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Pyelonephritis (Kidney Infections)

Kidney infection is a type of urinary tract infection (UTI) that commonly begins in your bladder and moves upstream to one or both of your kidneys.


A.Vogel Urinary Tract experts look at pyelonephritis. It's causes, symptoms and recommends solutions to help you. There's also a Q&A service where you can get answers to all your questions.

What Causes a Kidney Infection?

Usually, it starts with a bladder infection that spreads to the kidney.

Usually, bacteria called E. coli cause the infection to begin with. Other bacteria can also cause kidney infections.

It’s rare, but you can also have an infection that gets in through your skin, makes its way into your blood, then travels to your kidney.

You can get an infection after kidney surgery, too, but that’s very uncommon.

Acute Pyelonephritis

Infection and inflammation of the kidneys give rise to pain, often in the lumbar region. The symptoms of cystitis, accompanied by pain in the back or lumbar area, can indicate spread of the infection to the kidneys.

Despite the relatively long distance from the ureter, the ascending route of infection is the most common way for organisms to gain entry into the kidneys. Occasionally, infection of the kidneys can arise from microbes present in the blood. The most commonly infective organisms encountered in the kidneys are bacteria. 

When the kidneys are infected, inflammation and suppuration occurs with the production of pus and the destruction of nephrons. The outcome of this depends on the amount of tissue which has been damaged which is eventually replaced by fibrous tissue, causing scarring. 

This is the picture which is seen with ureteric reflux in young children. Urine, often infected, ‘shoots’ up the ureters and into the kidneys during micturition. This causes damage and scarring of the kidneys, which can then give rise to problems at a later age. If infection and inflammation does not resolve, or is recurrent, chronic pyelonephritis occurs.

Chronic Pyelonephritis

The phrase chronic pyelonephritis is used when repeated and recurrent episodes of infection occur, leading to chronic inflammation and scarring of the kidneys. 

The resultant damage to the kidneys causes a decline in kidney function, and the ability of the kidneys to control and manage fluids and electrolytes in the body is impaired. 

This situation is often described as renal impairment, and if allowed to progress, can lead to renal failure. The damage to renal tissue in chronic pyelonephritis can, in itself, lead to further recurrent attacks of infection, creating a ‘vicious cycle’. As well as this, it is not uncommon for this damage to give a predisposition to renal stones. 

Often, the infection and inflammation of the kidneys can go unnoticed for years. Scarring of the kidneys can be silent, and it is not uncommon for someone at the later stages of life to find that one of the two kidneys has stopped functioning, due to extensive scar tissue.

Symptoms of Pyelonephritis

Pyelonephritis may start with painful, urgent, or frequent urination. However, once the infection has spread to the kidney, signs of more severe illness usually result. They include:

  • Back pain or flank pain
  • Fever (usually present) or chills
  • Feeling sick (malaise)
  • Nausea and vomiting
  • Confusion (especially in the elderly)

Pyelonephritis may cause noticeable changes in the urine, such as:

  • Blood in the urine (hematuria)
  • Cloudy or foul-smelling urine
  • Pain when urinating
  • Increased frequency or urgency of urination

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