Biology Reference
In-Depth Information
CLINICAL MANIFESTATIONS
Sources of STEC
STEC infections are predominantly zoonotic in origin and food-borne in nature
with ruminants serving as the primary animal reservoirs. Cattle are the major
reservoir hosts of STEC and they exist as a part of the intestinal microbiota
of cattle largely without causing clinical symptoms (
Borczyk et al., 1987
;
Chapman et al., 1993
;
Laegreid et al., 1999
). STEC colonizes the bovine lym-
phoid follicle-dense mucosa of the recto-anal junction (
Naylor et al., 2003
;
Fox et al., 2008
). Prevalence of STEC in cattle ranges from 0.2% to 48.8%
in the US and Canada (
Pennington, 2010
). Although earlier surveys in cattle
indicated a low (e.g. 1.8%) prevalence of STEC in feces, possibly due to the
poor sensitivity of isolation methods (
Herriott et al., 1998
), more recent stud-
ies have indicated a markedly higher prevalence of STEC in cattle (
Heuvelink
et al., 1998
;
Jackson et al., 1998
). For example, one study indicated that 87%
of beef cattle had at least one
E. coli
O157-positive pre-evisceration sample
(
Elder et al., 2000
). Currently, STEC is considered ubiquitous in cattle farms
and the shedding rate in cattle farms is estimated to be greater than 10%,
sometimes approaching 100% (
Hancock et al., 2001
;
Besser et al., 2007
).
Prevalence of STEC also varies within farms. Calves with a functional rumen
appear to carry STEC at a higher rate compared to heifers and other types of
cattle (
Pennington, 2010
). Furthermore, individual animals also show tremen-
dous variation with some animals designated as 'supershedders' excreting far
more bacteria than others. A study by
Chase-Topping et al. (2008)
in Scotland
demonstrated that 80% of the
E. coli
O157 transmission originates from 20%
of the high-shedding animals in a farm. The type of cattle and animal stress due
to movement and weaning are considered risk factors for high colonization and
shedding (
Chase-Topping et al., 2007
). Interestingly, there is also a seasonal
effect in the prevalence of STEC in cattle with peak prevalence in summer and
early fall (
Hancock et al., 2001
). This corresponds with the peak in outbreaks
involving ground beef in summer (
Rangel et al., 2005
). In addition to cattle,
other domestic animals such as sheep, goats, and pigs also serve as reservoirs of
STEC. Wild animals such as deer can also harbor and transmit STEC (
Oliveira
et al., 2008
;
Oporto et al., 2008
).
Not all of the over 100 different serotypes of STEC that have been isolated
from cattle are equally pathogenic to humans. Based on the Stx-bacteriophage
insertion sites, STEC O157 strains identified in the US are classified into either
bovine-biased genotypes, which are primarily found in cattle and rarely isolated
from human cases, or clinical genotypes, isolated from both human cases and
cattle (
Besser et al., 2007
). It is believed that bovine-biased genotypes may have
evolved towards reduced human virulence. Supporting this hypothesis, a recent
study demonstrated that infection of piglets and Dutch belted rabbits with clini-
cal genotypes of
E. coli
O157 resulted in markedly severe disease compared to
infection by bovine-biased genotypes (
Shringi et al., 2012
). Similarly, in Europe
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