Agriculture Reference
In-Depth Information
(2007) showed that
Salmonella
was introduced to melon fi elds in Texas via irrigation
water and that fruit grown in the fi elds were contaminated with the bacterium.
Raspberries responsible for several outbreaks were grown in Guatemala where con-
taminated water is a signifi cant factor in the spread of
Cyclospora cayetanensis
(Mota
and others 2000; Mansfi eld and Gajadhar 2004).
Growing demand and competition for water to irrigate food crops and to satisfy
public or industrial uses has put increasing pressure on surface and groundwater sup-
plies in many parts of the world. Commercial-scale production of fruit crops is a
water-intensive activity, and natural supplies are used where the availability or cost
of treated water is restrictive. The microbiological quality of surface waters is change-
able, particularly near urban areas or where mixed agriculture is practiced. Subsurface
sources generally present less risk, although distant transport of human pathogens
through groundwater has been demonstrated and wells can become contaminated
(Bitton and others 1983; Powell and others 2003). Notwithstanding the source of
irrigation water, frequent testing is required to reduce the risk of crop contamination.
Microbiological standards based on fecal coliforms or
E. coli
tend to vary, but toler-
ances are increasingly restrictive in recognition of the associated safety risks. The
World Health Organization recommends that water applied to crops eaten raw contain
<
1,000 fecal coliform bacteria/100 ml (WHO 1989). Canadian standards, for example,
include the additional provision that samples contain
100
E. coli
/100 ml (CCME
1987). Although it is not possible to correlate the level of disease risk associated with
a given titer of indicator microorganisms, these guidelines will undoubtedly continue
to be used until economical technologies for the rapid, direct detection and identifi ca-
tion of human pathogens in water become available. Furthermore, it is diffi cult to
meet guidelines consistently in many producing regions, particularly where use of
reclaimed waters from sewage treatment plants is increasingly common (Steele and
Odumeru 2004). To date, little progress has been reported in the development of
practical, fi eld-deployable water-disinfection units that can accommodate volumes
needed for large-scale commercial fruit production.
Research in vegetable production systems suggests that overhead irrigation leads
to higher contamination levels on the crop compared with furrow or drip irrigation
(Keraita and others 2007). The effect of irrigation practice on the transfer of patho-
gens to fruit crops has not been examined in detail. Overhead irrigation would appear
to increase the risk due to the higher probability of direct contact with fruit.
Microorganisms deposited on plant surfaces are subjected to considerable stresses,
however, and some reports suggest that postirrigation die-off is rapid due to UV
radiation or dehydration (Brandl 2006). There is clearly a need to better understand
the role of different irrigation methods on the fate of human pathogens in fruit
production systems.
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Airborne Contamination
Research carried out in urban environments has shown that enteric bacteria such as
E. coli
can be present in air or dust disseminated by surface winds (Rosas and others
1997). Microbiological analysis of air at high altitudes and the historical record indi-
cate that transcontinental spread of human and plant pathogenic species is clearly
possible (Mohr 1997). The relationship between air quality within confi ned processing
