Emerging Infectious Diseases Volume 2 Issue 1

80 Pages·1996·1.6 MB·English

by  CDC

Emerging
Vol. 2, No. 1, January–March 1996
Infectious Diseases
Tracking trends and analyzing new and reemerging infectious disease issues around the world
Molecular Population Genetic James M. Musser
Analysis of Emerged Bacterial
Pathogens
Emergence of the Ehrlichioses as David H. Walker
Human Health Problems
Surveillance for Pneumonic Plague Curtis L. Fritz
Malaria Transmission Jane R. Zucker
Cluster of Lyme Disease Cases G. Thomas Strickland
Unexplained Deaths Due to Bradley A. Perkins
Possibly Infectious Causes
Trends in Bacteremic Infection Due Daniel M. Musher
to S. pyogenes
Infectious Diseases in Latin A. David
America and the Caribbean Brandling-Bennet
Microbial Threats and the Global Stuart B. Levy
Society
Xenotransplantation:Risks, Clinical Robert E. Michler
Potential, and Future Prospects
Another Human Case of Equine
Morbillivirus Disease in Australia
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Emerging Infectious Diseases
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Centers for Disease Control
Human Services and Health University of Virginia Medical Center
and Prevention (CDC)
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David Brandling-Bennett, M.D. Philip P. Mortimer, M.D.
Perspectives Editor
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Pan American Health Organization Central Public Health Laboratory
The Rockefeller University
World Health Organization London, United Kingdom New York, New York, USA
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Robert Shope, M.D.
Synopses Editor
Gail Cassell, Ph.D. Professor of Research
Phillip J. Baker, Ph.D.
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University of Alabama at Birmingham Galveston, TX National Institute of Allergy and Infectious Diseases
Birmingham, Alabama, USA National Institutes of Health (NIH)
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and Prevention (CDC) National Center for Infectious Diseases
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Contents
Emerging Infectious Diseases
Volume 2 • Number 1 January–March 1996
Perspective
Molecular Population Genetic Analysis of Emerged Bacterial 1 James M. Musser
Pathogens: Selected Insights
Synopses
Emergence of the Ehrlichioses as Human Health Problems 18 David H. Walker and J. Stephen
Dumler
Surveillance for Pneumonic Plague in the United States During 30 Curtis L. Fritz, David T. Dennis,
an International Emergency: A Model for Control of Imported Emerging Margaret A. Tipple, Grant L.
Diseases Campbell, Charles R. McCance, and
Duane J. Gubler
Changing Patterns of Autochthonous Malaria Transmission in the 37 Jane R. Zucker
United States: A Review of Recent Outbreaks
Dispatches
Cluster of Lyme Disease Cases at a Summer Camp in Kent County, 44 G. Thomas Strickland, Leena
Maryland Trivedi, Stanley Watkins, Margaret
Clothier, John Grant, John Morgan,
Edward Schmidtman, Thomas Burkot
Unexplained Deaths Due to Possibly Infectious Causes in the United 47 Bradley A. Perkins, Jennifer M. Flood,
States: Defining the Problem and Designing Surveillance and Richard Danila, Robert C. Holman,
Laboratory Approaches Arthur L. Reingold, Laura A. Klug,
Michael Virata, Paul R. Cieslak, Sherif
R. Zaki, Robert W. Pinner, Rima F.
Khabbaz, and the Unexplained
Deaths Working Group
Trends in Bacteremic Infection Due to Streptococcus pyogenes (Group A 54 Daniel M. Musher, Richard J. Hamill,
Streptococcus), 1986–1995 Charles E. Wright, Jill E. Clarridge,
Carol M. Ashton
Letter
PHLS Surveillance of Antibiotic Resistance, England and Wales: 57 David C. E. Speller, Alan P.
Emerging Resistance in Streptococcus pneumoniae Johnson, Barry D. Cookson,
Pauline Waight, Robert C. George
Commentary
Infectious Diseases in Latin America and the Caribbean: Are They 59 A. David Brandling-Bennett and
Really Emerging and Increasing? Francisco Pinheiro
Microbial Threats and the Global Society 62 Stuart B. Levy
Xenotransplantation: Risks, Clinical Potential, and Future Prospects 64 Robert E. Michler
News and Notes
Another Human Case of Equine Morbillivirus Disease in Australia 71
Social Science and the Study of Emerging Infectious Diseases 72 Johannes Sommerfeld and
Sandra Lane
WHO Establishes New Rapid-Response Unit for Emerging Infectious 72 Philippe Stroot
Diseases
Rotavirus Vaccine Workshop Held 73 Roger I. Glass
International Conference Addresses Preparedness for Emerging Strains 73 Dominick A. Iacuzio
of Pandemic Influenza
Course Offered on Clinical and Pathologic Features of Emerging Infections 74 C. Robert Horsburgh, Jr.
