Antimicrobial susceptibility patterns of Rhodococcus equi from necropsied foals with rhodococcosis.

This research investigates the resistance of the bacterium Rhodococcus equi, which causes rhodococcosis in foals, to commonly used antibiotics. The study found higher degrees of resistance in bacteria from foals that had been treated with combination therapy of rifampicin and macrolides, suggesting there may be a need for new treatment protocols.

Objective and Methodology

• The aim of the study was to examine antibiotic resistance in the R. equi bacterium, which causes rhodococcosis in foals. In particular, the research focused on any resistance to the common treatment of rifampicin and macrolide antibiotics.
• To do this, the researchers reviewed necropsy records of foals from January 2011 to August 2019. They looked for foals that had tested positive for R. equi and had details of minimum inhibitory concentrations (MICs) – the lowest concentration of a drug required to prevent bacterial growth.
• The study also aimed to understand if foals receiving the dual therapy before their death showed more resistance compared to those not treated with these drugs.

Findings

• The study found that of 256 R. equi isolates analysed from 256 necropsied foals with rhodococcosis, high resistance levels were noted to rifampicin, azithromycin, clarithromycin and erythromycin.
• The most effective antibiotics, or those with lowest MIC values, were found to be imipenem, doxycycline, amikacin and gentamicin.
• Importantly, the research found that R. equi isolates that were resistant to rifampicin and macrolides were significantly more isolated from foals that had received the dual therapy than from untreated foals.

Implications

• This study suggests that the use of the rifampicin and macrolide dual therapy may be contributing to the development of antibiotic resistance in R. equi. This is a significant finding, as it suggests that the current standard treatment for rhodococcosis in foals could be worsening the problem of antibiotic resistance.
• As a result, the research concludes that there may be a need to explore new treatment protocols for rhodococcosis in foals in order to limit the development of antibiotic resistance.