Minimum inhibitory concentrations of erythromycin and rifampin for Rhodococcus equi during the years 2007–2014

The aim of this project was to determine whether there was a trend of resistance occurring for Rhodococcus equi to the antimicrobials erythromycin and rifampin.

This analysis was carried out using isolates dating from 2007 to 2014 in order to establish if there was an increase in the Minimum Inhibitory Concentrations (MIC) over this timespan. This study was a follow on study from Buckley et al.,[10] who assessed the MIC values for R. equi isolates from pre- 2000 to 2006.

The 74 samples used in this study were analysed for their MIC values to erythromycin and rifampin using gradient strips for each antibiotic.

The MIC levels varied from year to year in this study. However, for erythromycin 75 % (6/8) of the mean MICs for all the years were above 0.5 μg/ml which indicates a trend of clinical resistance. According to Giguere et al., [7] the level of susceptibility for R. equi to erythromycin is ≤ 0.5 μg/ml . In the previous study conducted by Buckley et al., [10], only the mean MIC for the year 2006 was above this level at 0.583 μg/ml, indicating that the mean MIC levels may in fact be increasing with time. For rifampin in 2008, the mean MIC was above the susceptible level of 1 μg/ml. In the previous study, no levels for rifampin were above this level. Rifampin is always used in combination with another drug due to the fact that resistance to rifampin can occur quite quickly. Two isolates in this present study originated from soil samples and these samples had the highest MIC values of 2 μg/ml and 3 μg/ml for rifampin and erythromycin respectively. This is a worryingly high MIC value and may indicate that virulent R. equi is ubiquitous in the environment of farms. This indication warrants further research.

Buckley et al., [10] found that the MIC for rifampin increased from 1996 to 2006 and the MIC for erythromycin went from 0.258 μg/ml prior to the year 2000 to 0.583 μg/ml in 2006. The present study found that these MIC values have further increased in the last decade and the mean MIC values for 2014 were 0.657 μg/ml for erythromycin and 0.300 μg/ml for rifampin. This is a significant increase in MIC levels especially for a bacteria that is difficult to treat due to its intracellular nature and virulence plasmids.

The regions, where the isolates originated from, were also evaluated and the majority came from Kildare. This could be due to the fact that the majority of big stud farms are located in Co. Kildare. However, it could also point to a geographic link to R. equi virulence.

Data describing antimicrobial susceptibility for equine Rhodococcus equi isolates is scarce [11]. However, the incidence of resistant R. equi isolates is on the rise in recent years. A study by Boyen et al., [11] looked at antimicrobial resistance to R. equi. The study examined MIC values for R. equi to azithromycin, erythromycin , clarithromycin and rifampin. The results were similar to this present study in that of the 15 isolates tested for erythromycin, 10 were above 0.5 μg/ml, 5 were ≤ 1 μg/ml and for rifampin, one sample had an MIC of ≤ 1 μg/ml and 8 samples were ≤ 8 μg/ml. This suggests that resistance of R. equi to rifampin is increasing.

Takai et al., [9] described rifampin resistance back in 1997 and linked the resistance to the incorrect use of antibiotic therapy. Macrolide resistance seems to be more widespread than rifampin resistance. In this study, the mean MIC values were higher in every year (exception 2008) for erythromycin compared to rifampin. A study conducted by Burton [12], looked at clinical isolates from a Kentucky breeding farm to investigate macrolide and rifampin resistance after a screening program was initiated to detect subclinical R. equi pneumonia. The findings showed that seven years after the initiation of the program, which included prophylactic treatment of foals, the resistance to macrolides (including erythromycin) and rifampin, rose significantly. They found that 24 % of pre-treatment isolates showed resistance compared to a staggering 62 % of post-treatment isolates. This highlights the emergence of resistance over recent years and the need for correct diagnosis and treatment only when necessary.

This study highlights the acquired resistance of R. equi to the combination antibiotics of erythromycin and rifampin. Whilst the MIC values varied from year to year, this could be due to the relatively small sample size of 10 samples/year. Only four samples were analysed in 2014 due to the timing of the experiment. These were limitations of the study and further research should include a larger sample size.

The issue of antimicrobial resistance is a major concern for both human and animal public health. Due to the emergence of erythromycin and rifampin resistant R. equi isolates, alternative treatments must be researched further, as well as better management strategies for the issue of Rhodococcus equi infection. Better diagnostic methods should be researched further and avoidance of prophylactic treatments. Newer generation macrolides have been suggested as alternatives in combination with rifampin for the treatment of Rhodococcus equi. Of these, clarithromycin has been suggested as the best alternative for erythromycin. This was based on a study where the drug achieved satisfactory concentrations in the bronchoalveolar cells of foals and pulmonary epithelial lining fluid. This is a beneficial quality for a R. equi targeting antibiotic due to the intracellular activity of the bacterium [13]. Further research into the bioavailabity of antibiotics for the treatment of Rhodococcus equi is warranted.