spread of infection to the transplanted kidney is particularly risky because the person takes immunosuppressant drugs, which prevent rejection of the transplanted kidney but also weaken the body’s ability to fight infection.

People who have frequent episodes of pyelonephritis or whose infection returns after antibiotic treatment is finished may be advised to take a small dose of antibiotic every day as preventive therapy. The ideal duration of such therapy is unknown, but it is often stopped after a year. If the infection returns, preventive therapy may be continued indefinitely. If a woman of child-bearing age is taking an antibiotic, she should avoid pregnancy or talk to her doctor about whether to use an antibiotic that is safe during pregnancy in case she becomes pregnant.

Asymptomatic Bacteriuria

Asymptomatic bacteriuria is a condition in which larger than normal numbers of bacteria are present in the urine but symptoms do not result.

Asymptomatic bacteriuria is not normally treated because eradicating the bacteria can be difficult and complications are usually rare. Also, giving antibiotics can alter the balance of bacteria in the body, sometimes allowing bacteria to flourish that are more difficult to eliminate.

Did You Know…

Most people who have excess bacteria in the urine and have no symptoms should not be treated.

An exception is if the person has a condition that makes a urinary tract infection particularly risky. Such conditions may include pregnancy, a kidney transplant, taking drugs that suppress the immune system, or having a condition that suppresses the immune system (for example, AIDS, certain cancers, or having a low white blood cell count). For example, cystitis can seriously complicate pregnancy by ascending to the kidneys and causing pyelonephritis, leading to early labor. Also, a urinary tract infection can permanently damage one or both kidneys after a kidney transplant. A urinary tract infection can cause potentially fatal bloodstream infection in people whose immune system is suppressed by a drug or disorder. Sometimes, the immune system becomes suppressed after cancer chemotherapy. Asymptomatic bacteriuria is also sometimes treated in people who have certain kinds of kidney stones that cannot be eliminated and cause repeated urinary tract infections.