What are the risk factors for developing bacterial meningitis, particularly in terms of age, medical conditions, and vaccination history?

Risk Factors for Bacterial Meningitis

Bacterial meningitis risk factors are primarily determined by age, with specific pathogens predominating in distinct age groups, alongside immunocompromising conditions, anatomical defects, and vaccination status that collectively determine individual susceptibility.

Age-Related Risk Factors

Age is the single most important determinant of both susceptibility and causative pathogen in bacterial meningitis 1.

Neonates (0-6 weeks)

• Group B Streptococcus and E. coli cause approximately two-thirds of neonatal meningitis cases 2
• Early neonatal meningitis (first week) results from vertical transmission through the birth canal 2
• Late neonatal meningitis (weeks 2-6) occurs through nosocomial or horizontal transmission 2

Young Adults (16-25 years)

• Meningococcal disease shows a second peak in adolescents and early adults, with highest incidence between ages 16-25 1
• Viral meningitis is more common than bacterial in this age group, especially in women aged 20s-40s 1

Middle-Aged Adults (45-64 years)

• This age group has the highest overall incidence of bacterial meningitis at 1.21 per 100,000 population 1, 3
• S. pneumoniae becomes the predominant pathogen 1, 3

Older Adults (>50-60 years)

• Pneumococcal disease accounts for approximately 72% of cases in adults over 50 3
• Listeria monocytogenes emerges as a critical pathogen, particularly in those over 60 years 1, 2, 3
• Incidence increases by 3% annually in patients over 65 years 1
• Mortality reaches up to 30% in pneumococcal meningitis in this age group 1

Medical Conditions and Immunocompromised States

High-Risk Medical Conditions

• Immunosuppression (cancer, immunosuppressive therapy, HIV/AIDS) increases risk substantially 1, 4
• In immunocompromised patients, Listeria becomes the second most common pathogen after S. pneumoniae, found in 20-40% of high-risk cases 2, 3
• Alcoholism significantly increases susceptibility, particularly to pneumococcal and Listeria infections 1
• Diabetes mellitus increases risk, with Listeria being more common in diabetic patients 2, 3
• Chronic kidney disease and other chronic medical conditions elevate risk 4

Anatomical Defects

• Skull fracture or CSF leak creates direct access for pathogens, particularly pneumococcus, and represents a risk factor for recurrent meningitis 1
• Cochlear implants and other neurosurgical hardware increase susceptibility 4
• Anatomical asplenia or functional asplenia dramatically increases risk of encapsulated bacterial infections 4

Genetic Factors

• Host genetic factors are major determinants of susceptibility to meningococcal and pneumococcal disease 5
• Complement deficiencies (particularly terminal complement pathway defects) markedly increase meningococcal disease risk 4
• Properdin deficiency increases susceptibility to invasive meningococcal infections 4

Vaccination History

Incomplete or Absent Vaccination

• Lack of H. influenzae type b vaccination historically was a major risk factor, though this pathogen has virtually disappeared since 1990s vaccination programs 2
• Absence of pneumococcal conjugate vaccine increases risk, as vaccine introduction reduced incidence by 64% in previously healthy children 6
• No meningococcal C vaccination increases risk, as conjugated vaccine led to virtual disappearance of serogroup C disease 1
• Unvaccinated status against pneumococcus is particularly important in adults over 50 and those with chronic conditions 1

Post-Vaccine Era Considerations

• Following vaccine introduction, patients present with more premorbid conditions and different bacterial causes 6
• Serotype replacement has emerged as a concern, with non-vaccine serotypes causing disease after widespread vaccination 1
• The incidence in immunosuppressed patients increased by 3% annually post-vaccine introduction 6

Environmental and Exposure Risk Factors

Close Contact Exposure

• Household contacts of patients with meningococcal disease require chemoprophylaxis 7
• Crowded living conditions (military barracks, dormitories) increase meningococcal transmission risk 4

Recent Infections

• Co-existing upper respiratory tract infection increases risk, particularly for pneumococcal meningitis 1
• Recent viral infection may predispose to secondary bacterial meningitis 4

Clinical Pitfalls and Caveats

Critical warning: In patients over 60 years, immunocompromised patients, cancer patients, diabetics, and alcoholics, empirical ampicillin must be added to cover Listeria, as it becomes the second most common pathogen in these populations 2, 3. Failure to cover Listeria in these high-risk groups can result in treatment failure and increased mortality.

Important consideration: The epidemiology has dramatically shifted due to vaccination programs—historical data showing H. influenzae type b as a major pathogen is no longer applicable in vaccinated populations 2. Clinicians must adjust their empirical coverage based on current epidemiology rather than outdated textbook descriptions.

Age threshold nuance: While guidelines vary slightly, the threshold for adding Listeria coverage ranges from >50 to >60 years depending on the source 1, 3. The most conservative and safest approach is to add ampicillin for patients over 50 years with any additional risk factors, and routinely for all patients over 60 years 1, 3.