NASA Sponsors Symposium on Remote Sensing and Control of 74 Michael Braukus and Raj Khanna
Insect-Transmitted Diseases
CDC Convenes Meeting to Discuss Strategies for Preventing Invasive 75 The Working Group on Prevention
Group A Streptococcal Infections of Severe Group A Streptococcal
Infections
Regional Conference on Emerging Infectious Diseases Sparks Plan for 75 Bradford A. Kay
Increased Collaboration
Perspective
Molecular Population Genetic Analysis of Emerged
Bacterial Pathogens: Selected Insights
James M. Musser, M.D., Ph.D.
Department of Pathology, Baylor College of Medicine, Houston, Texas, USA
Research in bacterial population genetics has increased in the last 10 years. Population
genetic theory and tools and related strategies have been used to investigate bacterial
pathogens that have contributed to recent episodes of temporal variation in disease fre-
quency and severity. A common theme demonstrated by these analyses is that distinct
bacterial clones are responsible for disease outbreaks and increases in infection frequency.
Many of these clones are characterized by unique combinations of virulence genes or alleles
of virulence genes. Because substantial interclonal variance exists in relative virulence,
molecular population genetic studies have led to the concept that the unit of bacterial
pathogenicity is the clone or cell line. Continued new insights into host–parasite interactions
at the molecular level will be achieved by combining clonal analysis of bacterial pathogens
with large-scale comparative sequencing of virulence genes.
To avert the threat of resurgent and new micro- students of the molecular evolutionary processes
bial diseases, it is critical to gain insight into the in higher eukaryotic organisms. Bacteria were an
molecular mechanisms contributing to temporal attractive group of experimental organisms be-
variation in disease frequency and severity. Al- cause of their phenotypic diversity, short genera-
though comprehensive, unambiguous under- tion times, haploid chromosomal genomes, and
standing of the host and parasite factors accessory genetic elements. Hence, bacterial
mediating these processes is not available for any population genetic research was originated by
infectious agent, population genetic research in population geneticists interested in bacteria,
the last 10 years has provided noteworthy new
rather than bacterial geneticists or medical micro-
information about the bacterial side of the equa-
biologists interested in population genetics (1, 2).
tion. This review will summarize the insights ac-
In spite of its important implications for how the
crued from population genetic analysis of bacteria
field has developed over the last decade, this on-
responsible for disease outbreaks or increases in
togeny will not be discussed in detail here. How-
infection frequency and severity. One of the pri-
ever, the reader should recognize that bacterial
mary themes emerging from this research is that
population genetics is a discipline separate and
distinct bacterial clones have been responsible for
distinct from the study of the molecular epidemiol-
several infection outbreaks (Table 1). Moreover,
ogy of infectious agents. The research tools, meth-
the distinct clones are frequently characterized by
ods of data analysis, and general thought
unique combinations of virulence genes or alleles
processes are very different from the typological
of virulence genes. These observations have im-
thinking used by investigators of disease out-
portant implications for our understanding of in-
breaks or microbial pathogenesis (3-6).
fectious diseases and the public health measures
required to reduce their detrimental and poten- Early work on the clonal nature of bacterial
tially devastating effect on society. pathogens was conducted largely with Escherichia
coli, through a framework supplied by serotyping
of one or a few polymorphic surface antigens (7, 8).
