What Causes Pyelonephritis? Understanding the Key Risk Factors and Underlying Triggers

Pyelonephritis: A Serious Upper Urinary Tract Infection

Pyelonephritis is a potentially serious bacterial infection that affects the kidneys—specifically the renal pelvis and kidney tissue. Unlike simple bladder infections (cystitis), pyelonephritis is classified as an upper urinary tract infection (UTI) and requires prompt medical attention to prevent complications such as kidney scarring, sepsis, or chronic kidney disease.

Primary Cause: Ascending Bacterial Infection

The most common pathway for developing pyelonephritis is ascending infection—where bacteria travel upward from the urethra into the bladder, then through the ureters to reach the renal pelvis. Escherichia coli (E. coli) accounts for over 70–90% of cases, thanks to its ability to adhere to uroepithelial cells and evade immune clearance. Women are significantly more susceptible due to their shorter urethra, which allows easier bacterial access to the bladder—and subsequently, the kidneys—especially following untreated or recurrent cystitis.

Medical Procedures and Behavioral Risk Factors

Certain clinical interventions and lifestyle habits can compromise the body's natural defenses and increase vulnerability:

Urinary Catheterization

Indwelling catheters disrupt the sterile environment of the urinary tract, introducing pathogens directly into the bladder and facilitating rapid ascent toward the kidneys—particularly in hospitalized or long-term care patients.

Sexual Activity During Menstruation

Intercourse during menstruation may introduce bacteria and cause minor urethral or vaginal mucosal trauma, weakening local barriers and enabling bacterial entry. This risk is further elevated without proper hygiene practices before and after intercourse.

Anatomical and Structural Abnormalities

Structural irregularities in the urinary system create environments where urine stagnates—providing ideal conditions for bacterial colonization and proliferation:

• Congenital anomalies, such as ureteropelvic junction obstruction or vesicoureteral reflux (VUR), allow backward flow of infected urine from the bladder to the kidneys.
• Urethral strictures or kinks impede complete bladder emptying, increasing residual urine volume—a known risk factor for persistent infection.
• Kidney stones or urological tumors obstruct urinary outflow, leading to hydronephrosis and impaired immune surveillance in the renal pelvis.

Weakened Immunity and Systemic Vulnerabilities

While less common than ascending infection, hematogenous spread—where bacteria enter the bloodstream and seed the kidneys—is more likely in individuals with compromised host defenses:

Conditions such as uncontrolled diabetes mellitus, advanced HIV/AIDS, or active autoimmune disorders impair neutrophil function and antibody response. Similarly, prolonged use of systemic corticosteroids, chemotherapy agents, or other immunosuppressive medications significantly elevates the risk—not only of initial infection but also of recurrent or treatment-resistant pyelonephritis.

Prevention Starts With Awareness

Recognizing your personal risk profile—whether anatomical, behavioral, or immunological—is the first step toward prevention. Staying well-hydrated, practicing safe urinary habits (e.g., urinating after intercourse), seeking early treatment for lower UTIs, and discussing screening options with your healthcare provider if you have recurrent infections or known structural issues can all reduce your likelihood of developing this serious kidney condition.