Update on Antimicrobial Drugs in Horses

ANTIBIOTICS Fluoroquinolones The fluoroquinolone class of antibiotics work via inhibition of DNA gyrase (topoisomerase II). Resistance is mediated through chromosomal mutations in DNA gyrase which confers cross-resistance to other FQs. FQs distribute well to tissues and intracellularly. Elimination is via the kidney, and these drugs are often highly effective for treating resistant UTIs. The spectrum of activity includes Gram-negative bacteria and staphylococci. Enrofloxacin, and to a lesser extent marbofloxacin and orbifloxacin, continue to be used in horses. Ciprofloxacin pharmacokinetics were investigated following IV and oral administration to adult horses. This study confirmed the previously reported low oral bioavailability of the drug (avg 10.5%), and it also elucidated numerous adverse effects associated with both oral and IV administration, including mild transient diarrhea to severe colitis, endotoxemia and laminitis necessitating euthanasia of three out of the 8 study horses. Other adverse effects associated with IV administration were agitation and excitement followed by lethargy, sweating, muscle fasciculation, increased flehmen response, lip smacking, periorbital edema, patchy edema involving the face, neck and ⁄ or cranial thorax, and transient loss of appetite. The conclusions of this study were that the high incidences of adverse events preclude oral and rapid IV administration of ciprofloxacin. Levofloxacin pharmacokinetics have also been studied in stallions. Levofloxacin is the active enantiomer of ofloxacin, and has increased activity against topoisomerase IV. Bioavailability after IM administration approached 100% and IV or IM administration produced plasma concentrations expected to be therapeutic for bacteria with MICs ≤ 0.1 μg/mL following a dose of 4 mg/kg. Danofloxacin at 5 mg/kg IV or IM may also be considered as a potential therapeutic for MICs ≤ 0.25 μg/mL.