|Trends in antibiotic resistance associated
with E. coli calf scours
David G White, Michael Moen and Darren Huber
Department of Veterinary and Microbiological Sciences, North Dakota State University, Fargo, North Dakota, USA
Antibiotic-resistant pathogens in animals pose a concern
not only with respect to the health of animals but because of possible transmission to humans as food-borne pathogens.
The problem is compounded by the growing number of pathogens that are resistant to multiple, structurally unrelated
drugs, leading to the concern that there are likely to be few effective antimicrobials available by the end of
the decade. Accordingly, more attention is now being paid to the ease with which resistance to both single and
multiple antimicrobials can develop among bacterial pathogens. If the current trends continue, we may see bacterial
pathogens that are resistant to all currently available antimicrobials. The Food and Drug Administration and the
United States Department of Agriculture are currently implementing strategies to address this threat.
Bovine calf scours is a condition that afflicts young calves and is usually characterized by diarrhea; if not treated
properly, it can be fatal. Escherichia coli is recognized as the single most important bacterial cause of calf
scours. Numerous serotypes have been associated with the disease, but the majority are enterotoxigenic E. coli
(ETEC). Most causative strains possess the K-99 antigen (fimbriae that allow bacteria to adhere to the gut mucosa)
and secrete toxins into the intestine of the calf, causing the diarrhea. The most important treatment of calf scours
is correction of the accompanying dehydration, loss of electrolytes, and acidosis. Antimicrobial therapy is also
often implemented at the same time. In the past few years, strains of E. coli have become increasingly resistant
to most first-line antibiotics, including third-generation cephalosporins, aminoglycosides, and even fluoroquinolones.
Infections caused by drug-resistant organisms are a major and costly problem in animal health. These infections
prolong illness and, if not treated in time with more expensive, alternative antimicrobial agents, can cause loss
of stock. This potential problem will continue to have a large impact on the cattle industry both here in North
Dakota and across the country if not investigated and solved.
We are collecting data to determine the prevalence of multiple-antibiotic resistance among E. coli strains implicated
in bovine calf scours in North Dakota. By studying E. coli isolates from scouring calves, we hope to identify trends
in antibiotic resistance and to track several virulence factors that have recently been identified in pathogenic
E. coli strains. Susceptibility testing for amikacin, ampicillin, ceftiofur (Naxcel; Upjohn, Kalamazoo, MI), gentamicin,
tetracycline, sulfachlorpyridazine (Vetisulid; Solvay Animal Health, Mendota Heights, MN), sulfonamides, enrofloxacin
(Baytril; Miles Animal Health, Shawnee Mission, KS), lincomycin, neomycin, spectinomycin, and trimethoprim/sulfadiazine
(Tribissen; Mallinckrodt Veterinary, Mundelein, IL) is performed using standard disk-diffusion assays. E. coli
strains are screened for several virulence factors: F-5 (K-99) fimbriae, enterotoxin genes (lt, sta, stb), shiga-like
toxins (slt-I, slt-II), enterohemolysin gene (Ehly), the E. coli attaching and effacing gene (eae), cytotoxic necrotizing
factor genes (cnf-I, cnf-II), and cs31a, a fimbriae gene.
More than 280 isolates of E. coli, obtained from clinical cases, were submitted to the North Dakota State University
Veterinary Diagnostic Laboratory in 1996. The resulting antibiograms of antibiotic susceptibilities have illustrated
patterns of resistance among the strains and changes in resistance over time. Preliminary results show that resistance
to fluoroquinolones (enrofloxacin, which currently is not approved for use in large animals) and to cephalosporins
(ceftiofur) are emerging among these E. coli strains. The level of resistance to the antimicrobials tested ranged
from less than 1% of isolates to 100% of submitted strains (see Table). Isolates submitted to our laboratory are
usually the "worst of the worse," being unresponsive to all other treatments.
Our testing shows that the most available drugs have the highest rates of bacterial resistance. Unfortunately,
these drugs are often the first line of defense in combating calf scours. Amikacin, enrofloxicin, or ceftiofur
continue to be effective and with electrolyte fluid therapy constitute the best treatment options for calf scours
at the present time. However, enrofloxacin is not approved for large animal use; amikacin and ceftiofur are only
approved for extra-label use.
Eighteen of the 280 strains were positive for enterohemolysin and were further characterized by polymerase chain
reaction for virulence factors previously reported to be involved in pathogenesis. All 18 strains were negative
for lt, sta, and stb. Six strains were positive for slt-I, five were positive for slt-II, and three strains were
positive for slt-I and slt-II. Four strains were positive for eae, and five strains carried the cs31a fimbriae
gene. Although the K-99 antigen was found in 56% of strains, none of the strains positive for enterohemolysin expressed
this antigen. Four strains were positive for cnf-I, and one strain was positive for cnf-2. Two strains were positive
for slt-II and cnf-I, and one strain was positive for slt-I and cnf-I. All strains positive for slt-I and slt-II
were also positive for shiga-like toxins production.
Our data strongly suggest that resistance to first-line antibiotics used for calf scours is emerging at an alarming
rate. Multidrug resistance and the presence of several virulence factors in the strains of E. coli responsible
for this disease pose an increasing threat to the successful management of bovine calf scours.