Historically, there has been concern that administration of the fluoroquinolone antimicrobial, enrofloxacin, to a pregnant mare can result in damage to the foal’s cartilage and tendon tensile strength. Enrofloxacin and ciprofloxacin cross the equine placenta. Previous studies have demonstrated that enrofloxacin administered for 11 days IV in late pregnancy did not result in fetal lesions. However, it is speculated that side effects might only be evident when a foal becomes weight bearing.
To test this hypothesis, a study was conducted using enrofloxacin administration to late-term pregnant mares [Ellerbrock, R.E.; Canisso, I.F.; Roady, P.J.; et al. Administration of enrofloxacin during late pregnancy failed to induce lesions in the resulting newborn foals. Equine Veterinary Journal Apr 2019; 10.1111/evj.13131].
Nineteen healthy adult mares more than 280 days pregnant were used in the study:
- Five were untreated;
- Six received the recommended 7.5 mg/kg dose of oral enrofloxacin for 14 days;
- Six received a supratherapeutic dose of 15 mg/kg oral enrofloxacin for 14 days;
- Two mares were untreated during pregnancy, but the foals were treated postpartum with 10 mg/kg oral enrofloxacin—one foal received enrofloxacin once daily for two weeks starting at Day 7 and the other foal received 10 days of once-daily treatment starting at three weeks of age.
The last dose of enrofloxacin was given to pregnant mares at least 30 days prior to foaling. All foals were kept on pasture for five weeks, then euthanized for examination of cartilage and tendon tissue. In this study, administration of enrofloxacin in late pregnancy did not cause musculoskeletal disease in the foals from birth to 30 days of age. However, the two foals treated with enrofloxacin after birth developed clinical signs and pathology of cartilage and tendon tissues.
The study concluded: “While this study did not assess other stages of gestation or long-term foal outcomes, short-term administration of enrofloxacin to late-gestation, healthy mares did not result in macroscopic or microscopic lesions in the resulting foals by five weeks of age.”
It is suggested that enrofloxacin treatment of late-stage pregnant mares should be limited to life-threatening infections that necessitate using enrofloxacin based on bacterial culture and sensitivity results.