Agriculture Reference
In-Depth Information
Antibiotics have been detected in ground
and surface water, usually in ppb concentrations
(Sarmah
et al
., 2006). In a sampling survey of
rivers in 23 watersheds in Alberta, Canada,
parts per trillion concentrations of ten antibiot-
ics were detected in a total of 51% of samples
(Forrest
et al
., 2011). Monensin was the most
frequently detected (34% of samples) at concen-
trations to 853 ppt. These concentrations are
orders of magnitude lower than concentrations
considered ecotoxic, but data in this area are
scarce and indirect or cumulative effects on an
ecosystem are possible.
gene encoding resistance to each of the three
classes of antibiotics of interest (tetracylines, sul-
fonamides, erythromycins), and faecal samples
from all calves carried multiple types of antibiotic
resistant genes (ARGs) within the same sample.
Among the eight ARGs examined across three
classes,
tet
O was the only one to exhibit an
increase in response to antibiotic treatment, and
only when normalized to 16S rRNA genes. These
results suggest that removal of chlortetracycline
and neomycin from milk replacer will not elimi-
nate shedding of ARGs into the environment.
Similarly, McKinney
et al
. (2010) observed only
modest reduction of
tet
and
sul
ARG abundance
in waste lagoons on organic dairy farms relative
to those of conventional dairies.
While bacteria can adapt to ionophores
(Houlihan and Russell, 2003), genes coding for
resistance to any ionophore have not been iden-
tified. Also, the use of ionophores in cattle feed
does not necessarily lead to other types of antibi-
otic resistance (Houlihan and Russell, 2003).
Development of antibiotic resistance
in gut and faecal bacteria
Increasing antimicrobial resistance in manure
of mature cattle (Alexander
et al
., 2009) and
other livestock (Marshall and Levy, 2011) has
been observed with feed antibiotics. In calves fed
'waste milk' containing penicillin residues, resist-
ance of faecal bacteria to penicillin increased
with increasing dose fed (Langford
et al
., 2003).
Berge
et al
. (2006) found that milk replacer
containing neomycin sulfate and tetracycline
selected for
Escherichia coli
resistant to classes of
antimicrobials not used, including aminoglyco-
sides, chloramphenicol and sulfonamides.
In the Netherlands, resistance of
E. coli
to
antibiotics used regularly in poultry in the fae-
ces of broiler, turkey and layer indicated that
resistance was more prevalent in broiler and
turkey populations given antibiotics routinely
than in layer populations where antibiotics are
rarely used (van den Bogaard
et al
., 2001).
Resistance to nearly all antibiotics tested was
higher in faecal
E. coli
of turkey and broiler
farmers than in laying-hen farmers, and was
also elevated in people who slaughter turkey
and broilers. Genetic similarity between the
resistant
E. coli
in faeces of broilers, turkeys and
the people who work with them supported the
hypothesis that resistant clones and resistance
plasmids of
E. coli
may be transmitted from
poultry to humans.
In contrast, a recent culture-independent
(qPCR) evaluation of the effect of antibiotic addi-
tion in milk replacer on antibiotic resistance in
calf faeces through weaning showed little differ-
ence with antibiotic treatment (Thames
et al
.,
2012). Faeces of all calves carried at least one
Legal restrictions on feeding antibiotics
In the USA, the Food and Drug Administration
(FDA) recently issued new rules limiting anti-
biotic use in milk replacers for calves (21CFR
§ 520.1484; 21CFR § 520.1660d). The impact
of these changes is not yet clear, but the com-
plete ban on sub-therapeutic use of antibiotics
in the EU in the late 1990s provides a case
study of long term effects (Boerlin
et al
., 2001).
A major trigger for earlier bans in Denmark and
other Nordic countries was linkage of avoparcin
use in broiler chickens and swine with deadly
vancomycin-resistant
Enterococci
infection in
humans (Bager
et al
., 1997; Bates, 1997). In the
3 years following the European ban, a slight
decrease in antibiotic resistance was observed
in rectal swabs collected from food animals at
slaughter, and a documentable decrease in
acquired faecal enterococci resistance in humans
was observed (Casewell
et al
., 2003). Following
the Taiwan avoparcin ban in chickens in 2000,
the incidence of VRE decreased significantly
(Lauderdale
et al
., 2002). However, VRE still per-
sisted following the ban and resistance to other
classes of antibiotics, including tetracyclines and
macrolides, stayed the same or even increased. In
support of this observation, vancomycin and
macrolide ARGs have been noted to be present
