What is Pyelonephritis?

The urinary system is the drainage system of the human body for removing waste and extra water. It consists of the two kidneys, two ureters (which connect the kidneys and the bladder), a bladder and a urethra. Urinary tract infections represent the most common urologic disease with enormous direct and indirect costs of medical care for affected individuals.

Pyelonephritis can be defined as an infection of the renal pelvis and kidney that is a consequence of the ascent of pathogenic bacteria from the bladder to the kidneys via the ureters. This condition is a potentially life-threatening infection that can result in scarring of the kidney.

Types of pyelonephritis

Primary acute pyelonephritis represents the inflammation of the renal parenchyma and affects one or both kidneys. It is an uncomplicated form of disease which most commonly occurs in females as a result of the urinary tract infection, with 90% of cases caused by a common bacterium Escherichia coli.

This condition can present with low back or loin pain, localized flank, high fever, rigors and sweats, but also with a headache, nausea, vomiting, general malaise and renal swelling. Furthermore, symptoms of lower urinary tract infection may be present, and the manifest disease can develop promptly over one or two days.

Secondary acute pyelonephritis in women with urinary tract abnormalities, pregnant females, children and men is generally considered to be more serious condition than acute primary pyelonephritis. Predisposing factors can include catheterization of the bladder, immunosuppressive therapy following kidney transplantation, megaureter, neurogenic bladder, diabetes mellitus, polycystic kidney disease and certain tumors.

Presentation of secondary acute pyelonephritis (sometimes referred to as complicated acute pyelonephritis) resembles primary infections, albeit this disease can also be painless. Escherichia coli remains the most frequently encountered pathogen, but other Gram-negative bacilli and even Gram-positive organisms (namely in males) may be involved.

Chronic pyelonephritis represents diffuse, interstitial inflammatory disease of the kidney, which is not always infectious in origin. Emphysematous pyelonephritis is a serious, necrotizing renal parenchymal infection characterized by the production of intraparenchymal gas. Chronic destructive granulomatous process that stems from atypical, incomplete immune response to subacute bacterial infection is known as xanthogranulomatous pyelonephritis.

Epidemiology and risk factors

Epidemiologic data on the incidence of different types of pyelonephritis are limited. It is known that the incidence of acute pyelonephritis is highest in otherwise healthy women between 15 and 29 years of age, followed by infants and older persons. In the United States, this type of pyelonephritis accounts for at least 250,000 office visits and 200,000 hospital admissions every year.

All types of pyelonephritis are appreciably more common in females than in males, although this difference wanes with increasing age – especially in patients aged 65 years and older. In females the age distribution is trimodal (with peaks in newborn period, between 15-35 years of age and 80 years of age), while in males the age distribution is bimodal (i.e. there is no middle peak).

New studies indicate that approximately 20% of renal transplant patients suffer from acute pyelonephritis. Furthermore, between 30% and 50% of renal transplant patients suffer from acute pyelonephritis within 2 months after surgery. Pregnant women have a disease incidence of 1% to 2%, but possibly high as 30% in cases of untreated asymptomatic bacteriuria.

Individuals with urinary tract infection and those with a structural (or anatomic) problem in the urinary tract are at the highest risk for pyelonephritis. The single most important risk factor for the development of this condition is vesicoureteral reflux (VUR), which is the abnormal flow of urine from the bladder to the upper urinary tract.

Further Reading

Last Updated: Jun 13, 2023

Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.


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