What is pyelonephritis?

Quick Answer
Inflammation of the kidney as the result of a bacterial infection in the bladder.
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Causes and Symptoms

The urinary tract consists of the kidneys, ureters, bladder, and urethra. The kidneys serve as filters that excrete unnecessary substances from the blood and retain essential ones. For example, the kidneys excrete excesses of water, electrolytes, or metabolites of drugs and other ingested substances that might otherwise build up to harmful levels in the body. Conversely, the kidneys can also preserve these substances when necessary. This is the case in dehydration, in which the water in the bloodstream is actively reabsorbed to prevent further losses.

The excess water and waste products filtered from the blood becomes urine, which drains from the kidneys through narrow tubes known as ureters. These tubes connect the kidneys to the bladder, which is the storage reservoir for the urine. At the appropriate time, urine is released from the bladder and carried through the urethra, another small tube passing through the genitals, to exit the body.

Because of the differences in male and female anatomy, urinary tract infections (UTIs) arise more frequently in women. The female urethra is short, approximately four centimeters in length, and is located in close proximity to the vaginal and anal openings. In contrast, the male urethra travels the length of the penis and is distinctly separate from the anal opening.

Although harmless bacteria are normally found living in the vaginal area, it is typically bacteria from the bowel that cause UTIs. Given the proximity of the anal opening to the vagina, it is relatively easy for these bacteria to gain entry. Other factors that promote bacterial colonization of the vaginal and urethral openings include antibiotic use, genital infections, and use of contraceptives such as diaphragms. These bacteria may also be pushed into the bladder by pressure on the woman’s urethra that occurs during sexual intercourse. Although the simple act of urination often serves to flush out these organisms, the number of bacteria, the virulence of the strain, and the individual’s immune system are all important factors in whether an infection develops.

In men, the urethra passes through the prostate gland prior to entering the penis. The prostate gland typically enlarges with age and can encroach on the urethra, making drainage of urine more difficult. When urine is not drained completely from the bladder, any bacteria that have entered can proliferate, increasing the risk of infection. Therefore, men with prostate enlargement are predisposed to UTIs. Men who participate in anal intercourse are also at increased risk.

Other important risk factors for UTIs include pregnancy, neurologic diseases affecting bladder muscle function, presence of a urinary catheter, and (in children) the condition of vesicoureteral reflux.

In general, it is thought that pyelonephritis develops when bacteria present in the bladder are able to ascend upward toward the kidney. Often, the symptoms of a bladder infection, such as discomfort with urination, frequent urination, and sensation of an urgent need to urinate, precede such symptoms as fever, chills, flank pain, nausea, vomiting, and diarrhea. The latter symptoms are suggestive of pyelonephritis rather than simple bladder infection.

Treatment and Therapy

The majority of patients with pyelonephritis respond well to antibiotic therapy. Usually, oral antibiotics are appropriate; however, in cases where vomiting precludes their use, intravenous (IV) antibiotics are used. In cases where patients are severely ill or dehydrated, hospitalization may be required. Treatment for ten to fourteen days is standard. There are typically no long-term consequences once the infection is treated.

Certain factors may complicate the treatment of pyelonephritis, requiring more aggressive antibiotic therapy, hospitalization, and possibly invasive procedures. Patients who have an abnormal urinary tract because of the presence of a catheter or other external drainage tube are more likely to harbor antibiotic-resistant organisms. Similarly, patients with diabetes or immune system disorders are at risk for resistant germs.

Patients with an obstruction in the urinary tract as a result of kidney stones, cancer, prostate enlargement, or other anatomical abnormalities may require surgical intervention to remove the blockage. It is difficult to eradicate the bacteria completely unless infected urine is removed by drainage.

Patients with underlying kidney diseases require more aggressive care to ensure no further loss of renal function. Pregnant women have a higher incidence of complications from pyelonephritis and typically require hospitalization. Antibiotics are also selected differently for these women to avoid harm to the fetus.

Perspective and Prospects

The development of newer antibiotics that are highly active against bacteria responsible for most UTIs has made successful treatment of pyelonephritis straightforward. In addition, these antibiotics can achieve high levels in the bloodstream and kidneys when taken orally, making treatment at home rather than in the hospital safe and effective for many patients.

Procedures done with fiber-optic technology allow a urologist to extract an obstructing kidney stone without a surgical incision. Miniature cameras can be passed upward through the urethra and bladder and into the ureter, and miniature instruments can be used to remove the stone. In addition, specialized radiologists are able to pass a drainage tube directly into the kidney with x-ray guidance, without the need for an open surgical procedure. Both of these interventions are useful in cases of pyelonephritis complicated by obstruction.

Patients who experience frequent UTIs should use preventive measures such as copious fluid intake, urinating after intercourse, wiping “front to back” after urination, and urinating often throughout the day. Some women may require a prophylactic dose of antibiotic after intercourse.

A 2011 study funded by the National Center for Complementary and Alternative Medicine, part of the National Institutes of Health, found that, contrary to expectations, regular cranberry juice consumption failed to prevent UTI recurrence. The study was a double-blind, randomized, placebo-controlled trial conducted on otherwise healthy college-aged women and was the largest to test the efficacy of cranberry juice in preventing UTIs from recurring in this high-risk group.


Beers, Mark H., et al., eds. The Merck Manual of Diagnosis and Therapy. 18th ed. Whitehouse Station, N.J.: Merck Research Laboratories, 2006.

Longo, Dan Louis., et al., eds. Harrison’s Principles of Internal Medicine. 18th ed. New York: McGraw-Hill, 2012.

Parker, James N., and Philip M. Parker, eds. The Official Patient’s Sourcebook on Pyelonephritis.ICON Group, 2007.

National Kidney and Urologic Diseases Information Clearing House. "Cranberry Juice Fails to Prevent Recurrence of Urinary Tract Infections." National Kidney and Urologic Diseases Information Clearinghouses: Urologic Diseases Research Update, Summer 2011 , September, 2011.

Savitsky, Diane. "Kidney Infection." Health Library, November 26, 2012.

Shaikh, Nader and Nina Tolkoff-Rubin.. "Pyelonephritis: Kidney Infection." National Institutes of Health: National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC), June 11, 2012.