The success rate in treatment against Staphylococcus aureus mastitis with antibacterial drugs is marginal. Low antibacterial drug concentrations in the mammary tissue is part of the reason for this failure. Enrofloxacin has only been administered on few occasions through the intramammary route i.e., combined with gentamicin in an undisclosed vehicle or dose. A new solvate form of enrofloxacin (enrofloxacin hydrochloride-dihydrate [enro-C]) prepared as 1.5% suspension pH 5.5 was tested in a

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... mastitis outbreak due to coagulase-negative S. aureus (CNS) in F1 (Holstein/Zebu) cows aged 5-8 years destined to be culled. Enro-C was administered intramammarily daily (300 mg/infected quarter) for 8 days both to healthy and mastitic cows; milk and serum enro-C concentrations were determined on days 1 and 8. Maximum serum concentration (CMAX) values were similar in both groups (9.4 to 10.7 µg/mL). The area under the moment curve (AUMC) for enrofloxacin increased only by 11% in healthy cows and by 10% in infected cows from day 1 to day 8, a fact that suggests little accumulation with this dose regime. Peak concentrations in milk ranged from 18.5 to 19.8 µg/mL on day 1 and from 20.2 to 22 µg/mL on day 8. Cure rates on day 21 after treatment were 75% (12 out of 16 cows) or 69.4% (26 out of 36 glands). Somatic cell counts and California mastitis test showed a positive trend in cured animals. Uniquely high milk enrofloxacin and ciprofloxacin concentrations were obtained after intramammary administration of enro-C. These concentrations seem effective for treating CNS mastitis. The feasibility of incorporating this experimental pharmaceutical preparation of enrofloxacin is discussed.