Pyelonephritis is a bacterial infection of one or both kidneys.
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 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 Bacterial Infections in Infants and Children: Urinary Tract Infection in Children(UTI)). 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.
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.
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:
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 xanthogranulomatous 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.
Last full review/revision September 2007 by Stewart Shankel, MD