Agriculture Reference
In-Depth Information
could be treated (e.g. composted, anaerobically
digested) prior to land application, thus reduc-
ing subsequent risks associated with pathogens
(Lund
et al
., 1996; Tiquia
et al
., 1998).
significant knowledge gaps still exist with
respect to the fate and transport of bioaero-
sols, making it difficult to predicate the health
risks associated with aerosolized pathogens
accurately (Pillai and Ricke, 2002).
Aerosolization of pathogens
Fate, transport and negative impacts
of veterinary antibiotics in soil
Pathogens can potentially become aerosolized
during the land application of liquid and solid
manures, representing a potential risk if inhaled
in sufficient quantities or ingested after deposi-
tion on food crops and fomites (Brooks
et al
.,
2004; Dungan, 2010). When bioaerosols are
released from a source, they can be transported
short or long distances, eventually being depos-
ited (Brown and Hovmøller, 2002; Jones and
Harrison, 2004). Unlike zoonotic agents in
manures, soils and waters, aerosolized microor-
ganisms are highly susceptible to meteorological
factors such as relative humidity, solar irradi-
ance and temperature (Cox and Wathes, 1995).
In general, both laboratory and field studies
have shown that the viability of aerosolized
microorganisms decreases with decreases in rel-
ative humidity and increases in ambient tem-
perature and solar irradiance (Mohr, 2007).
Despite the potential for bioaerosol formation
during these activities, very few studies have
investigated the risk of human exposure to path-
ogens during the land application of animal
manures.
During the land application of swine and
cattle slurries by tanker and high-pressure
spray guns, airborne bacterial counts steadily
decreased with distance from the application
site and pathogenic bacteria such as
Salmonella
spp. and
Klebsiella pneumoniae
were not detected
(Boutin
et al
., 1988). During the spray irriga-
tion of swine slurry, a marker strain of
E. coli
was detected 125 m downwind in aerosols,
but not at 250 and 500 m downwind
(Hutchison
et al
., 2008). Using polymerase
chain reaction (PCR) to amplify 16S riboso-
mal RNA genes in air samples collected imme-
diately adjacent to the spreading of swine and
dairy cattle slurry, pathogens having an aero-
genic route of infection were not identified
(Murayama
et al
., 2010; Dungan, 2012).
While results from these and other studies
suggest a low risk for exposure to pathogens
(Brooks
Once in the soil, antibiotics can be transported
to surface and ground waters in a dissolved
phase or sorbed to soil particles and colloids
(Kay
et al
., 2004; Song
et al
., 2010). Tetracyclines
have been shown to strongly sorb to soils, while
macrolides, such as tylosin, have a weaker ten-
dency to sorb (Rabølle and Spliid, 2000; Allaire
et al
., 2006). In contrast, sulfonamides are likely
the most mobile of the antibiotics and have
been detected in groundwater at relatively high
concentrations (Hamscher
et al
., 2005; Batt
et al
., 2006). Despite such knowledge, there is
still little known about the occurrence, fate and
transport of VAs in the soil-water environment.
Recent research has shown that the addition
of pig manure to soil caused a temporary
increase in tetracycline resistance genes soon
after manure application (Sengeløv
et al
.,
2003). When manures are land applied, resist-
ant bacteria are also transferred, creating the
possibility of horizontal transfer of resistance
genes to the indigenous soil bacteria. The addi-
tion of nutrients to soils has been shown to
enhance horizontal transfer to bacteria by pro-
viding nutrients for activation of transfer as
well as mobilizing genetic elements (Top
et al
.,
1990; Heuer and Smalla, 2007). Furthermore,
antibiotics and their metabolites in the manures
might give resistant bacteria a selective advan-
tage after being land applied (Halling-Sørensen
et al
., 2001).
In addition to concerns over the prolifera-
tion of antibiotic-resistant bacteria, other con-
cerns over antibiotics in the environment are
related to negative impacts on water quality
and soil microbial communities. While detect-
able levels of antibiotics have been observed in
natural waters throughout the USA, much
needs to be learned about the chronic effects of
low-level exposures of pharmaceuticals on
human and environmental health (Kolpin
et al
.,
2002; Focazio
et al
., 2008). Currently, there
et al
., 2005; Tanner
et al
., 2005),
