Culturable Microbial Population From the Upper Respiratory Tract of 1,010 Clinically Healthy Horses in Southern Brazil.
Authors: Espíndola Julia Pires, Machado Gustavo, Diehl Gustavo Nogueira, Dos Santos Lucila Carboneiro, de Vargas Agueda Castagna, Gressler Leticia Trevisan
Journal: Journal of equine veterinary science
Summary
# Upper Respiratory Tract Microbiota in Clinically Healthy Horses: Implications for Diagnostic Interpretation Distinguishing pathogenic infection from normal upper respiratory tract colonisation remains a significant diagnostic challenge in equine practice, yet few reference datasets exist from genuinely healthy populations to guide interpretation of culture results. Espíndola and colleagues sampled nasal swabs from 1,010 clinically healthy horses across 341 farms in Southern Brazil, culturing on blood agar and identifying isolates to genus level to establish baseline microbiological profiles. The findings revealed a predominantly Gram-positive bacterial population dominated by Staphylococcus, Bacillus, Streptococcus and Corynebacterium spp., with five microbiological profiles accounting for approximately 49% of cases—most frequently Staphylococcus plus Gram-negative bacilli (12.67%), pure Staphylococcus (12.47%), and Staphylococcus with Bacillus (10.10%). Notably, potential pathogens including Streptococcus equi equi and Rhodococcus equi were isolated from 34 animals (3.37%), indicating these organisms can persist asymptomatically alongside commensals. For practitioners, these data underscore the importance of contextualising culture results within clinical presentation rather than treating any positive isolation as diagnostic of active infection, whilst highlighting that recovery of S. equi equi or R. equi warrants careful epidemiological consideration even in apparently healthy animals.
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Practical Takeaways
- •When interpreting nasal culture results, understand that Staphylococcus spp., Bacillus spp., and Gram-negative bacilli are normal commensals in healthy horses and their presence alone does not confirm infection
- •Be aware that pathogenic organisms like S. equi equi and R. equi can be present in clinically healthy horses; clinical signs and culture context must guide treatment decisions to avoid unnecessary antimicrobial use
- •Use this baseline data on normal URT microbiota profiles to improve diagnostic accuracy and develop targeted antimicrobial therapy protocols for horses with respiratory disease
Key Findings
- •Diverse Gram-positive and Gram-negative bacterial populations colonize the URT of healthy horses, with Staphylococcus spp. present in 35.24% of microbiological profiles
- •The five most common URT profiles account for 48.90% of cases, with Staphylococcus + Gram-negative bacilli being most prevalent at 12.67%
- •Pathogenic bacteria Streptococcus equi equi and Rhodococcus equi were detected in 3.37% (34/1010) of apparently healthy horses, indicating asymptomatic carriage
- •Gram-negative bacilli were significantly more prevalent than previously recognized in equine URT microbiota from healthy animals