Population Genetics and Clonal Analysis of
Only a few of the many possible O and H antigen
Bacterial Pathogens: Basic Concepts
serotypes were frequently associated with out-
Population genetic study of bacterial pathogens
breaks of infantile diarrhea in the United
arose largely as an offshoot of research designed
Kingdom and other countries, which suggested
to address questions of longstanding interest to
that isolates expressing these traits had special
virulence properties (7-9). Because serotype
Address for correspondence: James M. Musser, M.D., Ph.D.,
analysis of relatively few surface structures does
Department of Pathology, Baylor College of Medicine, One
Baylor Plaza, Houston, TX 77030 USA; fax [phone]; not provide robust data for estimating overall
1
Vol. 2, No. 1 — January-March 1996 Emerging Infectious Diseases
Perspective
Table 1. Representative Emerged Bacterial Pathogens Investigated with Molecular Population and other microbial
Genetic Strategies pathogens provides a
convenient strategy
Organism Disease Reference for indexing overall
Borrelia species Lyme disease 146-149,150,151 levels of chromosomal
Escherichia coli O157:H7 hemorrhagic colitis; diversity in the sample
hemolytic uremic syndrome 33-35 and for inferring ge-
Haemophilus influenzae Invasive disease 156 netic relationships
H. influenzae among strains. Be-
biogroup aegyptius Brazilian purpuric fever 25,26 cause of the strong cor-
Listeria monocytogenes Food-borne invasive disease 157, 158 relation of chromo-
Mycobacterium tuberculosis Tuberculosis 91,105 somal divergence in-
Neisseria meningitidis Sepsis, meningitis 52-63,66,69-73 dexed by multilocus
Salmonella species Food-borne illness 159-161 enzyme electro-
Staphylococcus aureus Toxic shock syndrome; phoretic data and
methicillin resistant strains 47,143,144 DNA-DNA hybridiza-
Streptococcus pyogenes Invasive disease, tion studies (15-18),
toxic-shock-like syndrome 82,83,85 several cryptic species
S.pneumoniae Otitis, pneumonia, sepsis, have been identified
meningitis 125-131,136 once their existence
Vibrio cholerae Cholera 153-155 was initially discov-
ered by population ge-
netic analyses that
levels of chromosomal diversity and relationships employed starch gel electrophoresis (16, 19-21). In
among strains, the primary research tool used to most bacterial species, the number of allelic vari-
examine the population genetics of emerging bac- ants is large, and it is unlikely that recombina-
terial pathogens has been multilocus enzyme elec- tional processes would, by chance, frequently
trophoresis (10, 11). This technique indexes allelic generate strains with the identical electromorph
variation in sets of randomly selected structural profile. Hence, organisms with the same electro-
genes located on the chromosome and provides a morph profile are generally thought to be similar
basis for estimating overall levels of genotypic by descent, rather than by convergence through
variation in populations, i.e., the sample of bacte- lateral gene flow.
ria chosen for analysis. The key concept underly- Recently, convenient, rapid, and relatively inex-
ing use of starch gel-based protein electrophoresis pensive large-scale DNA sequencing techniques
in population genetics is that electromorphs (mo- have also been adopted by several laboratories.
bility variants) of an enzyme can be directly Large-scale automated DNA sequencing has been
equated with alleles of the corresponding struc- used to rapidly and unambiguously identify a
tural gene. Moreover, electromorph profiles over a causative infectious agent and confirm or refute
sample of different enzymes, therefore, correspond the identity of isolates recovered from temporally-
to multilocus enzyme genotypes and are fre- linked patients thought to be involved in a disease
quently referred to as electrophoretic types or ETs. outbreak. In addition, sequence-based studies
The proteins analyzed are usually metabolic en- have been employed to define the nature and
zymes expressed by virtually all isolates of a spe- extent of allelic variation in toxin and other viru-
cies under the growth conditions used. The allelic lence factor genes and to rapidly identify muta-
variation detected is unaltered by environmental tions associated with antimicrobial agent
conditions such as culture conditions, laboratory resistance (22).
storage, anatomic site of recovery, or specific clini- Unless noted, data on the population genetic
cal disease. Allelic variation in these metabolic analysis of emerging bacterial pathogens summa-
enzymes is selectively neutral, or nearly so, which rized in this article were generated by multilocus
means that convergence to the same allele through enzyme electrophoresis, sometimes performed in
adaptive evolution is unlikely (4, 12-14). As a con- concert with automated DNA sequencing.
sequence, this approach to the study of bacterial
2
Emerging Infectious Diseases Vol. 2, No. 1 — January-March 1996
Perspective
Representative Insights spawned from a much larger base population of
diverse nonpathogenic precursor clones. Accord-
ing to this hypothesis, acquisition or loss of one or
Brazilian Purpuric Fever
more genes (or, perhaps, a shift in ecological niche)
Brazilian purpuric fever (BPF), a serious inva-
may produce a pathogenic form with the charac-
sive disease of children, was first characterized in
teristic viscerotropism for human conjunctivae.
