Bacterial Meningitis: Rarer, in Older Patients, but Equally Deadly

Abstract & Commentary

By Joseph E. Safdieh, MD, Assistant Professor of Neurology, Weill Cornell Medical College. Dr. Safdieh reports no financial relationships to this field of study.

Synopsis: With the advent of new vaccines, the incidence of bacterial meningitis has declined, particularly in children, but the mortality rate has remained the same.

Source: Thigpen MC, et al. Bacterial meningitis in the United States, 1998-2007. N Engl J Med 2011;364:2016-2025.

Bacterial meningitis is a feared medical illness that has a high morbidity and mortality rate. Meningitis can present at any age group, although the predominant pathogenic organisms do vary by age. Prior studies in the 1970s and 1980s have demonstrated that the most common causes of sporadic community-acquired bacterial meningitis in the United States are Haemophilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis, group B streptococcus (GBS), and Listeria monocytogenes. The introduction of the Hib vaccine reduced the overall incidence of bacterial meningitis by 55% by 1995. The authors of this study analyzed incidence patterns of bacterial meningitis in the United States from 1998-2007. A number of public health changes were introduced during the study period, including a meningococcal vaccine, a pneumococcal vaccine, and routine screening of pregnant women for GBS.

Coordinated by the CDC, the study authors reviewed a prospectively collected cohort of cases of bacterial meningitis in an infectious disease surveillance program at a number of geographically dispersed clinical sites, which included almost 8% of the United States population. Only cases of meningitis caused by one of the five organisms listed above were included in the study. The cases through the study period (1998-2007) were analyzed for trends and a latter portion of the group (2003-2007) was also analyzed for detailed epidemiology.

For the study period, 3188 cases of bacterial meningitis were identified with a mortality rate of 14.8%. The incidence of bacterial meningitis significantly decreased by 31% from the beginning to the end of the surveillance period (2 cases per 100,000 in 1998-1999 down to 1.38 cases per 100,000 in 2006-2007). The median age of patients significantly increased from the beginning to the end of the study period, from 30.3 years to 41.9 years. The overall case fatality rate over the study period remained unchanged. The most common organisms from 2003-2007 were S. pneumoniae (58%), GBS (18.1%), N. meningitidis (13.9%), H. influenzae (6.7%), and L. monocytogenes (6.7%). Through the study period, there were 4100 average annual cases of bacterial meningitis in the United States, with 500 annual deaths.

Rates of bacterial meningitis decreased most dramatically among children, causing the median age to rise. Overall, the pathogen with the most dramatic decrease over the study period was S. pneumoniae, which the authors suggest is due to pneumococcal vaccination. Of note, the rate of meningitis in children under age 2 months did not decrease, suggesting that GBS surveillance and treatment does not prevent late-onset disease, manifested as meningitis. L. monocytogenes did decrease in infants, which correlates with lower rates of maternal listeriosis, likely due to better education and a safer food supply.

Commentary

This study demonstrates a number of important points. Bacterial meningitis is certainly becoming rarer over time, due to vaccination as well as better public health measures. Because of the overwhelming success of pediatric vaccines in preventing meningitis, the median age of bacterial meningitis is increasing. It is worth noting that the absolute incidence of bacterial meningitis did decrease in adults older than age 65 in the study period, but not as dramatically as in children. Although bacterial meningitis is less common now than it was in the past, the mortality rate remains unchanged. This is even more important to remember in the setting of declining incidences, as younger physicians may not see as many cases in training and therefore may fail to recognize and rapidly treat patients with meningitis. There is more work to be done, including widespread adoption of the adult pneumococcal vaccine. However, it is certainly encouraging to see the profound positive effects that can occur as a result of public health policies.