Download Rational antibiotic use

APPLIEDSCIENCE By Daniel Scholl, Herman Barkema
and Julie Baillargeon
Rational antibiotic use
Consumer concerns about resistant
bacteria prompt research into ways
producers can use antibiotics more
effectively when treating mastitis
oug Morrison is the third generation on the family farm
tucked into the Kingsbury hills
between
Drummondville
and
Sherbrooke, Que. In Doug’s grandfather’s day, on-farm medicine cabinets
did not even contain antibiotics.
Now, 60 years after the development
of the first treatment against mastitis,
antibiotic resistance has become a
source of concern for the Morrisons
and dairy producers across Canada.
Doug, who operates Malboro
Holsteins with his wife, Hilda, has
been a member of the Canadian
Bovine Mastitis Research Network
(CBMRN) scientific committee for
the past four years. He and Hilda
have 125 Holsteins, including 40
milking cows.
The voice of dairy producers on the
CBMRN committee, Doug communicates their mastitis research needs
and priorities. During discussions, he
and his fellow producers in the network have stressed the importance of
addressing antibiotic resistance in the
CBMRN’s research programs.
Producer interest in antibiotic
resistance stems mainly from consumer concerns, he says. The public
is often exposed to media reports on
such issues as genetically modified
organisms, mad cow disease and a
bacteria linked to antibiotic resistance known as C. difficile, which can
cause serious intestinal illness.
Regardless of whether a problem is
real, media coverage has raised con-
D
34 | November 2007 | MilkPRODUCER
sumer awareness of these
topics. Ordinary Canadians are asking more
questions about the origin and quality of the
food on their plates.
Dairy producers are sensitive to
consumer opinions and do everything
they can to produce safe milk that
meets consumer expectations, Doug
says. “As a producer, I take great
pride in offering a high-quality prod-
uct. The quality of the milk really is a
priority for us, and it’s a topic that’s
raised at almost every one of our
(Quebec milk producer) federation
meetings.”
It’s highly improbable that dairy
products containing antibiotic-resistant bacteria could ever reach grocery
stores. Producers routinely discard
milk from cows that have clinical mastitis.
Moreover, pasteurization effectively
Quebec dairy farmer Doug Morrison voices producer concerns to researchers.
eliminates bacteria—even bacteria
that may be antibiotic resistant.
Antibiotic use on the Morrison
farm is mainly associated with mastitis, as it is on most Canadian dairy
operations. Whether they are used
for treating clinical cases or for dry
cow therapy, antibiotics are a valuable tool for helping producers better
control mastitis in their herds.
However, Doug has doubts about
their efficacy. “It seems to me that
we’ve been using the same antibiotics
for decades and we’re still having the
same problem controlling mastitis,”
he says. “Are the antibiotics becoming less effective?”
If mastitis were caused by a bacterium resistant to a particular antibiotic, any treatment using it or a similar antibiotic as the active ingredient
would be ineffective. The infected
cow’s immune system could still fight
the infection, but the antibiotic
would be of no help.
Yet the same bacterium could be
susceptible to other types of antibiotics with different active ingredients.
In theory, then, increased antibiotic
resistance in mastitis bacteria could
indeed limit the ability to treat and
prevent mastitis with antibiotics.
It is difficult to tell whether antibiotics really are less effective than they
What’s ahead for antibiotic use
At the CBMRN, dairy producers like Doug Morrison are working closely with
researchers to identify mastitis research priorities and needs. Researchers
will be working on several projects related to antibiotic resistance over a
five-year period that began April 2006. Their results will help them answer
these questions:
1. How can producers quickly
determine whether or not a
case of mastitis should be
treated?
Research underway at the CBMRN is aimed at validating a protocol that
uses an on-farm culture system to determine, in just over 24 hours,
whether a clinical case of mastitis needs to be treated.
2.What quantity of antibiotics are Canadian dairy farms currently
using, and is this antibiotic use related to susceptibility in mastitis
bacteria?
With the help of the National Cohort of Dairy Farms, the CBMRN will estimate the quantity of antibiotics used for various reasons, and determine
whether farms with more antibiotic-susceptible mastitis cases use fewer
antibiotics.
3.Does the treatment of clinical mastitis or dry cows cause bacteria to
develop antibiotic resistance?
