Agriculture Reference
In-Depth Information
Fate of pathogens in soil
In contrast to bacteria, protozoan parasites
(e.g. Cryptosporidium and Giardia ) can survive in
livestock manures for an extended period, which
is likely due to their ability to form cysts and
oocysts. The Cryptosporidium oocyst can resist
die-off over a wide temperature range while
remaining infective (Fayer et al ., 2000). In
unstirred swine slurries spiked with Crypto-
sporidium oocysts, a 1-log reduction in oocysts
was calculated to occur at 270 and 345 days
during cooler and warmer temperatures, respec-
tively (Hutchison et al ., 2005b). In contrast, the
survival of Giardia cysts is highly temperature
dependent. In a mixed human and swine
manure, 90% of Giardia lamblia cysts were non-
viable at 130 and 4 days when the respective
incubation temperatures were 5° and 25°C
(Deng and Cliver, 1992a).
Viruses are obligate intracellular para-
sites that are unable to replicate outside their
host and, as a result, their numbers do not
increase once released into the environment.
A variety of physical, chemical and biological
factors are responsible for the stability of
viruses in animal manure management and
treatment systems. Virus survival in manures
is likely influenced most by temperature, pH
(very high or low), NH 3 , microbial activity,
aggregation (virus clumping), encapsulation
or embedding in membranes or particles, and
indirectly through solids association (Deng
and Cliver, 1995a; Sobsey et al ., 2006). In vari-
ous animal manures, D 90 values (time, in days,
required for a 90% reduction of virus titre)
ranged from <7 days for herpesvirus to more
than 180 days for rotavirus (Pesaro et al .,
1995). With a bovine parvovirus and porcine
enterovirus, D 90 values in animal manures
were 200-300 days when the viruses were
kept at 5°C (Srivastava and Lund, 1980; Lund
and Nissen, 1983). In mixed swine and human
waste, poliovirus type 1 was more stable at
14°C than at 21°C, with respective D 90 values
of 52 and 19 days (Deng and Cliver, 1992b).
When dairy manure was mixed with human
waste, D 90 values for hepatitis A virus at 5°C
were 35 days compared with 8 and 7 days at
25° and 37°C, respectively (Deng and Cliver,
1995b). As with bacteria and protozoan para-
sites, livestock manures represent a potential
viral hazard when applied to agricultural land
without being treated (Sobsey et al ., 2006).
It has been demonstrated that pathogenic bacte-
ria survive longer when manures are immedi-
ately incorporated into soils than when left on
the surface for some time (Hutchison et al .,
2004). Pathogens on the surface may be exposed
to UV irradiation, temperature fluctuations and
desiccation that can potentially decrease their
ability to survive compared with soil-incorporated
pathogens. In soils, however, indigenous soil
microorganisms have been shown to increase
the inactivation rate of pathogens (Dowe et al .,
1997; Jiang et al ., 2002). The pH and tempera-
ture of soil also influences the survival of bacte-
ria, which is limited by low soil pH and higher
temperatures (Gerba et al ., 1975). Soil texture
may also enhance the survival of pathogens, as
E. coli O157 was reported to survive up to 2 months
longer in loam and clay soils than in sandy soil
(Fenlon et al ., 2000).
In sandy and clay loam soils amended with
various manures, Campylobacter, E. coli O157,
Listeria and Salmonella were found to survive as
long as a month or longer (Nicholson et al .,
2005). Similarly, Stanley et al . (1998) detected
Campylobacter for up to 20 days after the applica-
tion of contaminated dairy slurry, while Jones
(1986) reported survival times for Salmonella up
to 259 days in soils amended with animal faeces.
In addition, it was found that Salmonella may
persist in soils for a longer period in a viable non-
culturable state, thus avoiding detection via use
of traditional culture-based techniques (Turpin
et al ., 1993). Listeria are ubiquitous in the rhizo-
sphere, making them well adapted to survive for
extended periods in soils (Van Renterghem et al .,
1991). In dairy manure-amended soil, Listeria
monocytogenes survived for up to 43 and 14 days
when incubated at 5° and 21°C, respectively
(Jiang et al ., 2004). Dowe et al . (1997) showed
that chicken manure promoted better growth of
L. monocytogenes than did liquid hog manure,
but only when the competitive bacterial flora
was reduced by autoclaving.
In soil, Giardia cysts were inactivated after
incubation for 1 week at −4° and 25°C; however,
cysts were recoverable from soils for 2 months
when maintained at 4°C (Ziemer et al ., 2010).
Cryptosporidium oocysts are more environmen-
tally resistant and remained infective >3 months
in soil at −4° and 4°C, but at 25°C degradation
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