Longitudinal effects of oral administration of antimicrobial drugs on fecal microbiota of horses.
Authors: Gomez Diego, Toribio Ramiro, Caddey Benjamin, Costa Marcio, Vijan Stephanie, Dembek Katarzyna
Journal: Journal of veterinary internal medicine
Summary
# Antimicrobial-induced changes in equine intestinal microbiota Antimicrobial-associated diarrhoea remains the most frequently reported adverse effect in horses receiving oral drugs, yet the mechanisms driving dysbiosis in equine patients remain poorly characterised. Using 16S rRNA sequencing, researchers tracked faecal microbiota composition across 20 healthy horses administered one of four common oral antimicrobials (metronidazole, erythromycin, doxycycline, or sulphadiazine-trimethoprim) for five days, with sampling continuing for 30 days post-treatment. Erythromycin and doxycycline produced the most severe microbiotal disruption, significantly reducing richness and diversity whilst simultaneously suppressing beneficial genera (Treponema, Fibrobacter, and Lachnospiraceae) and promoting pathogenic blooms of Fusobacterium and Escherichia-Shigella; metronidazole showed intermediate effects, whilst sulphadiazine-trimethoprim caused minimal changes. The dysbiosis patterns observed in healthy treated horses paralleled those seen in clinical diarrhoea cases, suggesting these antimicrobials create a permissive intestinal environment for inflammation and secondary diarrhoea independent of direct antimicrobial toxicity. For equine practitioners, these findings support judicious antimicrobial selection when oral administration is necessary, consideration of pre- and post-treatment probiotic or prebiotic interventions in higher-risk cases, and heightened vigilance for gastrointestinal complications—particularly during erythromycin or doxycycline therapy—even when horses remain clinically asymptomatic during treatment.
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Practical Takeaways
- •When prescribing oral antimicrobials (particularly erythromycin and doxycycline), counsel owners that dysbiosis-like microbiota changes occur even in healthy horses and increase diarrhea risk; consider probiotic or prebiotic support during and after treatment
- •SMZ-TMP appears to have a more favorable microbiota profile compared to other commonly used oral antimicrobials in horses, which may influence drug selection when possible
- •Monitor treated horses closely for diarrhea development even after treatment cessation, as microbiota alterations persist beyond the 5-day administration period
Key Findings
- •Doxycycline, erythromycin, and metronidazole significantly reduced fecal microbiota richness and diversity at multiple time points post-administration, while SMZ-TMP did not
- •Most pronounced microbiota alterations occurred during days 2-5 of antimicrobial administration with decreased Treponema, Fibrobacter, and Lachnospiraceae and increased Fusobacterium and Escherichia-Shigella
- •Erythromycin and doxycycline produced the most dramatic dysbiosis-like changes, while metronidazole effects were less pronounced
- •Oral antimicrobial administration induced microbiota changes resembling pathological dysbiosis in healthy horses despite absence of clinical diarrhea during the study