| dc.description.abstract | Acute diarrhoeal diseases account for the
highest infant and childhood morbidity and mortality
in tropical developing countries. In recent
years, attention has been focused on enterotoxigenic
Escherichia coli (ETEC) which are
among the most commonly encountered enteropathogens.
The occurrence of ETEC in both animals
and man has posed the questions whether animals
could serve .as sources of infection for man
and to what extent transmission between different
hosts takes place.
This study deals with these issues. It
also aims at determining the prevalance of ETEC
in various animal populations, either as a cause
of disease, or as normal inhabitants of the
intestines of healthy animals. Some samples
of human origin have been examined for comparison.
One prerequisite for the causation of disease
is the production of enterotoxins, inherent in
the term "ETEC" itself. Another is the possession
of fimbriae which enable the organism· to adhere
to tissuess
ETEC elaborate a heat-labile enterotoxin
(LT) and at least two heat-stable enterotoxins
(ST) responsible for intestinal fluid hypersecretion.
Since the discovery of the functional, structural
and immunological similarities between LT and
the enterotoxin of Vibrio cholerae, assays which
were based on these concepts were developed concurrently~
A major problem in the diagnosis of ETEC
disease is the inherent difficulty in identifytng
enterotoxigenic strains among the many non-toxigenic
E. coli present in a faecal sample as a part of
the normal intestinal flora. There is also no
consensus of opinion on how many colonies should
be tested for toxin production in order to obtain
reliable results.
The bioassays, tissue culture techniques
and immunoassays which are available for the
detection of enterotoxins are time consuming,
expensive and require skilled technical assistanceo
The application of the staphylococcal coagglutination
technique (CAG) which allows the
examination of a large number of isolated colonies
of E. coli from a primary culture of a stool specimen
would . _present a major advantage. An evaluation
of this test to determine its suitability for
routine laboratory detection of LT was one of the
objectives of the present study. The sensitivity
and specificity of the coagglutination test were
compared with those of the enzyme immunoassay (ErA)
for the detection of LT. E. coli isolates were also
tested for the heat-stable enterotoxin, STA, by
means of the suckling mouse assay.
A total of 10,709 colonies of E. coli from man
and animals w~re examined for LT by means of the ErA
and the CAG test. The CAG test had a sensitivity of
93% and a specificity of 100% compared with the EIA,
This finding is in conformity with the sensitivity
and specificity reported for the previously described
tube coagglutination test for the detection
of LT.
An average of 20 colonies per stool sample were
tested for LT, and in most cases about 50% of these
were positive. In a few cases, only 1 to 3 colonies
were posiLive per culture. The CAG test can be
performed rapidly and easily for the screening of a
large nwnber of colonies from a primary culture, a
feature which may more than compensate for its lower
sensitivity compared with EIA.
It was also found necessary to effect the
release of LT from cultures of E. coli to achieve
uniform, reliable results in the CAG test. Triton
X-l00 at a dilution of 0.05% was found to serve as
an adequate substitute for the more expensive antibiotic
polymyxin B.
Enterotoxigenic ,&.. coli, defined as LT positive,
ST positive or LT/ST positive, were isolated from 41%
(135/329) of children with diarrhoea from Kenyatta
National Hospital, and 53% (9/17) of children with
diarrhoea from Muhimbili Medical Centre, Dar es Salaam,
Tanzania.
ETEC was also isolated from 19% (la/54) of
healthy pigs, 33% (18/55) of diarrhoeic pigs 22%
(12/55) of healthy cattle, 7% (8/110) of healthy sheep
and 1.4% (1/73) of healthy goats. A calf with
diarrhoea also harboured ETEC. Most pigs with diarrhoeal
disease were less than 12 weeks old. The ages of
other domestic animals were unavailable.
ETEC was most prevalent in children within the
0-2 year age group. This is in agreement with findings
in Ethiopian children.
ST-producing strains of E. coli were more
frequent than LT positive E. coli in both children
and animals. This finding is in conformity with
results of studies on diarrhoeic patients in
Bangladesh~ but in contrast to results in Ethiopian
children where LT producers were more frequent.
