overview- epidemiologie, supraveghere si biologia populatiei

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Overview 121

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From: Methods in Molecular Medicine, vol. 67: Meningococcal Disease: Methods and Protocols Edited by: A. J. Pollard and M. C. J. Maiden © Humana Press Inc., Totowa, NJ

8

Overview: Epidemiology, Surveillance,and Population Biology

Martin C. J. Maiden and Norman T. Begg

1. Introduction

A combination of data obtained by classical epidemiological techniques with

insights gained from the analysis of the population biology of Neisseria

meningitidis have proved to be critical in understanding the spread of menin-

gococcal disease. This is a consequence of the natural history and evolution of

this bacterium, which, despite its fearsome reputation as an aggressive patho-

gen (1), is ordinarily a harmless commensal inhabitant of the nasopharynx of

adult humans (2). Further, it has been established that “natural,” (that is to say,

carried) populations of meningococci are highly diverse, with a minority of

genotypes (the “hyperinvasive lineages”) being responsible for the majority of

disease (3). Finally, it is known that distinct hyperinvasive lineages tend to be

associated with particular epidemiological manifestations of meningococcal

disease (4) and some are especially associated with severe disease (the “hyper-

virulent” lineages) (5). These complexities have important implications for

public-health interventions, as different disease epidemiologies, caused by

genetically diverse meningococci, require distinct approaches to public-health

management. For example, the public-health response necessary to combat

large-scale meningococcal-disease outbreaks in Africa (6) is different from that

required during an institutional disease outbreak in Europe and North America,

and prolonged geographical outbreaks in these countries require a different

response again (7). In recognition of the importance of the multi-disciplinary

approach necessary to establish these insights, this section contains chapters

ranging from outbreak management through surveillance and isolate charac-

terization techniques to phylogenetic methods. Together the chapters provide



122 Maiden and Begg

the methodologies necessary for monitoring, understanding, and reacting to

the spread of meningococcal disease.

2. Meningococcal Epidemiology

As discussed in Chapters 18 and 21, meningococcal disease is a global prob-

lem that occurs in all human populations studied. This is presumably a reflec-

tion of widespread distribution of carried meningococci throughout the world.

The ubiquity of the disease, however, is accompanied by variation in its epide-

miology in different countries. There is a general pattern of a background level

of endemic disease interspersed by unpredictable epidemics, but within thispattern the precise epidemiology of meningococcal disease in any particular

time and place is dependent on a number of factors, with the genetic lineages

of meningococci circulating in the carrier state having a major influence

on disease rates. Indeed, meningococcal outbreaks often occur when a new

lineage is introduced into a susceptible population for the first time (see

Chapter 18).

The occurrence of meningococcal disease in epidemics has been recognized

for nearly 200 years (8), although it is intriguing for a disease with such promi-nent clinical features that the first probable description of a meningococcal

disease outbreak was as late as 1805, when Vieusseux described an epidemic

of cerebrospinal meningitis in Geneva, Switzerland. A major advance in men-

ingococcal disease epidemiology came during World War I, when it was estab-

lished that army recruit camps were at risk from meningococcal disease

outbreaks, especially during periods of overcrowding, which were accompa-

nied by high rates of carriage of meningococci. The vulnerability of military

recruits continues to this day and has proved to be a major motivator forresearch into the development of meningococcal vaccines. A number of other

factors that predispose individuals to meningococcal carriage, disease, or both

have since been identified, including passive smoking, recent influenza infec-

tion, hereditary complement deficiencies, and asplenia; these are discussed

briefly in Chapter 22.

A further milestone was the development of the serological typing of men-

ingococci (9), which led to the recognition of different capsules of the organ-

ism (10), and later to the immunological characterization of the highly diversesubcapsular antigens (11). The serological techniques that played such an

important part in the development of meningococcal biology are described in

Chapter 9. The differential serological properties of capsules of different

chemical composition is the basis of the serogrouping system for meningo-

cocci and the genetical basis for these differences, which has been established,

are discussed in Chapter 13. Molecular studies of the expression of one of the



Overview 123

most important subcapsular antigens for pathogenesis, the lipopolysaccharide

(sometimes referred to as lipooligosaccharide in the meningococcus) are

described in Chapter 14.

