Review
Meningococcal carriage in the African meningitis belt

https://doi.org/10.1016/S1473-3099(07)70288-8Get rights and content

Summary

In the African meningitis belt, epidemics of meningococcal disease occur periodically, although unpredictably, every few years. These epidemics continue to cause havoc but new efforts to control the disease, through the use of conjugate vaccines, are being made. Conjugate vaccines are likely to reduce meningococcal carriage, thus generating herd immunity, but to understand their potential impact we need to know more about the epidemiology of meningococcal carriage in Africa. We review published studies of meningococcal carriage in the African meningitis belt. A wide range of carriage prevalences has been reported, from 3% to over 30%, and the serogroup distribution has been variable. Factors influencing carriage include age, contact with a case, and the epidemic/endemic situation; however, season and immunisation with polysaccharide vaccine have little effect. Since the dynamics of carriage within a population are complex, longitudinal carriage studies are of great value; however, few such studies have been done. Carefully designed carriage studies are needed to measure and interpret the impact of meningococcal group A conjugate vaccines in Africa.

Introduction

The pattern of epidemic meningococcal disease in sub-Saharan Africa is well described.1, 2, 3 The meningitis belt, an area first defined by Lapeyssonnie1 in 1963 and updated by Molesworth and colleagues in 2002,4 stretches from Senegal to Ethiopia. The region is characterised by high levels of endemic meningococcal disease and most strikingly, by the occurrence of large epidemics in the dry season, periodically, although unpredictably, every 2–10 years. Asymptomatic pharyngeal meningococcal carriage is common in most populations, but invasive disease is usually rare. Thus, understanding the epidemiology of meningococcal disease requires study of carriage as well as study of invasive disease itself. Understanding the epidemiology of pharyngeal carriage is particularly important in view of plans for immunisation programmes using meningococcal conjugate vaccines, which probably exert their main effect by reducing carriage and inducing herd immunity. Here we review the Neisseria meningitidis carriage studies that have been done in and around the African meningitis belt.

Section snippets

Early carriage studies (before 1970)

The earliest published account of a meningococcal carriage study in Africa that we could find is by Chalmers and O'Farrell5 who described carriage among British troops stationed in the Sudan in 1915. The overall prevalence of carriage was 10% (86 of 847 soldiers), but the prevalence varied according to category of soldier. A study in the Sudan 15 years later by Davis,6 at the time of a meningitis epidemic, reported a 42·5% prevalence of meningococcal carriage, although only 47 individuals from

Characteristics of meningococcal carriage in the meningitis belt

The reported prevalence of meningococcal carriage within the meningitis belt has varied, with different study results ranging from 3% to 30%. Here we characterise some of the key factors that may have contributed to these heterogeneous findings. We have excluded studies that were done before 1970,5, 6, 7 because they used early microbiological techniques. We have also excluded studies from outside the meningitis belt,9, 15, 26, 27, 29, 30, 31, 41 with the exception of the analysis of vaccine

Dynamics of meningococcal carriage

Longitudinal studies are required to determine the dynamics of N meningitidis carriage. Factors that may be important in the appearance and disappearance of particular strains include the transmissibility of the strain, the duration of carriage, the development of immunity, and chance effects. Few longitudinal studies of carriage have been done in Africa. In northern Nigeria, the appearance and disappearance of the group A meningococcus was observed over a period of 8 months.17 In northern

Seroprevalence studies

Pharyngeal carriage of N meningitidis can induce naturally acquired immunity to meningococci.60 Seroprevalence studies are thus an additional means of measuring the patterns of exposure to meningococci. Few seroprevalence studies have been done in Africa. In northern Nigeria, the prevalence of participants with a titre of haemagluttinating antibody to serogroup A meningococcal polysaccharide 1:8 or more was found to be low in individuals less than 2 years. The prevalence then increases sharply

Conclusions

A few tentative conclusions can be drawn from the results of this Review of meningococcal carriage in the African meningitis belt. Carriage of N meningitidis is not necessarily rare in interepidemic periods and carriage strains are more diverse than disease-causing strains—for example, carriage of serogroup B meningococci is sometimes found even though this bacterial strain rarely causes disease. Patterns of carriage by age are not consistent across studies, probably because transmission is

Search strategy and selection criteria

We identified original papers and reports on meningococcal carriage studies in Africa published before July, 2007, by searching PubMed, Medline, and Web of Science databases using the keywords “meningococcal” or “Neisseria” and “carriage” and “Africa”. We also reviewed the citations of retrieved papers, reports, and theses, searched our own files, and asked expert colleagues if they knew of further studies. No language restrictions were imposed, but all the retrieved papers were in

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