1984 after an outbreak in Promissao, Sao Paulo
Although population genetic analysis did not
State, Brazil. Children with BPF have acute onset
provide a simple reason for the BPF outbreak, the
of fever and usually die within 48 h with dissemi-
demonstration that the causative clone of bi-
nated purpura, vascular collapse, and hypotensive
ogroup aegyptius was highly differentiated from
shock (23). BPF is caused by Haemophilus influ-
other phenotypically similar organisms provided
enzae biogroup aegyptius, an organism associated
an explanation for the unique infection manifesta-
with sporadic or epidemic conjunctivitis (24). Mul-
tions and the unique group of characters associ-
tilocus enzyme electrophoresis and other molecu-
ated with the clone (28-30). Moreover, population
lar techniques have demonstrated that isolates
genetic analyses demonstrated distinct medical
recovered from BPF patients represent a distinct
correlates to isolates classified as biogroup aegyp-
clone (25).
tius. The results of numerous subsequent studies
As a first step toward identifying the evolution-
have confirmed that, as a population, H. influen-
ary origin of this pathogenic H. influenzae bi-
zae biogroup aegyptius strains vary in their behav-
ogroup aegyptius clone, chromosomal variation
ior, as one would expect of a genetically diverse set
and genetic relationships were indexed among 17
of organisms.
biogroup aegyptius isolates, and 2,209 encapsu-
lated H. influenzae strains were recovered world-
wide (26). Biogroup aegyptius isolates form three Escherichia coli O157:H7
distinct evolutionary lineages of the species H. Strains of E. coli expressing serotype O157:H7
influenzae. Isolates of the case clone are only very were recognized in the early 1980s as important
distantly related to other isolates classified as causes of hemorrhagic colitis and hemolytic ure-
biogroup aegyptius; that is, the case clone was no mic syndrome in North America (31). Disease usu-
more related to other biogroup aegyptius isolates ally occurs after consumption of contaminated
than are (for example) two H. influenzae isolates beef or other food. Several large outbreaks have
selected at random from the species. The BPF case occurred, and more than 60 case clusters have
clone was genetically allied with H. influenzae been reported in the United States (32). Because
isolates expressing serotype c polysaccharide cap- several E. coli reference laboratories rarely iden-
sule, a result that explains an earlier observation tified organisms expressing this serotype before
(27) that BPF isolates, like serotype c strains, the early 1980s, reporting of these isolates has
produce type 2 IgA1 protease, whereas other iso- increased dramatically. Because of the medical
lates of biogroup aegyptius express type 1 IgA1 and economic importance of E. coli strains consid-
protease. Thus, the population genetic evidence erable effort has been directed toward elucidating
showed that biogroup aegyptius is polyphyletic genetic relationships among and between them as
and that the BPF organism is a genetically distinct well as between them and other members of the
clone unrelated to other isolates with the pheno- species; as a result extensive information is now
typic criteria of biogroup aegyptius. available about clonal relationships among these
important bacteria (33-35).
The genetic diversity in the sample of all bi-
ogroup aegyptius strains was approximately equal The observation that O157:H7 strains synthe-
to that recorded for entire species of certain patho- size one or more Shiga-like toxins and lack the
genic bacteria (16, 17). Therefore, the effective ability to rapidly ferment sorbitol initially sug-
population size of aegyptius must be large; how- gested that strains of this serotype had shared a
ever, this interpretation is difficult to reconcile recent common ancestor. To directly test this idea,
with the observation that strains in the biogroup multilocus enzyme electrophoresis was used to
are rare pathogens associated only with human assess genetic relatedness of 100 strains of E. coli
disease. A possible explanation for the relatively serotypes recovered from patients with hemor-
extensive genetic diversity among biogroup aegyp- rhagic colitis or hemolytic uremic syndrome (33).