Other studies report equal numbers of LT positive and
ST positive E. coli isolates.
All ETEC isolates from sheep and goats~ which
comprised 18% of the ETEC of animal origin, were
positive for LT only.
E. coli which produce both LT and ST are known
to cause more severe diarrhoea than producers of LT
only or ST only. The strains which are positive for
both enterotoxins comprised 3% of ETEC from children
with diarrhoea at Kenyatta National Hospital and '
6% of children with diarrhoea from Muhimbili Medical
Centre. These figures are similar to those reported
for villagers from the South Nyanza District of Kenya.
LT/ST positive E. coli strains comprised 4% of
ETEC from healthy pigs, 16% of diarrhoeic pigs and
5% of healthy cattle. An LT/ST positive strain was
also isolated from the calf with diarrhoea.
Adhesion fimbriae of ETEC exhibit considerable
host specificity in their attachment to the intestinal
mucosal cells of man and animals. Nevertheless, a
number of ETEC strains which do not possess the
characterised host-specific fimbriae may still cause
disease through as yet unknown attachment factors~ ,
F1, F2 and F3 fimbriae were detected orily in
E. coli isolated from humans, while F4, F5 and F6
were identified only in isolates from domestic
animals. F1 was present in ETEC as well as in non-
ETEC isolates from children, while F2 and r3 were
confined to only ETEC. Strains possessing F2 and F3
comprised 19% and 10% of al.L ETEC isolates fromchildren.
F4 fimbriae were identified in 29%, F5 in 18%
and F6 in 18% of ETEC isolates from pigs. The
remaining isolates (36%) were found to be nonfimbriate.
The only fimbrial type identified in
ETEC isolates from ruminants was F5, which was present
in 77% of ETEC from cattle and 38% of ETEC from sheep.
The only isolate of ETEC from a goat was also
positive for this fimbrial type.
A number of ETEC strains from both children and
animals were negative for fimbrial antigens. Due to a
lack of sera,these were not serotyped, and the
question of serologically identical strains of ETEC
being present in both man and animals could therefore
not be answered. It is also unclear whether
non-fimbriate ETEC may acguire the appropriate
fimbriae through plasmid transfer, which would
confer the ability to adhere to the intestinal
epithelium of a different species.
Strains of E. coli which colonize an individual
at infancy and become established as a part of the
normal microflora are non-toxigenic. It is however
conceivable that these non-toxigenic ~. coli, which
,may possess appropriate attachment factors, may
acquire the ability to produce enterotoxins through
plasmid transfer occurring in vivo following the
ingestion of ETEC of animal origin. The extent to
which this may occur will depend on whether the
ingested ~. coli of animal origin are capable of
transferring these plasmids, as well as the capability
of non-toxigenic human strains for acquiring the
plasmids which encode for enterotoxin production.
~fuile plasmids which encode for antibiotic
resistance are known to be readily transferable within,
as well as between species and genera of enteric
bacteria, it is largely unknown to what extent
transfer of plasmids that encode for the production
of enterotoxins and host-specific fimbriae may occur
in nature.
Most ETEC strains are restricted to a few 0
serogroups. Therefore, the frequency of transfer
tifplasmids encoding for fimbriae and enterotoxin
production may to some extent depend on the presence~
of E. coli of the appropriate serogroups wh lch can
acquire these plasmids.
This study has confirmed that ETEC strains of
human origin possess f'1 (somatic type 1) f'2 (Cf'A/1)
and f'3 (Cf'A/II) fimbriae which are specific for man~
ETEC which possess f'4 (K88) and [6 (987P) were limited
to pigs only, while ETEC strains with F5 (K99)
fimbriae o.1erecommon to pigs, sheep, goats and cattle.
Accordingly, the pattern which has emerged from
this study indicates that ETEC strains exhibit strict
host specificitY,the only exception being FS-fimbriated
ETEC which were found in pigs, sheep, goats and cattle.
It can therefore be concluded that cross-infections
are unlikely to occur between man and animals. Nevertheless,
the possibility of interchange of infective
plasmids between strains of animal and human origin
resulting in interspecies spread of ETEC disease
cannot be ignored. | en |
