Agriculture Reference
In-Depth Information
Cooley and others 2007). Similarly, recurrent outbreaks associated with tomatoes
were suspected of being grown on farms in Virginia and Florida, and outbreaks with
cantaloupes on farms in Mexico (Table 1.1).
Produce outbreaks linked to a region where a large amount of fresh produce is
grown is logical; however, a number of factors revealed by recent outbreak investiga-
tions are relevant to concepts of where, when, and how contamination occurs. As
noted, outbreaks have been associated with commodities grown in the same region
and with preharvest contamination rather than later in the distribution chain (e.g.,
transport or restaurant). Also, pathogen strains of the same serovar could be isolated
from watersheds in the vicinity of implicated fi elds, and for the fi rst time in recent
outbreak investigations,
E. coli
O157:H7 and
Salmonella
strains indistinguishable
from the clinical outbreak strains were isolated from environmental samples (CalFERT
2007b, 2007c, 2008; Cooley and others 2007; Greene and others 2008). Therefore,
accurate information about the fate and transport processes relevant to contamination
processes and the fi tness of pathogens near, on, or in produce plants in the fi eld is
critical for developing strategies for minimizing preharvest contamination of produce.
Incidence of Human Pathogens on Fresh Produce
How often are produce items contaminated with pathogens? The incidence is very low
generally, but any amount may be too much considering the low infectious dose for
some of the pathogens, especially
E. coli
O157:H7 on raw produce. The incidence of
major foodborne pathogens on different items of fresh produce and in animal hosts has
been reported in numerous studies, in addition to data relevant for assessing the survival
and fi tness of pathogens in agricultural environments such as manure, water, and soil.
These data are relevant to consider also for identifying potential point sources and
transport processes of pathogens in production environments linked to outbreaks.
Beuchat published in 1996 one of the fi rst and best reviews of reported incidence
of common foodborne pathogens on ready-to-eat vegetables, and the potential sources
of the pathogens and mechanisms of contamination (Beuchat 1996). The incidence,
growth, and survival of foodborne pathogens in fresh and processed produce has been
reported also in comprehensive reviews by Nguyen-the and Carlin (Nguyen-the and
Carlin 2000) and Harris and others (see Tables I-1 to I-7 in Harris and others 2003),
and other recent reviews (Johnston and others 2006b; Beuchat 2006; Mandrell and
Brandl 2004). Although distinctions between pre- and postharvest contamination are
not provided generally, these reviews provide useful summaries of the different
methods for isolating pathogens—for example,
Salmonella
,
Listeria
,
Yersinia
,
Campylobacter
species,
E. coli
O157:H7, and generic
E. coli
— from multiple types
of produce items that were grown in different regions of the world.
The incidence of pathogens reported in these separate studies often was between
0 and
20% reported
(Nguyen-the and Carlin 1994; Harris and others 2003; Mandrell and Brandl 2004).
Moreover, in the few studies reporting the concentration of pathogen per gram of
produce, the levels were low in most studies, even for generic
E. coli,
as a measure
of possible fecal contamination. For example, the percentages of positives out of 774
total samples tested for
Salmonella
on leafy vegetables or salad in eight separate
studies were 0 (0/151), 0 (0/63), 0.6 (1/159), 0.9 (1/116), 3.5 (2/57), 6.3 (5/80), 7.1
<
10% of all samples tested, with an occasional incidence of
>
