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M.M. Ford, K. Narfstrom, R.R. Dubielzig, E.A. Giuliano, C.P. Moore; Enrofloxacin and the Feline Retina . Invest. Ophthalmol. Vis. Sci. 2004;45(13):5072.
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Purpose: To characterize ophthalmoscopic, electroretinographic (ERG) and morphologic effects of oral enrofloxacin in healthy cats. Methods: Twenty four (12 male, 12 female), pathogen–free, DSH cats were equally randomized by gender into one of two study groups (experimental, control), then subdivided into 3 groups by duration of enrofloxacin administration (3, 5, or 7 d). Enrofloxacin was administered to the experimental study group once daily at 10 times the recommended dose (50mg/kg/d, PO) between d 1 and the day of sacrifice. Daily funduscopic examinations were performed by an investigator blinded to the treatment groups (KN). ERG studies were performed with cats under general anesthesia prior to, and every 2–3 d after onset of the study. A computerized ERG (ERG System Tor, Global Eye Program, Rejmyre, Sweden) with full–field white light stimulation was used. After 2 h of dark adaptation a scotopic low–intensity (–6.0 cd.s/m2 then light intensity increases in 0.5 log unit steps) and high–intensity (flashes up to 0.6 log cd.s/m2) light series was performed. After 10 min of light adaptation, photopic stimulation (5, 30, and 50 Hz, 0.0 cd.s/m2) was elicited. Control cats underwent identical dosing regimens with placebo, examinations, and testing procedures. Four animals per group were sacrificed on d 3, 5, and 7 and eyes were collected for light and electron microscopy. Results: Clinical, funduscopic, ERG, and morphologic abnormalities were identified in enrofloxacin treated cats only and included: absent menace response by d 4 (6 of 8 cats), seizures, ptyalism, and vestibular signs by d 5 (2 of 8 cats). A change in tapetal reflectivity and generalized retinal blood vessel attenuation was detected within 72 h. B–wave amp were decreased within 24 h of dosing, and were non–recordable by d 3. A–wave amp were non–recordable by d 5. These changes were reflected by a progressive decrease in b–/a–wave ratios. Morphologic changes included swelling and cytoplasmic vacuolation of the photoreceptors by d 3. By d 5, the number of photoreceptors was decreased and nuclear pyknosis replaced vacuolation. By d 7, few rod photoreceptors remained however, cone nuclei were still detectable. No changes to the inner retina were identified. Conclusions: Enrofloxacin administered at 50mg/kg/d PO to healthy adult cats is acutely toxic to rod and cone photoreceptors leading to significant reduction in ERG a– and b–wave amp by d 3 following treatment and profound atrophy of the outer retina by d 7. Decreases in b–wave ERG amp precede funduscopic changes. Rod photoreceptors appear to be more sensitive to enrofloxacin than cones.
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