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

OBJECTIVE

To determine whether isolation and virulence of Rhodococcus equi from soil and infected foals are associated with clinical disease.

DESIGN

Cross-sectional and case-control study.

SAMPLE POPULATION

R equi isolates from 50 foals with pneumonia and soil samples from 33 farms with and 33 farms without a history of R equi infection (affected and control, respectively).

PROCEDURE

R equi was selectively isolated from soil samples. Soil and clinical isolates were evaluated for virulence-associated protein antigen plasmids (VapA-P) and resistance to the beta-lactam antibiotics penicillin G and cephalothin. Microbiologic cultures and VapA-P assays were performed at 2 independent laboratories.

RESULTS

VapA-P was detected in 49 of 50 (98%) clinical isolates; there was complete agreement between laboratories. Rhodococcus equi was isolated from soil on 28 of 33 (84.8%) affected farms and 24 of 33 (72.7%) control farms, but there was poor agreement between laboratories. Virulence-associated protein antigen plasmids were detected on 14 of 66 (21.2%) farms by either laboratory, but results agreed for only 1 of the 14 VapA-P-positive farms. We did not detect significant associations between disease status and isolation of R equi from soil, detection of VapA-P in soil isolates, or resistance of soil isolates to beta-lactam antibiotics. No association between beta-lactam antibiotic resistance and presence of VapA-P was detected.

CONCLUSIONS AND CLINICAL RELEVANCE

On the basis of soil microbiologic culture and VapA-P assay results, it is not possible to determine whether foals on a given farm are at increased risk of developing disease caused by R equi.