Pneumococcal Meningitis Epidemics in Africa

Abstract

Outbreaks of meningococcal meningitis are not uncommon. In industrialized countries, epidemics increase and decrease in severity slowly over a number of years and rarely achieve very high incidence rates of disease. In sub-Saharan Africa, the situation is different. Within the African meningitis belt, which extends across the Sahel and sub-Sahel from Ethiopia in the east to The Gambia in the west, major epidemics of meningococcal disease occur every few years [1]. These epidemics show a characteristic association with season. They almost always start early in the dry season, build to a peak at the hottest and driest time of the year, and then abate rapidly with the onset of the rains [2]. The reason for this remarkable seasonality is unknown, but climatic factors—in particular, a low absolute humidity—almost certainly play a central role [3]. In contrast, pneumococci are not usually considered to be a cause of meningitis epidemics. Outbreaks of invasive pneumococcal disease are reported from time to time in industrialized countries among people living in poor social circumstances, such as centers for the indigent, prisons, and military camps, but these outbreaks usually involve only a handful of cases. Thus, the recent reports of major outbreaks of pneumococcal meningitis in countries located in the African meningitis belt that have many of the epidemiological features of classic meningococcal epidemics are of concern [4, 5]. Surveillance of cases of meningitis and longitudinal surveys of nasopharyngeal carriage of meningococci have been underway in the Kassena-Nankana region of northern Ghana since 1998. Carriage studies have shown fluctuations in the prevalence of meningococci of different serogroups and sequence types over time, including a wave of infections caused by meningococci belonging to serogroup X [6, 7]. Surveillance of admissions to the district hospital showed a marked increase in the number of cases of pneumococcal meningitis in 2002 and 2003; 1100 patients were seen during this 2-year period [4]. All age groups were affected, and the case fatality rate of 44% was high. A strong association with season was observed; an increase in the number of cases of pneumococcal meningitis occurred approximately a month before there was an increase in the number of cases of meningococcal meningitis. Examination of the pneumococci responsible for this outbreak showed that 76% of isolates belonged to serogroup 1. On multilocus sequence typing, most isolates belonged to a single sequence type, ST217, and its 2 single-locus variants ST303 and ST612. Recently, the number of cases of meningitis caused by pneumococci of this serotype has decreased in the study area (G. Plutschke, personal communication), which suggests a natural cycle of penetration of a susceptible population by a new strain of bacteria and its subsequent elimination as a result of naturally acquired immunity; no pneumococcal vaccination has been undertaken. Unusual events have also been taking