Effect of long-term administration of an injectable enrofloxacin solution on physical and musculoskeletal variables in adult horses

Objective-To evaluate clinical safety of administration of injectable enrof loxacin. Design-Randomized controlled clinical trial. Animals-24 adult horses. Procedures-Healthy horses were randomly allocated into 4 equal groups that received placebo injections (control) or IV administration of enrofloxacin (5 mg/kg [2.3 mg/lb], 15 mg/kg [6.8 mg/lb], or 25 mg/kg [11.4 mg/lb] of bod y weight, q 24 h) for 21 days. Joint angles, cross-sectional area of superf icial and deep digital flexor and calcaneal tendons, carpal or tarsal osteo phytes or lucency, and midcarpal and tarsocrural articular cartilage lesion s were measured. Physical and lameness examinations were performed daily. M easurements were repeated after day 21, and articular cartilage and bone bi opsy specimens were examined. Results-Enrofloxacin did not induce changes in most variables during admini stration or for 7 days after administration. One horse (dosage, 15 mg/kg) d eveloped lameness and cellulitis around the tarsal plantar ligament during the last week of administration. One horse (dosage, 15 mg/kg) developed mil d superficial digital flexor tendinitis, and 1 horse (dosage, 25 mg/kg) dev eloped tarsal sheath effusion without lameness 3 days after the last admini stration. High doses of enrofloxacin (15 and 25 mg/kg) administered by bolu s injection intermittently induced transient neurologic signs that complete ly resolved within 10 minutes without long-term effects. Slower injection a nd dilution of the dose ameliorated the neurologic signs. Adverse reactions were not detected with a 5 mg/kg dose administered IV as a bolus. Conclusions and Clinical Relevance-Enrofloxacin administered IV once daily at the rate of 5 mg/kg for 3 weeks is safe in adult horses.