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et al., 2000
;
Fernandez-Brando et al., 2011
). However, STEC does not
appear to induce a sufficient antibody response in humans to confer protec-
tive immunity (
Orth et al., 2008
). Indeed, antibody response to Stx is not
universal among STEC-infected individuals (
Karmali et al., 1994
), and the
apparent low amount of Stx antibodies may reflect of the small amount of
this highly potent toxin produced during human infection. It is also possible
that the altered regulation of innate immune response by EHEC type III
effectors interferes with the development of an effective adaptive immune
response.
Control and prevention
Control and prevention of STEC infection is of tremendous importance
because of the lack of specific treatments for this potent pathogen. Attempts
to develop human vaccines against STEC based on Stx, T3SS proteins, and
STEC LPS antigens have been unsuccessful, and current preventive measures
focus primarily on breaking the transmission cycle by controlling the contami-
nation of food. Most of the past STEC outbreaks were associated with a fail-
ure to adhere to the proper control measures. The 1994 declaration of
E. coli
O157:H7 as an adulterant in ground beef in the US resulted in a zero tolerance
policy and magnified the legal liability of the meat industry (
Dodd and Powell,
2009
). Since then there have been important advances in pre- and post-harvest
control measures in the meat industry, regulated by the USDA Food Safety and
Inspection Service (FSIS), that have contributed to a decline in beef-associated
outbreaks in the US (
Koohmaraie et al., 2007
;
CDC 2007
). Recently, a policy
update from the FSIS classified six new non-O157 STEC serogroups as adul-
terants in beef that require testing. These strains, known as 'Big Six
E. coli
',
include the serogroups O26, O45, O103, O111, O121, and O145 (
Kalchayanand
et al., 2012
).
Pre-harvest intervention both diminishes beef carcass contamination, which
is closely associated with STEC fecal shedding and hide prevalence, and also
reduces the environmental prevalence of STEC, thereby diminishing the risk
of contamination of other food products such as leafy vegetables (
Berry and Wells,
2010
) (
Table 5.5
). Vaccines that prevent STEC colonization of cattle are an
emerging pre-harvest control measure. Current cattle vaccines licensed for use in
the US include Epitopix, which targets siderophore receptors and porin proteins
and has been shown to reduce fecal shedding of
E. coli
O157:H7 (
Thornton
et al., 2009
). Vaccination of feedlot cattle with T3SS proteins, which are essen-
tial for cattle colonization, reduced environmental contamination with STEC
and the frequency of hides that were positive for
E. coli
O157:H7. This vaccine
is commercially available in Canada (Econiche™, Bioniche Life Sciences Inc)
and is currently being licensed for use in the US (
Smith et al., 2009
). Other
experimental vaccines targeting intimin, EspA, and a combination of T3SS pro-
teins are being explored (
Berry and Wells, 2010
).
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