Enterohemorrhagic and other Shigatoxin-producing Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 140

where E. coli strains are classified based on phage types (PT), strains of PT8,
PT2, PT28/21 predominate in human isolates, whereas PT21 is a common PT
isolated from cattle but found less frequently in clinical cases ( Mora et al., 2004 ).

Transmission
Transmission of STEC from animal reservoirs to humans occurs predominantly
through a variety of food vehicles ( Table 5.4 ). One important factor that dis-
tinguishes STEC, particularly E. coli O157:H7, from many other food-borne
pathogens is its very low infective dose. E. coli O157:H7 has an estimated infec-
tive dose of 10-100 organisms ( Keene et al., 1994 ; Tuttle et al., 1999 ; Strachan

TABLE 5.4 Major multistate STEC outbreaks in the United States between
2006-2012

Number
of states
affected

Vehicle/Source of
infection

Number
infected Year Serotype

2012 O26 Raw clover sprouts 25 (as of

03/08/12)

8

2011 O157:H7 Romaine lettuce 60 10

O104:H4 Travel to Germany 6 -

O157:H7 Bologna 14 5

O157:H7 In-shell hazel nuts 8 3

2010 O157:H7 Cheese 38 5

O145 Shredded romaine

lettuce

26 5

O157:H7 Beef 21 16

2009 O157:H7 Beef 26 8

O157:H7 Beef 23 9

O157:H7 Prepackaged cookie

dough

72 30

2008 O157:H7 Beef 49 7

2007 O157:H7 Pizza 21 10

O157:H7 Ground beef patties 40 8

2006 O157:H7 Lettuce 71 5

O157:H7 Fresh spinach 199 26