Association of disease with isolation and virulence of Rhodococcus equi from farm soil and foals with pneumonia.

The research aims to establish whether the isolation and virulence of Rhodococcus equi, a bacterium, from soil samples and infected foals is associated with clinical disease. However, it concludes that based on the current indicators, it is impossible to determine if foals on a particular farm are at heightened risk of developing disease caused by Rhodococcus equi.

Methodology

• The study was carried out using cross-sectional and case-control methods. The selected samples were R equi isolates from 50 foals diagnosed with pneumonia, as well as soil samples from 33 farms with a history of R equi infection (termed “affected”) and 33 farms without such a history (termed “control”).
• R equi bacteria were selectively isolated from the soil samples. These soil and clinical isolates were then evaluated for virulence-associated protein antigen plasmids (VapA-P) and resistance to specific beta-lactam antibiotics – penicillin G and cephalothin.
• The researchers conducted microbiologic cultures and VapA-P assays, with these procedures independently repeated at two separate laboratories.

Results

• The VapA-P was detected in around 98% of clinical isolates, with both laboratories in complete agreement about this finding.
• However, when it came to R equi isolation from soil, the agreement between the two labs was noticeably poor. The bacterium was found in almost 85% of soils from affected farms and around 73% of soils from control farms.
• Virulence-associated protein antigen plasmids were found in around 21% of farms (14 out of 66), but the two laboratories only agreed on one instance of a VapA-P-positive farm.
• The researchers found no significant correlation between a farm’s disease status and either the isolation of R equi from the soil, detection of VapA-P in soil isolates, or resistance of soil isolates to beta-lactam antibiotics. Similarly, there was no observed association between beta-lactam antibiotic resistance and the presence of VapA-P.

Conclusion

• The study concluded that, despite the significance of soil microbiologic culture and the VapA-P assay results, they alone do not sufficiently indicate whether foals on a specific farm are at a higher risk of developing a disease caused by R equi.