Spread of Multidrug-Resistant Rhodococcus equi, United States

Area of Science:

• Veterinary Microbiology
• Antimicrobial Resistance
• Genomics

Background:

• Multidrug resistance (MDR) in Rhodococcus equi, an animal and zoonotic pathogen, has been identified in US equine farms.
• This emergence follows widespread macrolide/rifampin antibioprophylaxis.

Purpose of the Study:

• To investigate the emergence and genetic basis of MDR R. equi.
• To understand the role of plasmid pRERm46 and chromosomal mutations in resistance.
• To assess the potential for zoonotic transmission and international spread.

Main Methods:

• Phylogenomic analyses of R. equi isolates.
• Plasmid characterization (pRERm46) and resistance profiling.
• Identification of specific mutations in chromosomal genes (e.g., rpoB).

Main Results:

• MDR R. equi emerged due to acquisition of the conjugative plasmid pRERm46, conferring resistance to macrolides, lincosamides, streptogramins, and tetracycline.
• Phylogenomic data revealed a dominant MDR clone (R. equi 2287) linked to coselection of pRERm46 and an rpoB^S531F mutation.
• A novel MDR clone (G2016) arose from pRERm46 transfer to other R. equi genotypes, associated with an rpoB^S531Y mutation.

Conclusions:

• Overuse of antimicrobial prophylaxis in animal agriculture can generate MDR pathogens with zoonotic potential.
• MDR R. equi and pRERm46-mediated resistance are actively disseminating within the US.
• International spread is probable due to global horse movements, necessitating surveillance and control measures.