Identification of macrolide- and rifampicin-resistant Rhodococcus equi in environmental samples from equine breeding farms in central Kentucky during 2018.

The research conducted tests on environmental samples from horse-breeding farms in central Kentucky to look for antimicrobial-resistant Rhodococcus equi, a bacterium that can cause pneumonia in foals and potentially, humans. The results revealed the presence of resistant R. equi that did not considerably increase during the foaling season, prompting concerns about the risk of disease and bacteria’s potential to spread resistance genes.

Key Objectives of the Research

• The main purpose of the study was to investigate the presence and evolution of antimicrobial-resistant Rhodococcus equi in horse breeding farms that had a record of foal pneumonia caused by antibiotic-resistant R. equi.
• Another aim was to examine the differences in concentrations between resistant and virulent strains of R. equi in the environment, specifically the ground (soil or stall bedding) and air.
• The researchers also sought to determine if these concentrations would increase in number over the course of the foaling season.

Research Methodology

• Over the course of 2018, the researchers conducted the study on 10 separate horse-breeding farms of central Kentucky. These farms had a known history of foal pneumonia caused by macrolide- or Rifampicin-resistant R. equi.
• They collected both ground and air samples from stalls and paddocks in January, March, May, and July.
• Environmental samples were cultured with and without macrolides and Rifampicin to distinguish and select for the resistant organisms.
• The collected data were analyzed utilizing linear mixed-effects and Hurdle models, statistical analysis models frequently used in ecological studies.

Major Findings

• The presence of total R. equi was significantly higher in the bedding or air of stalls in January than in any other months of the testing period.
• A significant number of resistant R. equi were identified in soil samples from paddocks compared to stall beddings.
• Air samples from paddocks had a significantly higher proportion of resistant isolates than samples from stalls.
• Half of the resistant soil isolates and a third of the resistant air isolates were identified as virulent through the detection of the vapA gene, an indicator of virulence in R. equi.
• Contrary to their initial hypothesis, the concentrations of the resistant R. equi isolates did not increase over the course of the foaling season.

Conclusion

• The study concluded that horse breeding farms with a history of pneumonia attributed to R. equi could house and sustain robust populations of antimicrobial-resistant R. equi within their environment. This causes an ongoing risk of infection, and potentially even the spread of resistance genes within bacterial populations.
• Although the resistant isolates do not appear to increase during the foaling season, they remain a consistent threat.