Prevalence of Rhodococcus equi from the nasal cavity of 1010 apparently healthy horses.
Authors: Gressler L T, Machado G, da Silveira B P, Cohen N D, Corbellini L G, Leotti V B, Diehl G N, Dos Santos L C, de Vargas A C
Journal: Equine veterinary journal
Summary
# Rhodococcus equi in Healthy Horse Populations: Prevalence and Environmental Links Rhodococcus equi remains a significant pathogen in foal pneumonia, yet its epidemiology in healthy adult horses—particularly as a potential environmental reservoir—remains incompletely understood. Researchers cultured nasal swabs from 1,010 apparently healthy horses across 341 farms, identifying isolates through multiplex PCR, assessing virulence profiles and antimicrobial susceptibility, and examining seven environmental variables for associations with carriage using logistic regression and spatial clustering analysis. Only 1% of individual horses (10 animals) and 3% of farms yielded R. equi, all isolates were antimicrobial-sensitive, and just one (10%) displayed virulent characteristics—findings suggesting this organism occupies a minor niche in the normal nasal flora of immunocompetent pasture-kept horses. Red-Yellow Argisol soils (classified as PVA/PV in the Brazilian system) showed significant association with isolation (OR 8.02), whilst well-drained soils appeared protective (OR 0.85), pointing to soil composition and drainage as potential environmental risk factors warranting further investigation. For practitioners, these results support continued vigilance regarding foal pneumonia prevention on specific soil types, whilst reassuring that detection of R. equi in apparently healthy adults is uncommon and rarely represents a virulent threat—though longitudinal studies exploring soil microbiology and seasonal carriage patterns would strengthen evidence for farm-level management recommendations.
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Practical Takeaways
- •R. equi carriage in healthy adult horses is rare (1%) and typically non-virulent, so nasal detection alone in healthy horses does not predict foal pneumonia risk
- •Farm management should consider soil composition and drainage: properties with iron-rich, poorly-drained soils (Red-Yellow Argisol) may require enhanced biosecurity measures for foal protection
- •Identifying R. equi-positive farms allows targeted management strategies for pregnant mares and foals, even though most carrier horses pose minimal risk
Key Findings
- •R. equi was isolated from only 1% (10/1010) of apparently healthy horses across 341 farms, with only 10% of isolates classified as virulent
- •Red-Yellow Argisol soils were significantly associated with R. equi isolation (OR 8.02; 95% CI 1.98-32.50), while well-drained soils were protective (OR 0.85; 95% CI 0.76-0.96)
- •All isolated R. equi demonstrated antimicrobial sensitivity, suggesting these represent commensal organisms rather than pathogenic strains in healthy horses
- •Three percent of farms (10/341) had at least one horse positive for R. equi, indicating environmental clustering of the organism