Abstract
Dear Editor:
A
The World Health Organization has labeled the quinolone antimicrobials, of which enrofloxacin is a member, as a “Critically Important” agent among other relevant antimicrobials. Within the group of “Critically Important” antibiotics, quinolones are further distinguished as the “Highest Priority.” 2 As such, the use of these agents in food-producing animals can have serious repercussions for human health. Suggested by the Swann report in 1968, and confirmed by numerous other studies since, it has been established that use of antimicrobials at subtherapeutic dosages and regimens in food animals can lead to development of antibiotic-resistant bacteria, posing a hazard to both human and animal health. An example is fluoroquinolone-resistant campylobacter, which has been identified as persisting in a cattle reservoir. 3
The traditional thought of fluoroquinolone resistance is that mutants emerge after exposure to lethal levels of the antibiotics of this class. 4 Exposure to low concentrations of the fluoroquinolones exerts a selection pressure for resistance. This has been shown when ciprofloxacin, the active metabolite of enrofloxacin, has been exposed to Escherichia coli at low dosages. 4 Fluoroquinolone-resistant mutants spontaneously occur in Campylobacter populations and use of fluoroquinolone antimicrobials selects and enriches these mutants. 3 Owing to these concerns, the use of enrofloxacin beyond the approved label claim for bovine respiratory disease is strictly prohibited in some countries, such as the United States. Although the amount of enrofloxacin added to the semen in this study was sufficient to lead to inhibition of growth of bacteria after cryopreservation, once artificial insemination is performed, the concentrations within the nonsterile uterine environment could rapidly decline to subtherapeutic levels.
The bovine uterine microbiome is a complex environment, and home to numerous bacterial species. Campylobacter can inhabit the female bovine reproductive tract. Salmonella can be present under certain circumstances. Other bacteria of human health concerns, such as E. coli and Staphyloccus, have been isolated from the bovine uterus and vagina under some conditions. Thus, the introduction of small amounts of enrofloxacin in the form of a semen cryopreservative could present a path for antimicrobial resistance in potential zoonotic bacteria. With millions of cattle artificially inseminated every year, there is potential for increased emergence of fluoroquinolone resistance with this practice. Exposure of these millions of food animals to subtherapeutic dosages of “Critically Important” antibiotics for human health could represent multiple risks to the food supply.
The article mentions that exposure of B. indicus semen to gentamicin, tylosin, lincomycin, and spectinomycin increases fertility rate. Since the relative importance of those antimicrobials is lower than that of enrofloxacin, perhaps combinations of lesser priority drugs would present an alternative to enrofloxacin. Similarly, alternative practices in semen processing, such as colloid centrifugation, 5 could also decrease bacterial burden in semen without using critically important antimicrobials.