Enrofloxacin concentrations in the vitreous of horses with equine recurrent uveitis (ERU) after repeated intravenous administration

Abstract

Equine recurrent uveitis (ERU) is hallmarked as a recurrent inflammation of the uvea with acute serofibrinous or serohaemorrhagic uveitic attacks and their sequelae. An acute uveitic attack has been and still is treated conservatively, with mydriatics and anti-inflammatories. Vitrectomy, performed in quiescent phases of the disease was successfully used to prevent recurrences of ERU. In vitreous and aqueous humour samples of equine eyes with typical recurrent uveitis, obtained during vitrectomy, leptospira spp. and/or specific antibodies to leptospira can be detected nearly regularly. Although antimicrobial therapy of leptospira-associated uveitis appears as a promising therapeutic option for this ailment (Divers et al. 2008), such a therapy has yet not been established. The objective of this study was therefore to examine the effects of repeated intravenous administration of enrofloxacin on positive culture results for leptospira spp. of vitreous samples of horses with typical ERU. Furthermore, enrofloxacin concentrations in vitreous humour and serum samples were analysed. Twenty-five horses diagnosed with typical ERU received repeated administrations of enrofloxacin in a dosage of 7.5 mg/kg body weight intravenously. In this group, serum and vitreous humour concentrations of enrofloxacin were determined by ELISA. Enrofloxacin concentrations in the vitreous were related to the severity of damage of the eyes and to the previous course of the disease. Additionally, the vitreous samples were subjected to MAR and ELISA testing for the presence of intra ocular infection with leptospira and, if positive, were cultured for leptospiral growth. Samples of 35 horses diagnosed with ERU, but not having received enrofloxacin, served as control group. Samples were cultured for leptospira only after positive testing for intraocular infection with leptospira spp. by MAR and ELISA. Cultures for leptospiral growth among horses receiving enrofloxacin were positive in 30% of vitreous humour samples, and in 54% of the untreated control group, the difference not being significant. Mean enrofloxacin concentration in the vitreous humour was 1.06 mu g/ml (0.47-2.20 mu g/ml). Mean serum enrofloxacin concentration was 5.48 mu g/ml (3.08-8.21 mu g/ml). There were significant differences in the enrofloxacin concentrations of horses that had suffered from 2 episodes of uveitis. There was a trend for higher enrofloxacin concentrations in eyes with numerous inflammatory products (vitreous floaters). The concentrations of enrofloxacin in serum and in vitreous humour were high above the minimal inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC) for leptospira as determined by in-vitro studies. The relatively high proportion of positive leptospira cultures from horses treated with enrofloxacin may indicate that leptospira spp. in vitreous humour have an increased insensibility to intra ocular enrofloxacin, or that the bacteria do not sufficiently come into contact with the antibiotic. Increasing enrofloxacin concentrations in eyes that had experienced several inflammatory episodes, and in eyes with many inflammatory floaters in the vitreous chamber may be the result of a disruption of the blood-ocular barrier. With the therapeutic regimen used in this study systemic enrofloxacin administration is not suitable as a sole treatment for leptospira-associated ERU and may not prevent recurrences of inflammatory bouts.