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Kidney infection (pyelonephritis) is a specific type of urinary tract infection (UTI) that generally begins in your urethra or bladder and travels up into your kidneys. If not treated properly, kidney infection can permanently damage your kidneys or spread to your bloodstream and cause a life-threatening infection. Prompt medical attention is required.

Your kidneys filter waste from your blood, adjust blood levels of many substances, and conserve or excrete water from your system depending on your body's needs. Urine is the product of this filtration and renal tubular function.

From the kidneys, urine passes through tubes called ureters, which lead to the bladder. Your bladder stores urine until it exits your body through the urethra. Kidney infection usually begins when bacteria enter the urinary tract through the urethra and then start to multiply.

Antibiotics are the typical treatment for kidney infection. You can take a number of steps to help prevent a kidney infection in the first place.

Kidney infection may cause one or more of these signs and symptoms:

• Frequent urination
• Strong, persistent urge to urinate
• Burning sensation or pain when urinating
• Abdominal pain or pressure
• Cloudy urine with a strong odor
• Pus or blood in your urine (hematuria)
• Inability to urinate (urine retention)
• Need to urinate during the night (nocturia)
• Back, side (flank) or groin pain
• Fever

Severe kidney infection also may involve:

• High fever - body temperature of 101 F (38.3 C) or greater
• Shaking chills
• Night sweats
• Extreme fatigue
• Nausea or vomiting
• Confusion

In infants and young children, signs and symptoms of kidney infection may include:

• Irritability
• Loss of appetite
• Loose bowel movements
• New episodes of bed-wetting (enuresis)

In older adults, mental changes, such as confusion, sometimes are the only signs of kidney infection.

Kidney infection typically occurs when bacteria enter your urinary tract through the urethra and begin to multiply. Bacteria from an infection elsewhere in your body also can spread through your bloodstream to your kidneys. Kidney infection is unusual through this route, but it can occur in some circumstances — for instance, when a foreign body, such as an artificial joint or heart valve, gets infected. Rarely, kidney infection results after kidney surgery.

Bacteria in the urine don't always signify an infection. Some people, especially older adults, may have bacteria in the bladder that don't cause any signs or symptoms or harm, and therefore doesn't require treatment. This condition is known as asymptomatic bacteriuria.

Your kidneys have infection-fighting properties that ordinarily inhibit the growth of bacteria. However, certain factors increase the chances that bacteria will enter your urinary tract, take hold and multiply into a full-blown infection.

• Female anatomy. Women have a greater risk of kidney infection than do men. A woman's urethra is much shorter than a man's, so bacteria have less distance to travel from outside the body to the bladder. The proximity of the urethra to the vagina and anus also creates more opportunities for bacteria to enter the bladder. Once in the bladder, any infection can spread to the kidneys.
• Urinary obstruction. Anything that impedes the flow of urine or reduces your ability to completely empty your bladder when urinating, such as a kidney stone, structural abnormalities in your urinary system, or, in men, an enlarged prostate gland can increase your risk of kidney infection.
• Weakened immunity. Medical conditions that impair your immune system, such as cancer, diabetes or the human immunodeficiency virus (HIV), increase your risk of kidney infection. Certain medications, such as drugs taken to prevent rejection of transplanted organs, have a similar effect.
• Sensory impairment. Nerve or spinal cord damage may block the sensations of a bladder infection soon enough to keep it from becoming a kidney infection.
• Prolonged catheterization. Urinary catheters are tubes used to drain urine from the bladder. You may have a catheter placed in your bladder during and after some surgical procedures and diagnostic tests. A catheter may be used continuously if you are confined to a bed.
• Vesicoureteral reflux. In this condition, small amounts of urine flow from your bladder back up into your ureters and kidneys. People with vesicoureteral reflux may have frequent kidney infections during childhood and are at higher risk of kidney infection during both childhood and adulthood.

If you have signs or symptoms of kidney infection, contact your doctor as soon as possible. If your doctor suspects you have kidney infection, he or she will likely ask for a urine sample to determine whether bacteria, blood or pus is in your urine. There's no simple test to differentiate kidney infection from bladder infection (cystitis), but the presence of fever and upper back pain suggest that the infection extends into your kidney.

