Rational Selection of Antimicrobials for Use in Horses

Antibiotics frequently play an important central or adjunctive role in the therapeutic management of horses and foals with a variety of illnesses, including those requiring critical care, because diseases caused by primary or secondary bacterial infection are commonly encountered and may contribute to failure of single or multiple organs. However, it should be understood that supportive therapy usually plays a role at least as important as antimicrobials in promoting a positive outcome, and that the adverse effects of antimicrobial drugs individually or in combination may actually lead to negative consequences. Antibiotic use should be based on sound rational principles involving thorough patient evaluation, good clinical judgment, overall medical knowledge, information regarding the individual patient and the infecting agent(s), selection of an appropriate drug, and formulation of a dosage regimen appropriate to the patient and its caretaker after assessment of the potential benefits and risks of that therapy. The ultimate aim of antibiotic treatment is to inflict an insult on infecting bacteria sufficient to kill the organism or render it susceptible to inactivation by natural host defenses or the local microenvironment without adversely affecting the patient. The following discussion emphasizes important aspects of antimicrobial use in horses, including the bacterial species likely to be involved in particular disease syndromes, susceptibility profiles of bacterial isolates, and the antimicrobial spectrum, mode of action, indications, dose, and adverse effects of selected commonly used antibiotics. Knowledge regarding adverse effects is particularly important because the relative sparsity of antimicrobials approved for parenteral and oral use in horses frequently makes extra-label use necessary. Consequently, much of the responsibility for adverse events rests with the prescribing clinician.