tius strains is that they represent cell lineages Although 25 distinct multilocus enzyme genotypes
3
Vol. 2, No. 1 — January-March 1996 Emerging Infectious Diseases
Perspective
were identified, cluster analysis found that Staphylococcus aureus Toxic Shock Syndrome
O157:H7 isolates are closely related organisms. Toxic shock syndrome (TSS) was described in
The results were interpreted to mean that 1978 (43) as a severe acute illness (characterized
O157:H7 organisms recovered from epidemiologi- by high fever, erythematous rash, hypotension or
cally unassociated North American outbreaks be- shock, multiorgan involvement, and desquama-
long to a single geographically widespread tion of the skin) of young children associated with
pathogenic clone with specific virulence properties infection with Staphylococcus aureus. Two years
(33). Subsequent analysis of O157:H7 strains by later, it was recognized that TSS is a geographi-
pulsed-field gel electrophoresis has supported this cally widespread disease affecting mainly young,
idea (36). healthy, menstruating women, especially those us-
To delineate clonal relationships among ing certain high absorbency tampons (44). Most
O157:H7 organisms and other E. coli strains that vaginal isolates of S. aureus recovered from pa-
cause hemorrhagic colitis and infantile diarrhea, tients with TSS produce a chromosomally encoded
1,300 isolates representing 16 serotypes from pa- toxin, designated as toxic shock syndrome toxin-1
tients with these diseases were studied by multilo- (TSST-1) (45). Evidence implicating TSST-1 as a
cus enzyme electrophoresis and probing for genes major virulence factor in the pathogenesis of TSS
encoding Shiga-like toxins (34). The O157:H7 has accumulated (46). Almost all strains recovered
clone was closely related to a clone of O55:H7 from patients with menstrual TSS, which account
strains that has a long history of worldwide asso- for approximately 90% of TSS cases, synthesize
ciation with outbreaks of infantile diarrhea (34). TSST-1, whereas only 50%-60% of isolates from
The data strongly suggested that the O157:H7 and cases of nonmenstrual TSS and 5%-25% of strains
O55:H7 clones have recently radiated from a com-
causing other diseases produce this protein.
mon ancestral cell. The O157:H7 clone arose from
Several questions of importance to both medical
an O55:H7-like ancestor, perhaps through hori-
bacteriology and evolutionary genetics were ad-
zontal transfer and recombination events adding
dressed in a study of 315 TSST-1-producing
Shiga-like toxin genes and adhesion genes to an E.
strains of S. aureus (47). It was discovered that the
coli genome preadapted for causing diarrheal dis-
organisms responsible for most cases of TSS with
ease (34, 35). If, as the multilocus enzyme electro-
a female urogenital focus are members of a single
phoretic data indicate, O157:H7 and O55:H7
distinctive clone (designated as ET 41), a result
organisms have shared a recent common ancestor,
that explains the observation that isolates recov-
it is likely that the close genetic affiliation would
ered from patients with TSS share many traits
be reflected at the nucleotide level. To test this
(48, 49). The investigation also showed that TSST-
notion, the gene (eae) (34) encoding intimin, a
1 is expressed by isolates of a great variety of
protein involved in bacterial attachment to entero-
clones representing virtually the full breadth of
cytes and subsequent effacement of the microvilli,
genotypic diversity in the species as a whole. In
was sequenced from representative isolates of
addition, isolates of ET 41 represented 24% of a
these two serotypes. The resulting sequence data
sample of TSST-1-producing strains recovered be-
were consistent with the hypothesis that O157:H7
fore 1978, which meant that the tst gene encoding
and O55:H7 organisms share a close genetic affin-
the toxin neither evolved nor was acquired re-
ity and thereby provide a plausible explanation for
cently by this species. The failure to recover iso-
the observation that these bacteria cause similar
lates of ET 41 from non-human hosts effectively
attaching and effacing lesions in cells grown in
eliminated the likelihood that animals are impor-
culture (38) and in animal models (39). Because
conventional serotyping of E. coli does not provide tant in the transmission of this clone.
