Antimicrobial Susceptibility of Fecal Escherichia coli Isolates in Dairy Cows Following Systemic Treatment with Ceftiofur or Penicillin

The existence of resistance mechanisms of bacteria against the
action of antimicrobial drugs is a well documented fact and has
complicated treatment of bacterial disease since the beginning of
antimicrobial therapy in the last century. Drug resistance is a
problem of veterinary and human medicine alike, especially when
considering infections with food-borne pathogens such as
Salmonella. Fecal E. coli isolates were used as a model to measure
the effect of treating dairy cows systemically with ceftiofur on
antimicrobial resistance of this important pathogen. Ceftiofur, a
third-generation cephalosporin, is an attractive treatment for
certain diseases in dairy cattle such as metritis and interdigital
necrobacillosis since withholding times for meat are short and milk
does not need to be withheld from marketing. Third-generation
cephalosporins are also used in human medicine, for example in the
treatment of systemic non-typhoidal salmonellosis in children.
Therefore, the emergence and spread of resistance towards this
class of antimicrobials has been a focus of concern. Effects of
ceftiofur treatment on a group of cattle were compared to a
penicillin treated group and a healthy control group not receiving
treatment. Reduction of susceptibility of E. coli was tested for
ampicillin, tetracycline and three different concentrations of
ceftiofur. A significant decrease in the concentration of fecal E.
coli on days 2 and 7 post-treatment was detected in animals treated
with ceftiofur. Treatment did not have a significant effect on the
proportion of isolates expressing reduced susceptibility to
ceftiofur. The result of the reduction of total E. coli count in
the samples during and after ceftiofur use is consistent with
results reported by other authors. The large number of isolates
analyzed provided a high possibility of detecting major effects of
ceftiofur treatment on E. coli susceptibility. However we did not
observe a net increase in reduced susceptible E. coli isolates with
the exception of decreased tetracycline susceptibility in the
ceftiofur treated group on day 2. The results of this work lead to
the conclusion that the emergence and spread of resistance-encoding
genes in a bacterial population under the influence of ceftiofur
are rare and that the effect could not be measured even though we
were able to draw information on a large number of isolates
analyzed compared to other investigations previously published on
this topic. The results are applicable to commercial dairy herds
with typical antimicrobial treatment practices comparable to those
used in the US, including prior use of ceftiofur for treating
individual sick animals.