Suspect novel adverse drug reactions to trimethoprim-sulphonamide combinations in horses: a case series.

Summary

# Trimethoprim-Sulphonamide Combinations: Emerging Neurological Concerns Whilst trimethoprim-sulphonamide antibiotics are routinely prescribed in equine practice for their broad-spectrum activity, adverse reactions—including gastrointestinal and dermatological complications—are well recognised across veterinary species. Stack and Schott's 2011 case series documents five horses (four at therapeutic doses, one overdosed foal) that presented with unexpected neurological signs during treatment with these combinations, including hypermetric gait, agitation and erratic behaviour. Notably, all clinical signs completely resolved following drug withdrawal, with no lasting neurological sequelae, and alternative diagnoses were systematically excluded in each case. This cluster of cases suggests a previously unreported neurotoxic adverse reaction to commonly used antimicrobial combinations that practitioners should remain alert to. Farriers, physiotherapists and coaches encountering otherwise unexplained neurological deterioration in treated horses should prompt veterinary consideration of drug aetiology, whilst veterinarians should maintain a lower threshold for discontinuing trimethoprim-sulphonamides if ataxia or behavioural changes emerge during treatment, particularly given the rapid and complete resolution observed once medication ceased.

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Key Findings

• •Five horses (4 at normal dosage, 1 at overdose) developed neurological signs including hypermetric gait, agitation, and erratic behaviour during trimethoprim-sulphonamide treatment
• •All neurological abnormalities resolved completely upon medication withdrawal with no residual deficits
• •No alternative diagnoses could be identified, suggesting a novel adverse drug reaction to this antimicrobial combination in horses

Conditions Studied