Antimicrobial use continues even in situations where it is not deemed necessary, resulting in adverse effects on One Health due to the prevalence of multidrug resistance. In a collaborative study, researchers at the University of Georgia, Texas A&M University, and Hagyard Equine Medical Institute reported on the effects of antimicrobial use on drug resistance.
While this article was published a few years ago, it bears mentioning again because the same concerns still apply about injudicious use of antimicrobial drugs. It is relevant for equine practice decisions to keep up with the ever-changing landscape of antimicrobial resistance. The authors reported on the use of a macrolide with rifampin given prophylactically to “apparently healthy foals with pulmonary lesions identified with thoracic ultrasonography,” likely due to Rhodococcus equi infection. Despite evidence that most foals with a subclinical R. equi pneumonia will not develop clinical signs of pneumonia, treatment with a macrolide and rifampin still occurs.
Study: Antimicrobial Use in Foals
In the study, researchers compared 38 subclinical pneumonic foals treated with the macrolide erythromycin with 19 foals treated with gallium maltolate and 19 untreated control foals. They evaluated the foals before treatment and again 14 days later. They also obtained environmental samples from 76-100 horse breeding farms in Central Kentucky.
The study showed that oral macrolide treatment results in “dysbiosis, significantly decreased fecal microbiota abundance and diversity, and an expanded abundance and diversity of antimicrobial resistance genes in feces.” In contrast, gallium maltolate did not have adverse effects on fecal bacteria or environmental selection of bacterial resistance genes to R. equi. Gallium mimics iron, which bacteria need to survive and replicate. The bacteria then absorb the gallium, disrupting their metabolic functions and ultimately causing cell death without promoting drug resistance.
The researchers also evaluated the persistence of and selection for multidrug resistance genes in experimentally infected soil plots. From Weeks 4-24, macrolide-resistance isolates were significantly higher in macrolide-treated soil compared to gallium-maltolate-treated soil.
Enterococcus spp. and Streptococcus spp. also experience resistance to macrolide treatment. Oral treatment with the macrolide for 14 days increased the number of resistance gene copies but did not increase the number of antimicrobial resistance genes. Even healthy, untreated foals can be exposed to potential multidrug resistance genes through soil contamination and coprophagia of their dams’ feces.
The researchers identified that macrolides “increased abundance and diversity of antibiotic resistance genes to multiple antimicrobials: macrolides, aminoglycosides, glycopeptides, tetracycline,” to name a few.
Study Conclusions
The authors concluded, “Our results indicate that macrolide use promotes multidrug resistance in R. equi and commensals that are shed into their environment where they can persist and potentially infect or colonize horses and other animals.” Gallium maltolate shows good antimicrobial activity against R. equi with reduced development of macrolide resistance genes against R. equi and does not alter fecal microbiota or contaminate the soil with macrolide-resistant genes.
Reference
Alvarez-Narvaez S, Berghaus LFj, Morris ERA, Willingham-Lane JM, Slovis NM, Giguere S, and Cohen ND. A Common Practice of Widespread Antimicrobial Use in Horse Production Promotes Multi-Drug Resistance. Nature – Scientific Reports 2020, vol. 10, article # 991
Related Reading
- Translational Medicine in Human & Equine Sport
- Antimicrobial Stewardship With Honey
- Disease Du Jour: Antimicrobial Stewardship in Equine Practice
Stay in the know! Sign up for EquiManagement’s FREE weekly newsletters to get the latest equine research, disease alerts, and vet practice updates delivered straight to your inbox.