a reliable basis for analyzing population structure Twenty-eight percent of isolates of S. aureus
and can be grossly misleading as to genetic rela- cultured from the introitus, vagina, or cervix of
tionships among isolates (40-42), many important unassociated healthy carriers or women with non-
medical correlates of the population structure will TSS urogenital symptoms were ET 41 or closely
not be recognized and understood fully until E. coli allied clones; no other single multilocus enzyme
isolates are sorted out along clonal lineages. genotype accounted for more than 12% of normal
vaginal isolates. These observations led to the
hypothesis that isolates of ET 41 are more readily
able to colonize the human vagina and, hence, are
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Emerging Infectious Diseases Vol. 2, No. 1 — January-March 1996
Perspective
widely dispersed in an ecological niche of great Serogroup B ET-5 Complex Organisms
consequence in TSS. Under this “adapted clone” Caugant et al. (52) demonstrated that an epi-
hypothesis, isolates of ET 41 are responsible for demic of serogroup B meningococcal disease that
most vaginal cases because this clone has a special began in the 1970s in Norway and subsequently
affinity for the cervicovaginal milieu, perhaps (but spread through much of Europe was caused by a
not necessarily) as a consequence of variation in group of 22 very closely related clones, designated
regulation of toxin- or other virulence-gene expres- as the ET-5 complex, that have no close genetic
sion. In summary, data derived from clonal analy- relationship to other clone groups. Clones of this
sis of TSST-1-producing S. aureus are consistent complex were traced intercontinentally to Chile
with the notion that the “bloom” in TSS cases and South Africa, where they also caused contem-
happened because of a change in the character of porary outbreaks of invasive disease. Clonal
catemenial products (perhaps associated with de- analysis also showed that a severe epidemic of
creasing levels of anti-TSST-1 antibody in human meningococcal disease in Cuba (characterized by
populations), not because of a new S. aureus a high attack rate and incidence of septicemia) was
strain. due to ET-5 complex organisms. The recovery of
Two additional points are noteworthy regarding these same bacteria from outbreaks in Miami,
population genetic analysis of S. aureus strains Florida, in 1980 and 1981, strongly suggested that
producing TSST-1. First, if the gene encoding Cuban refugees imported the clones to Miami.
TSST-1 were evolutionarily old, allelic variants Members of the ET-5 complex have seldom been
differing in nucleotide and, perhaps, amino acid recognized as important pathogens in the United
sequence would exist in natural populations. This States. However, ET-5 complex organisms were
prediction was borne out by the identification of a responsible for a recent increase in meningococcal
variant of TSST-1 associated with goat, sheep, and disease rates in Washington and Oregon (53). In
occasionally bovine mastitis that is encoded by a
addition, a serogroup B epidemic in greater Sao
gene which differs from the “human” form by 14
Paulo, Brazil, was also caused by ET-5 complex
nucleotides, resulting in 9 amino acid changes.
members (54).
The variant toxin retains mitogenic activity for
Recently, clonal analysis has been used to study
mouse splenocytes but differs significantly in
serogroup B meningococcal isolates that caused
other functions ascribed to TSST-1, including abil-
invasive disease in The Netherlands between 1958
ity to induce a TSS-like disease in rabbits (50).
and 1986 (55). Significant temporal variation in
Second, if the rapid increase in TSS cases were
the clonal composition of meningococcal popula-
caused by a change in host character rather than
tions was identified. Recent disease episodes were
by the rapid spread of a single, new, hypervirulent
caused predominantly by isolates of three clonal
clone, subclonal heterogeneity would be present
lineages (designated I, III, and VI) that were not
among isolates classified as ET 41. Examination
represented in samples collected before 1975. In
of RFLP patterns for the gene (coa) encoding co-
addition, an epidemic in 1966-1967, and a hyper-
agulase has shown at least three distinct sub-
endemic disease wave in 1972 were caused mainly
clones of ET 41 (51).
by two closely related clones (ET-11 and ET-17)
expressing serotype 2b protein. Strong statistical
Neisseria meningitidis
deviation in the sex ratio was recorded for disease
Extensive work in the last 10 years has exam-
caused by clones of two lineages. Clones of lineage
ined the molecular population genetics of Neisse-
V were cultured far more frequently from female
ria meningitidis, predominantly by clonal
analysis, and more recently by DNA sequencing of than for male patients; whereas, clones of lineage
putative virulence genes. This work suggests that IX were recovered from disease in male patients
temporal variation in disease frequency and sever- approximately four times more often than aver-
ity is usually associated with clonal replacement age. The cause(s) of these differences are unknown
much like influenza epidemics are driven by anti- but warrant further investigation.