Of the 13 recognized serogoups, only 5—serogroups A,B,C, Y, and W-135—

are associated with disease (1), implicating the capsules defined by these

serogroups as major virulence determinants. Serogroup A meningococci caused

the disease outbreaks in the First and Second World Wars, but meningococci

expressing this serogroup have not caused significant disease in Western

Europe and North America since then (12). These meningococci, however,

continue to be a major cause of disease in Asia and Africa, especially in the

“meningitis belt” of the Sahel region of sub-Saharan Africa, which was

defined in a seminal monograph by Lapeyssonnie in 1963 (13). The epidemiol-

ogy of meningococcal disease in this region has since been extensively studied

and is discussed in Chapter 21. It is characterized by explosive, localized epi-

demics that invariably occur in the dry winter months (the harmattan) and that

often appear and wane over a period of a few weeks. Although most epidemics

in the meningitis belt are caused by serogroup A strains, more localized

serogroup C epidemics also occur (14).Two serogroup A lineages, known as subgroup I and subgroup III, have

been responsible for successive pandemics of meningococcal disease, which

have spread from Asia to Africa and on some occasions beyond (15–17). These

outbreaks remain the most serious manifestation of meningococcal disease,

with attack rates as high as 1,000 cases per 100,000 during intense epidemics.

In the latest outbreak in Africa during 1996, which was caused by subgroup III,

meningococci a total of 109,580 cases and 11,717 deaths were recorded in

Nigeria alone (18). The annual Haj pilgrimage to Mecca has been associatedwith outbreaks of serogroup A disease, notably the spread of serogroup A;

subgroup III organisms in 1987 (19). During 2000, members of the ET-37 com-

plex expressing serogroup W135 capsules have spread by this means (20) as

well. Some of these outbreaks are discussed in Chapters 20–22.

In the Americas (Chapter 22), Europe (Chapter 20), and Australasia (21),

meningococcal disease occurs at substantially lower rates and is more common

in winter than in summer, this seasonal pattern being particularly marked in the

Northern hemisphere. In these areas, the annual incidence of meningococcaldisease is usually between 1 and 10 per 100,000 population, with localized

outbreaks occurring from time to time, where attack rates may rise to as high as

50–100 per 100,000. In addition, some geographic areas experience elevated

incidence of disease, or hyperendemics, over a period of several years. Attack

rates are usually highest in children under 5 yr of age and there is sometimes a

smaller, second peak in disease incidence in teenagers. During outbreaks, the



124 Maiden and Begg

age distribution can change, with a shift toward older children and young

adults. The majority of disease is caused by meningococci expressing

serogroup B or C capsules, with disease caused by serogroup B organisms

generally the most common. Disease outbreaks caused by meningococci

expressing each of the disease-associated capsules have been described,

although they are commonly caused by organisms expressing a serogroup

C capsule. During the 1990s, serogroup C meningococcal disease outbreaks

occurred in several countries, including Canada (5), the UK (22), Spain (23),

and the Czech Republic (24). Serogroup Y infections have emerged as a sig-

nificant cause of morbidity in the US in the last few years (see Chapter 22),

and other serogroups may sometimes predominate (for example, serogroup A

infections are common in Russia [25] ). The case-fatality rate is usually about

10%, although it may be higher, particularly during outbreaks.

The application of multi-locus enzyme electrophoresis (MLEE, described in

Chapter 18) for analysis of genetic relatedness and monoclonal antibodies

(MAbs) for the improved characterization of subcapsular protein antigens

enabled the underlying epidemiology of meningococcal disease to be eluci-

dated. In a groundbreaking study, Dominique Caugant and colleagues estab-lished that meningococcal populations were genetically highly diverse, indeed

they were the most diverse bacterial population examined up to that time (26).

