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in Bangladesh (
Evans and Evans, 1973
;
Nalin, 1975
;
Ryder et al., 1976
;
Sack
et al., 1977
) later corroborated these early studies.
Soon after the initial discovery of enterotoxin-producing
E. coli
in patients
with severe cholera-like syndromes came reports of travelers' diarrhea linked to
E. coli
. While travelers' diarrhea was clearly a previously known entity, Rowe
and Taylor first described an outbreak of diarrhea related to particular
E. coli
serotype O148:H28 in British soldiers deployed to the United Kingdom of
Aden (now Yemen) (
Rowe and Taylor, 1969
), and interestingly, ETEC of the
same serotype was shortly thereafter identified in soldiers deployed to Vietnam
(
Rowe et al., 1970
;
DuPont et al., 1971
). One of these isolates, B7A, was used
in early volunteer challenge studies which firmly established the pathogenic
nature of ETEC (
Levine et al., 1979
). Following these initial reports, came the
publication of a number of studies establishing the importance of ETEC as the
principal etiologic agent of travelers' diarrhea (
Shore et al., 1974
;
Merson et al.,
1976
).
Evolution
Enterotoxigenic
E. coli
strains exhibit both phenotypic and genetic diversity.
This likely relates to the fact that genes for both ST and LT are encoded on plas-
mids (
Gyles et al., 1974
). Several lines of evidence support the idea that ETEC
have arisen through independent acquisition of these essential toxin genes by
a genetically diverse population of
E. coli.
First, while some serotypes appear
more commonly in collections of ETEC than others, enterotoxigenic
E. coli
are
represented by multiple O and H serotypes (
Wolf, 1997
). Some phylogenetic
comparisons based on multi-locus sequence typing (MLST) would likewise
suggest that the chromosomal background of ETEC strains is not highly con-
served (
Turner et al., 2006
). Nevertheless, in the most extensive phylogenetic
analysis of ETEC to date, performed on over 1000 ETEC isolates, Steinsland
et al. determined that strains segregated into distinct clonal groups represented
the majority of ETEC (
Steinsland et al., 2010
). Their analysis suggested that
the population of currently circulating ETEC strains likely emerged on several
occasions from distinct established globally distributed lineages. One implica-
tion of these later studies is that while the evolution of ETEC has certainly been
complex, the existence of distinct pathogen lineages may facilitate the identifi-
cation of conserved antigens useful in vaccine development.
Epidemiology and global impact
Diarrheal illnesses are a leading cause of death in developing countries where
more than a fifth of all deaths in children under the age of 5 years can be attributed
to infectious diarrhea (
Kosek et al., 2003
). Collectively, diarrheal pathogens are
estimated to cause between 1 and 2 million deaths annually (
Kosek et al., 2003
;
Boschi-Pinto et al., 2008
). ETEC contributes significantly to mortality associated
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