Canada and the U.S. are conducting a joint study to determine whether
bacteria present in cows’ intestines are more resistant to antibiotics following dry cow treatment than those in the intestines of cows that do not
receive dry treatment.
All Canadian dairy producers are participating financially in the research
program and are involved at all decision-making levels of the CBMRN. To
learn more or to share your ideas and comments, visit the CBMRN’s website www.mastitisnetwork.org or contact Julie Baillargeon at
[email protected].
were 10 or 30 years ago. Methods
used to measure antibiotic susceptibility have changed so much over the
years results can’t be compared. A
study conducted over two to three
years comparing antibiotic susceptibility of mastitis bacteria in first-calf
heifers and older cows after calving
found no difference between the two
groups. There is no scientific data to
suggest clinical mastitis is less responsive to antibiotic treatment now than
before.
The theoretical principle behind
the prevention of antibiotic resistance
is to reduce the likelihood antibioticresistant bacteria could have a competitive advantage over antibioticsusceptible bacteria. In practice, this
means using the right antibiotic for a
long enough time and at a high
enough dose to kill the bacteria.
To ensure a treatment is both long
enough and powerful enough, however, it’s recommended producers follow instructions on product labels or
dosages prescribed by veterinarians.
It is important never to reduce the
treatment to shorten withdrawal time
or limit treatment cost.
When one of his cows contracts
clinical mastitis, Doug always asks
himself whether he should treat her.
It’s a good question since research
has clearly shown available antibiotics are ineffective against certain
types of clinical mastitis. “It would
be really useful to have a test that
would quickly show us whether or
not antibiotics should be used in a
given situation,” Doug says.
That test could arrive soon.
CBMRN researchers are currently
working on validating methods for
on-farm mastitis bacteria identification (see The Milk Producer, July
2007). Not treating mastitis cases
that are unlikely to respond to treatment would reduce unnecessary
antibiotic use, unjustified withdrawal
times and the risk of increasing
antibiotic resistance.
While waiting for the new tools,
Doug is evaluating a number of
strategies for rational antibiotic use
in mastitis treatment. For clinical
MilkPRODUCER | November 2007 | 35
mastitis, he uses a treatment protocol
developed in collaboration with his
veterinarian.
Although he is comfortable with
using antibiotics when necessary,
Doug is aware of consumer concerns
and risks associated with antibiotic
resistance. He relies heavily on
CBMRN research to answer his questions on antibiotic resistance and to
develop strategies for rational antibiotic use. “For me, antibiotics are a
management tool that has to be used
properly but that we can’t get along
without,” Doug says.
Daniel Scholl is director of the CBMRN
and professor at the faculty of veterinary
medicine at the University of Montreal;
Herman Barkema is CBMRN mastitis
monitoring research team leader and
professor at the University of Calgary,
faculty of veterinary medicine; and Julie
Baillargeon is agronomist and CBMRN
transfer manager.
Resistance 101
What is it?
Antibiotic resistance is the ability
of bacteria to grow and multiply in
the presence of an antibiotic.
Resistance develops in bacteria
with genes enabling them to produce proteins that protect against
the effects of the antibiotic
molecule.
Is it reversible?
In theory, bacterial populations
that have become resistant to a
certain class of antibiotics can
become susceptible to them again
after a prolonged discontinuation
of antibiotic use.
How does resistance develop?
1. By selection pressure. Bacterial
populations often contain some
cells susceptible to a given antibiotic and others more or less resistant. If an antibiotic administered
at a low dose kills susceptible cells
but fails to kill moderately resistant bacteria, the surviving bacteria
will remain in the mammary
gland, free to multiply and
become the dominant strain. The
bacterial population would then
become more resistant.
2. By gene transfer. A number of
the bacterial genes associated
with antibiotic resistance are carried by plasmids, small DNA molecules found in a variety of bacterial species. Plasmids can be transferred between bacterial cells.
When plasmids containing antibiotic-resistant genes are transferred
to other bacteria, the recipient
bacteria acquire resistance to
antibiotics.
3. By induction. Bacterial genes
must be turned on or induced to
express the proteins required for
antibiotic resistance. Factors that
induce antibiotic resistance are not
well understood, and are the focus
of several research projects, especially in human medicine.
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