Abstract
Anaplasmosis is an economically-significant, hemolytic, tick-borne disease of cattle caused by Anaplasma marginale which can cause clinical anemia and death. Current control options are limited, and FDA-approved antimicrobial control options do not have a defined duration of use. A practical and routinely used anaplasmosis control method involves feeding free-choice chlortetracycline (CTC)-medicated mineral to pastured cattle for several months. Constant antimicrobial use poses the risk of expediting the development and dissemination of antimicrobial resistance in off-target commensal bacteria in the bovine gastrointestinal tract. The objective of this study was to determine the CTC-susceptibility of Escherichia coli isolated from anaplasmosis endemic beef cattle herds provided different FDA-approved free-choice CTC-medicated mineral formulations, all intended to provide cattle a dosage of 0.5 to 2.0 mg CTC/lb bodyweight per day. A closed-herd, comprised of Hereford-Angus cows, naturally endemic for anaplasmosis, were grazed in five different pastures with one herd serving as an untreated control group. The other cattle herds were randomly assigned one of four FDA-approved CTC-medicated mineral formulations (700, 5000, 6000, and 8000 g CTC/ton) labeled for “the control of active anaplasmosis” and provided their respective CTC-medicated mineral formulation for five consecutive months. Fecal samples were collected monthly from a subset of cows (n = 6 or 10) per pasture. Fecal samples were cultured for E. coli isolates and the minimal inhibitory concentration of CTC was determined. Baseline CTC-susceptibility of E. coli was variable among all treatment and control groups. The susceptibility of E. coli isolates was significantly different between study herds over the treatment period (p = 0.0037 across time and 0.009 at the final sampling time). The interaction between study herds and treatment period was not significant (p = 0.075).
Highlights
The objective of this study was to determine the CTC susceptibility of E. coli isolated from anaplasmosis endemic beef cattle herds provided different FDA-approved free-choice CTC-medicated mineral formulations in a pasture setting
Study animals consisted of cows and heifers from a natural anaplasmosis-endemic cow-calf herd routinely maintained on pasture from May to October
While there was considerable age variability within treatment groups and between the treatment and control groups, there was no statistical difference between age and CTC-medicated mineral formulation or minimal inhibitory concentrations (MIC) values
Summary
The use of medically important antimicrobials in food-producing animals has raised concerns about promoting antimicrobial resistance (AMR) [1]. In 2019, the total amount of antimicrobial drugs actively marketed was approximately 11.4 million kilograms [2]. Of the total drugs marketed, 6 million kilograms were medically important antimicrobial drugs sold and distributed for food-producing animals, of which 2.5 million kilograms were for use in cattle [2]. Tetracycline antimicrobials are considered highly important and have consistently been the largest portion (67%) of all antimicrobials sold in the U.S for Microorganisms 2021, 9, 2495.
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