How do you differentiate between bacterial and viral orchitis?

Differentiating Bacterial from Viral Orchitis

The differentiation between bacterial and viral orchitis relies primarily on patient age, clinical presentation timing, associated symptoms, and targeted microbiological testing—with bacterial orchitis typically showing gradual onset with urinary symptoms or STI risk factors, while viral orchitis (predominantly mumps) presents with acute onset following parotitis and positive mumps serology. 1

Age-Based Pathogen Distribution

Patient age is the single most important initial discriminator:

• Men <35 years with bacterial orchitis: Most commonly caused by sexually transmitted pathogens (Chlamydia trachomatis and Neisseria gonorrhoeae), presenting with gradual onset and urinary tract symptoms 1

• Men ≥35 years with bacterial orchitis: Typically caused by gram-negative and gram-positive organisms (similar to UTI pathogens like E. coli), often by extension from epididymitis 1

• Viral orchitis: Most frequently mumps virus, occurring across age groups but particularly in unvaccinated individuals 1

Clinical Presentation Patterns

Bacterial orchitis characteristics:

• Gradual onset of scrotal pain over days 2
• Associated lower urinary tract symptoms (dysuria, frequency, urgency) 2
• Often concurrent epididymitis (epididymo-orchitis) 1
• Fever present but typically develops with the scrotal symptoms 2
• History of recent UTI, urologic instrumentation, or sexual exposure 3

Viral orchitis characteristics:

• Acute onset of scrotal swelling and pain 4
• Preceding parotitis in 82% of mumps orchitis cases, with average 10-day interval between parotitis and orchitis 4
• High fever (>38.5°C) at presentation 4
• Absence of urinary tract symptoms 4
• Bilateral involvement more common than in bacterial cases 5

Diagnostic Testing Algorithm

For suspected bacterial orchitis:

1. In men <35 years: Obtain nucleic acid amplification tests (NAATs) for C. trachomatis and N. gonorrhoeae—these are the most sensitive and rapid diagnostic procedures 1

2. In men ≥35 years:

   • Midstream urine culture (closed sterile container, transport within 1 hour or refrigerate at 4°C) 1
   • Consider urethral culture if discharge present 1

3. If surgical tissue obtained: Culture for bacterial pathogens with antimicrobial susceptibility testing 1

For suspected viral orchitis:

1. Mumps serology: IgM antibodies for acute infection, or acute and convalescent IgG serology 1

2. Consider testing for other viral causes if mumps negative: Coxsackie virus, rubella virus, Epstein-Barr virus, and varicella zoster virus 1

Laboratory Findings That Help Differentiate

Bacterial orchitis:

• Positive urine culture (≥10,000 CFU/ml) in 20% of cases 6
• Elevated urine white blood cells, bacteria, leukocyte esterase, and nitrites 6
• Positive NAAT for STIs in 13.8% of diagnosed cases, with chlamydia (12.3%) more common than gonorrhea (3.1%) in men <35 years 6

Viral orchitis:

• Significantly elevated C-reactive protein (mean 140 mg/L in mumps orchitis) 4
• Positive mumps IgM or rising IgG titers 1
• Negative bacterial cultures 4

Critical Pitfalls to Avoid

Common diagnostic errors:

• Undertesting for STIs: Only 50% of men diagnosed with epididymo-orchitis receive appropriate STI testing, missing 13.8% of cases 6

• Assuming all cases in young men are bacterial: Viral orchitis can occur at any age, particularly in unvaccinated individuals 4

• Premature antibiotic administration: While bacterial orchitis cannot be excluded at initial presentation, obtaining appropriate cultures and NAATs before antibiotics is essential 1, 4

• Missing mumps in vaccinated populations: Despite vaccination, mumps has not been eradicated and mini-epidemics still occur 4

Histopathologic Differentiation (When Biopsy/Orchiectomy Performed)

Viral orchitis shows:

• Preserved testicular architecture with hemorrhage and edema 5
• Patchy lymphohistiocytic infiltrate within seminiferous tubules (intratubular infiltrate predominates) 5
• Mixture of CD68+ histiocytes and CD3+ T cells, with few B cells and granulocytes 5

This pattern helps avoid misdiagnosis as intratubular germ cell neoplasia or lymphoma when specimens are submitted for pathologic evaluation 5.