Antibiotic resistance in faecal bacteria isolated from horses receiving virginiamycin for the prevention of pasture-associated laminitis.

This research investigates the relationship between the use of the antibiotic virginiamycin on horses and antibiotic resistance in faecal bacteria, finding that the use of this antibiotic for the prevention of pasture-associated laminitis does not seem to trigger an increased frequency of resistance to the antibiotic streptogramin quinupristin-dalfopristin (Q/D).

Study Introduction and Background

• Enterococcus faecium is a bacteria often responsible for serious hospital-acquired infections in humans. It is known for its ability to develop resistance against several antibiotics. Because many antibiotics are ineffective against this bacteria, the antibiotic Q/D stands as one of the few still effective treatments.
• The use of virginiamycin, another type of antibiotic belonging to the group streptogramins, was banned as a growth promoter in animals within the European Union in 1999. This ban was enacted because some believed that its use was promoting the emergence of E. faecium that are resistant to Q/D.
• Despite the ban, virginiamycin is still recommended for preventing pasture-associated laminitis, a common and painful disease in horses. However, the impact of this application on the development of Q/D resistance was not known prior to this study.

Methodology

• The research team collected and analyzed faecal samples from three different horse groups – those receiving virginiamycin, those that were co-grazing with treated horses, and those not exposed to virginiamycin.
• They then cultured E. faecium from these samples to see if the bacteria would grow in the presence of streptogramins, which would indicate the E. faecium were resistant to the antibacterial effects of these antibiotics.
• The presence of certain genes (ermB and vatD) that promote antimicrobial resistance was also checked using real-time PCR method.

Results

• They found streptogramin-resistant E. faecium in the majority of samples from all three groups. Significantly, there were no major differences in the rates of resistant bacteria between these groups.
• Certain antimicrobial resistance genes (ermB and vatD) were detected in a similar fashion from all three groups.
• The gene load of vatD was found to be significantly higher in unexposed animals compared to those treated with virginiamycin.
• A significant finding was that the antimicrobial resistance genes were present only in samples from each of the three groups where Q/D resistant E. faecium had been cultured.

Conclusion

• This study concludes that virginiamycin usage in the prevention of laminitis in horses does not appear to increment the frequency of streptogramin (Q/D) resistance in E. faecium found in horse faecal matter.
• However, given the high rate of resistance across all groups, horses can potentially act as a reservoir of Q/D resistant genes and clones, which poses a risk of transient transmission to humans.