L. monocytogenes, which together account for approximately 75 percent of sporadic cases as noted by Thigpen and coworkers. As discussed in the introduction, the following are less frequent causes: Staphylococcus aureus and group A (Streptococcus pyogenes) and group D streptococci, usually in association with brain abscess, epidural abscess, head trauma, neurosurgical procedures, or cranial thrombophlebitis; E. coli and group B streptococci in newborns; Pseudomonas and the Enterobacteriaceae, such as Klebsiella, Proteus, which are usually a consequence of lumbar puncture, spinal anesthesia, or shunting procedures to relieve hydrocephalus. Less-common meningeal pathogens include Salmonella, Shigella, Clostridium,
Neisseria gonorrhoeae, Bacillus cereus, and Acinetobacter calcoaceticus. In endemic areas, mycobacterial infections (to be considered further on) are as frequent as those caused by other bacterial organisms and they now assume greater importance as the number of immunosuppressed persons increases.
Pneumococcal, influenzal (H. influenzae), and meningococcal forms of meningitis have a worldwide distribution, occurring mainly during the winter and early spring and, in the case of the first two, also in the fall, and predominating slightly in males. Each has a relatively constant incidence, although epidemics of meningococcal meningitis seem to occur roughly in 10-year cycles. Drug-resistant strains appear with varying frequency, and such information, gleaned from surveillance reports issued by the Centers for Disease Control and Prevention and from reports of local health agencies and hospital infection surveillance, are of great practical importance.
H. influenzae meningitis, formerly encountered mainly in infants and young children, has been nearly eliminated in this age group as a result of vaccination programs. Noteworthy is the report of Schuchat and colleagues, who found that in 1995, some 5 years after the introduction of the conjugate H. influenzae vaccine, the incidence of bacterial meningitis in the United States had been halved. However, the incidence of meningitis from this organism continues to be common in developing nations, where it is now occurring with increasing frequency in adults.
Meningococcal meningitis occurs most often in children and adolescents but is also encountered throughout much of adult life, with a sharp decline in incidence after the age of 50 years. Pneumococcal meningitis predominates in the very young and in older adults. Perhaps the greatest change in the epidemiology of bacterial meningitis, aside from the one related to H. influenzae vaccination, has been the increasing incidence of nosocomial infections, accounting for 40 percent of cases in large urban hospitals (Durand et al); staphylococcus and gram-negative bacilli account for a large proportion of these.
The yearly incidence rate (per 100,000 population) of the responsible pathogens is approximately as follows:
S. pneumoniae, 1.1; N. meningitidis, 0.6; group B streptococcus (mainly in newborns), 0.3; L. monocytogenes, 0.2; and
H. influenzae, 0.2. In an epidemiologic survey of bacterial meningitis in the United States from 1998 to 2007, Thigpen and colleagues found the relative order of the various organisms to be much the same and again emphasized the decrease in incidence of the disease due mainly to the
H. influenza vaccination program. They estimated the recent overall incidence of bacterial meningitis in the United States to be 4,100 cases annually, resulting in 500 deaths. Their article is recommended for its detailed analysis of age, race, and underlying medical conditions.