These data permitted the categorization of the many genotypes into lineages.

Although the members of these lineages clearly shared a recent common

ancestor, there was much genetic diversity even within a given lineage, leading

to the concept of the clonal complex, a group of related strains that had begun

to diverge. Examination of the serological properties of the individual com-

plexes established that serological classification of meningococci could bemisleading as genetically related organisms could be antigenically diverse. This

high diversity and the mobility of variants by horizontal genetic exchange has

important implications for the development and introduction of vaccines

against this organism.

In practical terms, the application of these techniques enabled the global

tracking of particular hyperinvasive meningococci, such as the members of the

ET-5 complex (27). It is now well-established that a few clonal complexes

cause most meningococcal disease (28) and that these change over time. Oftenincreases in disease incidence accompany the introduction of a new lineage

(clonal complex) into a given human population. The epidemic spread of dif-

ferent lineages is discussed in Chapter 18. More precise characterization of

isolates also enabled the diversity of carried meningococci to be identified and

established that the disease-associated meningococci were a minority compo-

nent of the meningococcal population as a whole (29).



Overview 125

The advent of nucleotide-sequence determination for the characterization of

bacterial genes, which is described in Chapters 11 and 12, have provided more

precise identification of genetic and antigenic variants. Nucleotide-sequence

data also provided a wealth of evidence to suggest that diverse and dynamic

nature of meningococcal populations was at least in part a consequence of the

transformable nature of the organism. Neisseria are competent for transforma-

tion throughout their growth cycle and high rates of carriage presumably pro-

vide many instances of co-colonization, which permit horizontal genetic

exchange within and among lineages and among the different species of men-

ingococci (30). In addition, it is now clear that antigenic diversity is being

continually generated in several surface proteins, presumably driven by

immune selection (31). Some of the phylogenetic techniques that are employed

to establish and measure the genetic diversity of meningococci are discussed in

Chapter 23.

Molecular techniques that enable the comparison of genetically and patho-

genically distinct meningococci, for both epidemiological and research pur-

poses, are becoming ever more sophisticated. A particularly exciting prospect

is the ability to compare whole genomes. The first technique that was able toachieve this was pulsed-field gel electrophoresis (PFGE) (32), a technique that

permits the resolution of whole chromosomes on an agarose gel. This has been

used to map meningococcal chromosomes and also in outbreak investigation

to identify relationships among isolates and identify outbreak strains, and is

discussed in Chapter 10. An alternative approach to the comparison of menin-

gococcal genomes, representational difference analysis, is discussed in Chap-

ter 16. This technique enables the gene content of different meningococci to be

compared. Perhaps the most dramatic contribution of molecular techniquesto the study of the meningococcus was the determination of two complete

genomes for different meningococci a serogroup A subgroup IV-1 (33) and an

ET-5 complex isolate (34), genome sequencing and annotation are described

in Chapter 15. The prospect of ever more detailed comparisons of meningo-

coccal genomes associated with particular pathogenic and epidemiological

characteristics, provides a tantalizing prospect for an incremental improvement

in our understanding of the epidemiology of meningococcal disease.

3. Public-Health Responses to Meningococcal Disease

In many industrialized countries, the investigation and control of an out-

break of meningococcal disease is one of the most challenging tasks for a

public-health practitioner. An outbreak usually occurs without warning; pre-

dictions about its evolution are difficult if not impossible, and the reaction of

the public, the media, and sometimes health professionals is often irrational



126 Maiden and Begg

and even hysterical. Moreover, the evidence base for control measures is weak,

so it can be difficult to justify action taken. For this reason, control policies

vary greatly from country to country. The level of intervention will depend on

many factors, including the setting of the outbreak, the organism responsible,

the resources available, and the level of public concern. The investigation and

management of outbreaks is considered in Chapter 17. Carriage studies are

often helpful in outbreak investigations; their planning and execution in both

outbreak and nonoutbreak situations is described in Chapter 19. Further,

high-quality surveillance of meningococcal disease and appropriate carriage

studies backed up by accurate characterization are all critical to inform control

strategies.

