Sandra Godden on AB resistance 11-2009
I’m no expert on antimicrobial resistance but will share what I’ve read. No, pasteurization
does not ‘inactivate’ antibiotics that may be in nonsaleable milk (usually at very low
concentrations if present).
One study, which fed calves milk artificially spiked with varying concentrations of penicillin,
reported a dose response, with increased shedding of penicillin-resistant bacteria in feces as
concentrations of penicillin fed in milk were increased (Langford et al., 2003). However
observational studies have reported no obvious increase in antibiotic resistance of intestinal
bacteria in calves fed non-saleable milk (Wray et al., 1990; Berge et al., 2005). The Berge
study reported increased resistance in faecal E. coli in calves raised on farms that fed
medicated milk replacer (as compared to farms that fed non-medicated milk replacer or farms
that fed nonsaleable milk).
Studies to date have not typically followed calves beyond the weaning event to investigate
whether resistant bacteria ‘go away’ after exposure to antibiotics through the milk diet is
removed (hypothesis: resistant bacteria will fade into the background as the susceptible
bacteria ‘bloom’ up again). We are in the midst of an observational study to investigate if this,
in fact, occurs.
Given growing public concern about antibiotic use in food animals, subtherapeutic or
otherwise, the practices of feeding either i) non-saleable milk with potential for residues or ii)
medicated milk replacer are likely to receive continued scrutiny.
Sandra
Langford, F.M., D.M. Weary, and L. Fisher. 2003. Antibiotic resistance in gut bacteria from
dairy calves: A dose response to the level of antibiotics fed in milk. J. Dairy Sci. 86:39633966.
Wray, C., S. Furniss, and C.L. Benham. 1990. Feeding antibiotic-contaminated non-saleable
milk to calves – effects on physical performance and antibiotic sensitivity of gut flora. Br.
Vet. J. 146:80-87.
Berge, A.C.B., E.R. Atwill, and W.M. Sischo. 2005. Animal and farm influences on the
dynamics of antibiotic resistance in faecal Escherichia coli in young dairy calves. Prev.
Vet. Med. 69:25-38.
Sandra Godden DVM, DVSc
Associate Professor
Department of Veterinary Population Medicine
College of Veterinary Medicine, University of Minnesota
225 VMC, 1365 Gortner Ave.
St. Paul, MN 55108
Tel: 612-625-8177
Fax: 612-625-6241
Email: godde002@umn.edu
Sandra,
on your statement of "whether resistant bacteria ‘go away’ after exposure to antibiotics
through the milk diet is removed", you are right about the lack of research following these
calves over time, but all studies consistently show much lower proportion of resistance of E.
coli and Salmonella (the 2 bugs that are usually studied in dairy cattle) in adult cows than in
calves.
We did a longitudinal study at CSU on this, and the results are consistent. Resistance drops
after weaning.
NOTE, the point of using antimicrobials is to KILL SUSCEPTIBLE bacteria. Therefore, if you
sample while there are still antimicrobial residues in the body, you should expect to find
only/mostly resistant bacteria, you did kill the susceptible bacteria! Once the susceptible
bacteria grow back (uptake form environment) they overpower the resistant bacteria
and proportion of isolation is usually back to normal "background" levels. Resistance is
usually accompanied by a "fitness cost".
Aurora
Aurora Villarroel, DVM, MPVM, PhD, Dipl ACVPM
Epidemiologist, Assistant Professor
221 Magruder Hall
Corvallis, OR 97331
Ph: (541) 737 2858