If you have a persistent kidney infection, your doctor may order X-ray tests to look for abnormalities in your urinary system.

If left untreated, kidney infection can lead to potentially serious complications, such as permanent damage to your kidneys or blood poisoning (septicemia), which can be life-threatening. Your kidneys filter waste from your blood and then return your blood to the rest of your body. If you have a kidney infection, the bacteria can spread as the kidneys return blood to circulation.

Young children, older adults and people with weakened immune systems are at greater risk of complications caused by kidney infection. Women who develop kidney infection during pregnancy may have an increased risk of delivering low birth weight or premature babies. Young children and older adults have the greatest risk of kidney damage from kidney infection because their symptoms are often overlooked or mistaken for other conditions.

If your symptoms are typical and you're generally in good health, antibiotics are the first line of treatment for kidney infection. Which drugs you use and for how long depends on your health condition and the bacteria found in your urine tests.

Drugs commonly recommended for treating kidney infection include:

• Amoxicillin (Amoxil, Trimox)
• Quinolones such as ciprofloxacin (Cipro) and norfloxacin (Noroxin)
• The antibiotic combination sulfamethoxazole and trimethoprim (Bactrim, Septra)

Be sure your doctor is aware of any other medications you're taking or any allergies you have. This will help him or her select the best treatment for you.

Usually, the signs and symptoms of kidney infection begin to clear up within a few days of treatment. But you may need to continue antibiotics for a week or more. Take the entire course of antibiotics recommended by your doctor to ensure that the infection is completely eradicated.

Recurrent and severe infections
Recurrent kidney infections may result from an underlying medical problem, such as a structural abnormality. Your doctor may refer you to a kidney specialist (nephrologist) or urinary surgeon (urologist) for an evaluation to determine if urologic abnormalities may be causing your infections. A structural abnormality may need to be surgically repaired.

For severe kidney infection, hospitalization and treatment with intravenous antibiotics may be necessary. When recurrences of kidney infection are frequent or the infection becomes chronic, your doctor will likely recommend that you seek medical care from a specialist who can identify underlying and potentially treatable causes.

You can take steps to reduce your risk of kidney infection. Women, in particular, may benefit from these suggestions:

• Drink plenty of fluids, especially water. Drinking plenty of liquids can help remove bacteria from your body when you urinate. In particular, cranberry juice may have infection-fighting properties. However, don't drink cranberry juice if you're taking the blood-thinning medication warfarin (Coumadin). Possible interactions between warfarin and cranberry juice can lead to bleeding.
• Urinate frequently. Avoid holding back when you feel the urge to void.
• Empty your bladder after intercourse. Urinating as soon as possible after intercourse helps to clear bacteria from the urethra, reducing your risk of infection. Also, drink a full glass of water to help flush bacteria from your urinary system.
• Shower rather than bathe. If you're susceptible to infections, taking a shower instead of a tub bath can help prevent excess moisture that can contribute to infection.
• Wipe carefully. For women, wiping from front to back after urinating and after a bowel movement helps to prevent bacteria in your anal region from spreading to your urethra.
• Wash gently. Wash the skin around your vagina and anus carefully every day. But don't use harsh soaps or wash vigorously. The delicate skin around these areas can become irritated.
• Avoid using feminine products in your genital area. Using feminine products, such as deodorant sprays or douches, in your genital area can irritate your urethra.

These tips may help to reduce pain or discomfort as you recover from kidney infection:

• Try heat. Place a heating pad on your abdomen, back or side to reduce feelings of pressure or pain.
• Use pain medicine. For fever or discomfort, take a nonaspirin pain reliever that contains acetaminophen (Tylenol, others) as directed by your doctor, or take a prescription medication that he or she provides.
• Stay hydrated. Drinking plenty of fluids will help to flush bacteria from your urinary tract. Avoid citrus juices, coffee, alcohol and soft drinks with caffeine until your infection has cleared. These products can aggravate your frequent or urgent need to urinate.

If you have recurrent kidney infections, let your doctor know. Together, you can develop a strategy to reduce recurrences.