The surveillance of meningococcal disease is much more challenging in

developing countries where resources for case ascertainment and investigation

are limited. Most cases will be clinically diagnosed without laboratory confir-

mation, and disease incidence is usually based on estimates derived from ad

hoc studies. The surveillance of meningococcal disease in developing coun-

tries is considered in Chapter 21. In developed countries, surveillance is usu-

ally based on both notifications of clinically diagnosed cases and laboratoryreports. The level of case ascertainment is generally high, particularly where

meningococcal disease is perceived as a significant problem, and therefore all

potential cases are investigated. In recent years, notification data and labora-

tory data have become increasingly divergent (35). The surveillance of menin-

gococcal disease has been greatly enhanced by the advent of nonculture-based

diagnostic techniques, especially detection of meningococcal DNA, the tech-

niques for which have been discussed in Chapter 3. At the individual country

level, the number of cases of meningococcal disease may be too small toprovide robust information. International surveillance schemes can add public-

health value by pooling data from several countries; an example of an interna-

tional scheme is described in Chapter 20.

Vaccines against the meningococcus are the subject of a companion volume

to this text, Meningococcal Vaccines, also published Humana Press, and will

be discussed only briefly here. Childhood vaccination at the population level

would be an effective policy for meningococcal disease control; however, there

is as yet no comprehensive meningococcal vaccine that is effective from earlychildhood (36). Meningococcal disease-surveillance data show that, in prin-

ciple, the formulation of vaccines against the meningococcus should be rela-

tively straightforward to develop and implement. Possession of one of the five

disease-associated capsules (corresponding to serogroups A, B, C, Y, and

W-135) is an absolute requirement for pathogenesis and it was established in

the late 1960s that vaccines against meningococcal capsules can protect against

infection (37). Consequently, a five-valent vaccine would protect against all men-



Overview 127

ingococcal disease. Unfortunately, the plain polysaccharide vaccines are ineffec-

tive in infants and may induce tolerance in older age groups. The development of

protein-polysaccharide vaccines overcame these problems, rendering polysaccha-

ride vaccines effective in stimulating amnestic responses, even in small children.

While such vaccines are a realistic prospect from serogroups A, C , Y, and W-135,

problems remain for the development of protein-conjugate serogroup B polysac-

charide vaccines, owing to its particularly poor properties as an immunogen (36).

These are very likely to be related to its chemical and immunological identity to

host polysaccharides (38), raising regulatory issues if vaccines to this antigen were

developed. Attempts to develop comprehensive protein-based vaccines have been

frustrated by the high variability of these proteins and the mobility of genes encod-

ing variants by horizontal genetic exchange. A number of large-scale trials or popu-

lation-scale interventions with partial meningococcal vaccines have been

undertaken (39 , 40) or are currently being planned and the long-term effect of such

introductions will have to be carefully monitored using many of the techniques

described in this section.

4. ConclusionsMeningococcal epidemiology is intimately related to the complex natural his-

tory of this intriguing organism. The meningococcus has evolved numerous mecha-

nisms that enable it to survive as an obligate inhabitant of human mucosal surfaces,

mostly as a harmless commensal. Despite recent advances made in our understand-

ing of meningococcal biology, many of which have been achieved by combining

epidemiological and molecular approaches, there are many mysteries surrounding

meningococcal populations; for example, why are meningococcal populations so

diverse? Of particular concern is our lack of understanding of the dynamics of meningococcal populations and the effects of the introduction of vaccines that do

not offer comprehensive protection against meningocococcal disease. It is prob-

able that further advances are also likely to depend on an integrated and multi-

disciplinary approach. The ultimate challenge is to translate this understanding into

comprehensive and effective public-health interventions.

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