Shift seen in burden of bacterial meningitis since 1998

Immunizing 2- to 23-month-old children with the pneumococcal conjugate vaccine (PCV7; Prevnar, Wyeth) has significantly reduced bacterial meningitis among this pediatric population.

It is still, however, unknown how the meningococcal conjugate vaccine (MCV4; Menactra, Sanofi Pasteur) and implementation of universal group B streptococcal (GBS) screening among pregnant women will affect the epidemiology of the disease, according to Michael C. Thigpen, MD, of the CDC, and colleagues.

“Bacterial meningitis is a feared illness, often killing or disabling otherwise healthy people,” Thigpen said at the 43rd Annual Meeting of the Infectious Diseases Society of America.

Historically, the five pathogens that cause most bacterial meningitis are Haemophilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis, GBS and Listeria monocytogenes. The disease is of special concern because consequences are potentially devastating and because many of these isolates have reduced susceptibility to medications.

Trend analysis

Thigpen and colleagues analyzed trends in the incidence of overall and pathogen-specific bacterial meningitis between 1998 and 2003.

The researchers used data from the Active Bacterial Core surveillance system (ABCs), an active, population-based surveillance system for culture-confirmed, invasive bacterial infections. The surveillance area encompassed almost 25 million people, they said.

The case definition of bacterial meningitis in this system is the isolation of one of the above five disease-causing pathogens from cerebrospinal fluid or other normally sterile sites associated with the diagnosis of meningitis in a resident from the surveillance areas.

Between 1998 and 2003, there were 2,039 cases of bacterial meningitis caused by any of the five pathogens, Thigpen said.

Cases in people of all ages declined from nearly 2 cases per 100,000 population to 1.5 cases per 100,000 population between 1998 and 2003. Thigpen said this was statistically significant for both linearity and trend (P <.05).

In 1986, the median age of people with bacterial meningitis was 15 months, he said. The median age rose to 25 in 1995 and 39 in 2003.

S. pneumoniae causes most cases of bacterial meningitis, which is not surprising, since S. pneumoniae has been on the rise in the United States since 1987. Even with antibiotics and intensive care, the bacteria causes excess morbidity and mortality in young children, the elderly and patients with medical problems.

Rates of disease changed among children, but rates have not dramatically changed, Thigpen said, among older people.

The incidence of pneumococcal meningitis among children aged 2 to 23 months declined from approximately 10 cases per 100,000 population – prior to PCV7 – to 3 cases per 100,000 population in 2003.

“This was statistically significant; however, no such decline in pneumococcal meningitis was seen in older case patients as has been seen with other types of invasive pneumococcal disease after the introduction of [PCV7],” he said.

The incidence of GBS meningitis among children aged younger than 2 months is 40 to 80 times higher than in other age groups, according to Thigpen. Disease declined to its lowest level, 42 cases per 100,000 population, in 2003.

“It’ll be important to evaluate disease trends in 2004 and 2005 to assess whether this decline is sustained,” he said.

Between 2002 and 2003, 781 cases of bacterial meningitis occurred in the surveillance area.

Of these cases, S. pneumoniae caused 60% of the cases and 76% of the 127 reported deaths.

Meningococcal meningitis was the most common cause of disease in the 11- to 17-year-old population. There were no cases of L. monocytogenes meningitis in children younger than 2 years.

Using this data to estimate the national burden of bacterial meningitis, these five pathogens caused approximately 4,400 cases of bacterial meningitis each year during 2002 and 2003, according to Thigpen.

Future of bacterial meningitis

“In summary, the incidence of bacterial meningitis in the United States declined from 1998 to 2003,” Thigpen said.

Associated with that decline is the reduction of pneumococcal disease and the cyclical changes in meningococcal disease, he added.

“PCV7 made a significant impact on pneumococcal meningitis among the 2- to 23-month-old target population but no overall reduction among older case patients,” Thigpen said.

Immunizing 2- to 15-month-old children with H. influenzae type b (Hib) vaccine also helped bring H. influenzae meningitis down, he said.

“We’ve had some success over the last two decades preventing bacterial meningitis,” Thigpen said. According to the WHO Web site, “in developing countries, where the vast majority of Hib deaths occur, pneumonia accounts for a larger number of deaths than meningitis.”

However, WHO officials said that Hib meningitis is a serious problem in such countries with mortality rates several times higher than seen in developed countries; it leaves 15 to 35% of survivors with permanent disabilities such as mental retardation or deafness.

It is unknown, Thigpen said, how screening pregnant women for GBS and administration of MCV4 will affect bacterial meningitis cases in the future.

The study has limitations. The ABCs system identifies only culture-confirmed, invasive infections. Also, the relatively short time period included in the study and the rarity of bacterial meningitis limit interpretation of trends, he said.

“As bacterial meningitis has become even more a disease of adults, with S. pneumoniae causing disease across all ages, further reductions using a targeted approach may be difficult,” Thigpen said. “Therefore, broad strategies – including new vaccines – to prevent adults cases of bacterial meningitis should be considered.”