genic shift. For most bacterial pathogens, few data are
available regarding the frequency with which dis-
tinctive clones are recovered in asymptomatic per-
sons. Caugant et al. (56) studied the clonal
composition of meningococcal isolates cultured
5
Vol. 2, No. 1 — January-March 1996 Emerging Infectious Diseases
Perspective
from the nasopharnyx of healthy carriers in Nor- A group led by M. Achtman assembled 423
way and discovered that the frequency of recovery serogroup A meningococcal isolates, recovered pri-
of clones (ET-5 complex and ET-37 complex) caus- marily from invasive episodes, and representing
ing 80% of disease episodes were represented by organisms responsible for 23 epidemics or out-
only 7% and 9%, respectively, of carrier isolates. breaks between 1915 and 1983. Thirty-four dis-
This same study demonstrated that the clones tinctive clones were assigned to four complexes
most commonly represented among carrier iso- representing groups of related clonal genotypes
lates (19%) have never been recovered from pa- (61). Most epidemics were caused by a single clone,
tients with invasive meningococcal infection. The and the same clone often was responsible for con-
data reinforce the concept that bacterial clones current epidemics in contiguous countries. For
vary dramatically in virulence potential. example, serogroup A clone I-1 caused a pandemic
that began in North Africa and certain Mediterra-
Serogroup C Disease nean countries in 1967 and spread throughout
An increase of invasive disease due to serogroup West Africa in the subsequent 2 years; clone III-1
C N. meningitidis strains has been reported in has been responsible for disease outbreaks in Fin-
several countries in recent years (57-60). Study of land, Brazil, Nepal, and China.
121 isolates recovered from patients in Greater
Recently Achtman’s group has extensively char-
Sao Paulo, Brazil, between 1976 and 1990 identi-
acterized more than 300 serogroup A isolates from
fied a striking increase in isolates assigned to ET
patients or carriers in one epidemic in The Gambia
11 complex (58). The percentage of invasive dis-
in 1982-1983 and in 1984-1985 after an immuni-
ease episodes caused by complex 11 organisms
zation program at the end of 1983. Analysis of a
increased from 8% in 1988 to 66% in 1990. Out-
representative subgroup of 64 isolates showed
breaks of serogroup C meningococci have also been
that all were assigned to clone IV-1 (64, 65). Iso-
recently reported from distinct regions of the
lates of this clone were examined for subclonal
United States (59) and Canada (57, 60). Analysis
variation with SDS-PAGE profiling, LPS profiling,
of organisms collected from 13 U.S. outbreaks
and genomic restriction endonuclease profiling,
identified five distinct multilocus enzyme types,
and rare variants were detected. Two cell-surface
all very closely allied in overall chromosomal re-
antigens (class 5 outer membrane protein and pili)
latedness (59). Moreover, strains causing 4 of these
were unusually variable, and the hypothesis was
13 outbreaks were identical in multilocus enzyme
formulated that variation in the class 5 OMP
type (designated ET-15) to organisms responsible
occurs as a consequence of recombinational events
for outbreaks in eastern Canada (60). Canadian
affecting the translational reading frame. The
investigators have reported (60) that ET-15 organ-
role, if any, of this subclonal microheterogeneity in
isms had a significantly higher case-fatality ratio
serogroup A meningococcal epidemics is being as-
than other invasive meningococcal disease iso-
sessed. Clonal analysis has provided a framework
lates, which may be due to a lower herd immunity
that is being exploited to rationally select strains
to the newly emerged clone.
for further characterization by molecular and se-
rologic techniques that may provide insight into
Serogroup A Disease
the forces driving a bacterial epidemic (66-68).
Unlike other serogroups of Neisseria meningi-
Clonal analysis also has demonstrated that se-
tidis, which are usually associated with endemic
rogroup A isolates are a restricted phylogenetic
disease, isolates expressing serogroup A capsular
subpopulation of the species N. meningitidis (69).
polysaccharide are unusual in that they may cause
This result may mean that the genotype bestowing
large epidemics. For example, serogroup A organ-
the epidemic phenotype has arisen a single time
isms have been responsible for epidemics of inva-
and that it has not been successfully transferred
sive disease in Africa, China, Iran, Greece,
Finland, Brazil, and Nepal (61). Major epidemics horizontally to unrelated phylogenetic lineages of
every 5-10 years in the Sahel region of sub-Saha- the species.
ran Africa have led to the description of a “menin- Moore et al. (70) employed clonal analysis to
gitis belt” and to detailed studies by clonal analysis document the intercontinental spread of an epi-
of the molecular epidemiology of serogroup A or- demic group A meningococcal clone complex by
ganisms responsible for these and other outbreaks Muslim hajis pilgrims in 1987. Apparently this
(62, 63). clone was carried from South Asia (Nepal and/or
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Emerging Infectious Diseases Vol. 2, No. 1 — January-March 1996