There have been ongoing concerns of adverse effects on a fetus when giving a mare fluroquinolone antibiotic during pregnancy, especially in the first trimester. This prompted a study to determine what effects might occur. Early gestation is a particularly critical time for skeletal development, and the concern is that there could be toxicity of cartilage and possibly tendon tissue.
A study evaluated the effects on 45-60-day old equine fetuses following two weeks of enrofloxacin administration to nine pregnant mares [Ellerbrock, R.E.; Canisso, I.F.; Podico, G.; Roady, P.J.; Uhl, E.; Lima, F.S.; Li, Z. Diffusion of fluoroquinolones into equine fetal fluids did not induce fetal lesions after enrofloxacin treatment in early gestation, The Veterinary Journal (2019), doi: https://doi.org/10.1016/j.tvjl.2019.105376].
Five pregnant mares served as the controls; four pregnant mares were treated with 7.5 mg/kg of oral enrofloxacin once daily for 14 days. Then, at 60 days of gestation and 24 hours after the last dose of enrofloxacin, abortion was induced for histologic studies of fetal tissues. These four mares were bred back, not given any medication and assigned as their own controls.
In later gestation, ciprofloxacin (a metabolite of enrofloxacin) accumulates in allantoic fluids from fetal urine excretion, whereas in early gestation, fetal kidneys are not fully functional so there is far less accumulation of the drug. The researchers suggested that in the first 60 days of gestation, there might be less risk of cartilage toxicity because this timing coincides with very early stages of chondrocyte development.
The study concluded that while enrofloxacin and its metabolite ciprofloxacin diffuse to fetal fluids early in gestation, there were no detectable abnormalities found in any fetus after two weeks of mare treatment. In early gestation, i.e., the initial two months, a mare experiencing a life-threatening bacterial infection might benefit from fluroquinolone treatment when that is the best choice based on bacterial culture and sensitivity.