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Epidemiology of bacterial meningitis

With almost 8000 cases and 2000 deaths occurring annually, bacterial meningitis continues to be a significant source of morbidity and mortality. The attack rate per year in the United States is reportedly 0.6-4 cases per 100,000 population.

Meningococcal meningitis is endemic in parts of Africa, India, and other developing areas. Periodic epidemics occur in the so-called sub-Saharan "meningitis belt," as well as among religious pilgrims traveling to Saudi Arabia for the Hajj. In parts of Africa, widespread epidemics of meningococcal meningitis occur regularly. In 1996, the biggest wave of meningococcal meningitis outbreaks ever recorded arose in West Africa. An estimated 250,000 cases and 25,000 deaths occurred in Niger, Nigeria, Burkina Faso, Chad, and Mali.

The incidence of neonatal bacterial meningitis is 0.25-1 case per 1000 live births. In addition, the incidence is 0.15 case per 1000 full-term births and 2.5 cases per 1000 premature births. Approximately 30% of newborns with clinical sepsis have associated bacterial meningitis.

The frequency of H influenzae type B (HIB) disease has been markedly reduced, but N meningitidis causes approximately 4 cases per 100,000 children aged 1-23 months. The risk of secondary meningitis is 1% for family contacts and 0.1% for daycare contacts. The rate of meningitis caused by S pneumoniae is 6.5 cases per 100,000 children aged 1-23 months.

Previously, HIB, N meningitidis, and S pneumoniae accounted for more than 80% of cases of bacterial meningitis. Since the late 20th century, however, the epidemiology of bacterial meningitis has been substantially changed by multiple developments.

An increased incidence of HIV infection worldwide resulted in a correspondingly increased frequency of meningitis caused by encapsulated organisms (primarily S pneumoniae).^[13]

Even so, the overall incidence of bacterial meningitis declined from 1.9 to 1.5 cases per 100,000 between 1998 and 2003.^[14] This was partially due to the widespread use of the HIB vaccination, which decreased the incidence of HIB meningitis by more than 90% (see Table 3 “Changing Epidemiology of Acute Bacterial Meningitis in the United States,” below), nearly eliminating it in many developed countries where routine HIB vaccination is used.

Because the frequency of bacterial meningitis in children has declined, the condition is becoming more of a disease of adults. The median age for persons with bacterial meningitis was 25 years in 1998, while in 1986, it was 15 months.^[15]

A total of 255 cases of invasive H influenzae disease among children younger than 5 years were reported to the CDC in 1998, in contrast to 20,000 cases among children in 1987. This shift has reportedly been less dramatic in developing countries, where the use of the HIB vaccine is not as widespread.

Table 3. Changing Epidemiology of Acute Bacterial Meningitis in the United States* (Open Table in a new window)

The introduction of vaccines against S pneumoniae has substantially reduced the incidence of pneumococcal meningitis in children. The routine screening of group B streptococcus in pregnant women may have also reduced the incidence of meningitis due to S agalactiae. Routine vaccination against meningococcus with the use of serogroup C meningococcal conjugate vaccine may also reduce the incidence of N meningitidis infections. These efforts, together with the use of the HiB vaccine, has reduced the incidence of meningitis in recent years. During a 1998-2007 survey, the incidence of meningitis declined by 31%.^[16]

Excluding meningococcal meningitis, patients younger than 5 years or older than 60 years are at increased risk for bacterial meningitis, despite the above-mentioned shift in median age for persons with the disease.

Newborns are at highest risk for acute bacterial meningitis. After the first month of life, the peak incidence is in infants aged 3-8 months. In addition, statistics show an increased incidence in persons aged 60 years and older, independent of other factors. Annual incidences are 1.7-7.2 cases per 100,000 adults, and the mean annual incidence has been reported as 3.8 cases per 100,000 adults. Of patients with bacterial meningitis, 61% had no previous or present accompanying diseases that may have predisposed them to meningitis.

Depending on their age, individuals are also predisposed to other etiologic agents (see Table 4 “The Most Common Bacterial Pathogens Based on Age and Predisposing Risks,” below). E coli K1 and S agalactiae meningitis are common among the neonatal group, and L monocytogenes meningitis is common among neonates and elderly individuals. (The development of neonatal meningitis is related to labor delivery; it results from colonized pathogens in the maternal intestinal or genital tract, immaturity, and environment.)

The attack rate for bacterial meningitis is reportedly 3.3 male cases per 100,000 population, compared with 2.6 female cases per 100,000 population. (In meningitis caused by the mumps virus, males and females are affected equally.) In neonates, the male-to-female ratio is 3:1.

The epidemiology of bacterial meningitis continues to evolve as preventive strategies are implemented.

Table 4. The Most Common Bacterial Pathogens Based on Age and Predisposing Risks (Open Table in a new window)

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