Prevalence of Rhodococcus equi from the nasal cavity of 1010 apparently healthy horses.

Background: Rhodococcus equi is an important cause of foal pneumonia. While its isolation from different sources has been widely evaluated, there is a need to better understand the R. equi epidemiology from samples of the nasal cavity of healthy horses.

Objectives: To determine the prevalence of R. equi from the nasal cavity of healthy horses, along with its virulence profile, antimicrobial susceptibility and environmental variables associated.

Study Design: Cross-sectional study.

Methods: Swabs from the nasal cavity of 1010 apparently healthy horses from 341 farms were submitted for bacteriological analyses. The identity and virulence profile of the R. equi isolates were assessed by multiplex PCR; antimicrobial susceptibility was determined by the disk-diffusion method. The occurrence of R. equi was calculated at the level of both animal and farm. The association of seven specific environmental factors with R. equi isolation was assessed using logistic regression and by a spatial scan statistical method to determine the presence of local clusters.

Results: Antimicrobial-sensitive R. equi was isolated from 10 (1%) of 1010 horses ranging between 3 and 29 years old. Ten farms (3%) had at least one positive horse. Only one R. equi isolate (10%) was classified as virulent. Red-Yellow Argisol (PVA/PV) soils were significantly associated with R. equi isolation (odds ratio (OR) 8.02; CI , 1.98-32.50, P = 0.01), and areas with well-drained soil were less likely to be test positive (OR 0.85; CI , 0.76-0.96, P = 0.03).

Main Limitations: The use of culture-based method instead of PCR-based assay and the lack of soil sampling.

Conclusions: Antimicrobial-sensitive R. equi may be considered a minor part of the normal bacterial flora in the nasal cavity of healthy and immunologically functional horses breeding on pasture. Further studies are warranted to determine if soils rich in iron and well-drained are, in fact, associated with the occurrence of R. equi.