What are the common causes of meningitis in the general population, considering factors such as age, medical history, and geographic location?

Causes of Meningitis

Bacterial Meningitis

Streptococcus pneumoniae is the dominant bacterial pathogen causing meningitis in adults, accounting for approximately 59% of all bacterial meningitis cases and up to 72% in adults over 50 years of age. 1, 2, 3, 4

Age-Specific Bacterial Causes

Neonates (0-6 weeks):

• Group B Streptococcus and Escherichia coli together cause approximately two-thirds of neonatal meningitis cases 5, 2
• Early neonatal meningitis (first week) results from vertical transmission through the birth canal, while late neonatal meningitis (weeks 2-6) occurs through nosocomial or horizontal transmission 5, 2
• Listeria monocytogenes causes only 2% of neonatal cases despite historical emphasis 2

Children and Adolescents (>6 weeks to young adults):

• S. pneumoniae and N. meningitidis serogroup B are now equally common as leading causes 2
• Haemophilus influenzae type b has virtually disappeared since 1990s vaccination programs 5, 2
• Meningococcal disease shows a second peak in adolescents and early adults, with highest incidence between ages 16-25 1, 5
• Neisseria meningitidis causes 93% of infections in the 16-20 year age group 6

Adults (18-50 years):

• S. pneumoniae causes the majority of cases 2, 3
• Middle-aged adults (45-64 years) have the highest overall incidence at 1.21 per 100,000 population 5, 3
• N. meningitidis (mostly serogroup B) is found primarily in adolescents and young adults 2

Older Adults (>50-60 years):

• S. pneumoniae becomes increasingly predominant, accounting for approximately 72% of cases in adults over 50 2, 3
• Listeria monocytogenes emerges as a critical pathogen in those over 60 years, though it causes only 5% of cases overall 1, 2, 3
• Critical caveat: In immunocompromised patients, cancer patients, diabetics, and alcoholics, Listeria becomes the second most common pathogen after S. pneumoniae, found in 20-40% of high-risk cases 5, 2, 3
• Incidence increases by 3% annually in patients over 65 years 5

Other Bacterial Causes

Less common but important bacterial pathogens include: 1

• Streptococcus pyogenes
• Enterococcus species
• Non-type B Haemophilus influenzae (increasing in recent years) 4
• Gram-negative bacteria (Enterobacter, Klebsiella, Pseudomonas) in specific risk groups
• Mycobacterium tuberculosis in those with appropriate risk factors

Risk Factors for Specific Bacterial Pathogens

Pneumococcal meningitis is associated with: 1

• Age over 50 years
• Co-existing upper respiratory tract infection (otitis media, sinusitis)
• Skull fracture or CSF leak (also increases risk of recurrent meningitis)
• Asplenia

Listeria meningitis risk factors include: 1, 5

• Age over 60 years
• Immunocompromise from illness or medication
• Alcohol dependency
• Diabetes mellitus
• Malignancy
• HIV infection (though less common than cryptococcal meningitis in this population)

Meningococcal disease risk factors include: 1

• Adolescence and early adulthood (ages 16-25)
• Complement deficiency
• Asplenia

Viral Meningitis

Viral meningitis is more common than bacterial meningitis, especially in young adults, particularly women in their 20s-40s. 1

Common Viral Causes

• Herpes simplex virus (HSV) is the most commonly diagnosed cause of viral encephalitis, with an annual incidence of 1 in 250,000 to 500,000 1
• Most HSV encephalitis is due to HSV type 1, but approximately 10% is due to HSV type 2 1
• HSV-2 is the commonest cause of recurrent lymphocytic meningitis 1
• Enteroviruses most often cause aseptic meningitis but can also cause encephalitis 1
• Varicella zoster virus (VZV) is a relatively common cause, especially in immunocompromised patients 1
• Cytomegalovirus (CMV) occurs almost exclusively in immunocompromised patients 1

Geographic Viral Considerations

Travel history may determine rarer viral causes including: 1

• Toscana Virus (Mediterranean)
• Tick Borne Encephalitis Virus (Central and Eastern Europe)
• West Nile Virus (USA)

Fungal Meningitis

Cryptococcal meningitis is the most common fungal cause, particularly in HIV-positive patients with CD4 count <100 × 10⁶, though it should be considered in anyone with CD4 count <200 × 10⁶ or <14%. 1, 7

• Cryptococcus neoformans is the predominant fungal pathogen causing CNS infections 8, 7
• Other fungal genera are capable of causing CNS infections but are less common 7
• Fungal meningitis has increased in relevance due to the rise in immunocompromised populations, including HIV/AIDS patients and those on immunosuppressive medications 7

Epidemiologic Shifts and Current Trends

The epidemiology of bacterial meningitis has dramatically shifted due to vaccination programs, and clinicians must adjust their empirical coverage based on current epidemiology rather than outdated textbook descriptions. 5, 2

• Meningococcal serogroup C cases sharply decreased following Men C vaccine introduction 2
• Pneumococcal conjugate vaccine (PCV13) introduction reduced incidence by 64% in previously healthy children 5
• Overall bacterial meningitis incidence declined from 1.3 per 100,000 (2008-2009) to 0.7 per 100,000 (2020-2021), followed by resurgence to 1.0 per 100,000 (2022-2023) 4
• In 34%-74% of cases, no pathogen is identified despite thorough investigation 1

Mortality and Prognosis

• Overall mortality rate for community-acquired bacterial meningitis is approximately 20% for all causes 1
• Pneumococcal meningitis carries mortality up to 30%, increasing with age 1, 3
• Among bacterial meningitis cases with outcome data, 11% died in recent U.S. surveillance (2008-2023) 4
• Case fatality ratios have remained stable despite advances in medical care 4