PLASMA DISPOSITION OF AMIKACIN AND INTERACTION WITH GASTROINTESTINAL MICROFLORA IN EQUIDAE FOLLOWING INTRAVENOUS AND ORAL-ADMINISTRATION

Amikacin was detectable (> 0.02 mug/ml) in plasma for 12 h in horses a nd donkeys and for 8 h in ponies following intravenous (i.v.) administ ration at a dose rate of 6 mg/kg bodyweight. The elimination half-life (harmonic mean) of amikacin was 2.8, 1.6 and 1.9 h in horses, ponies and donkeys, respectively, and the mean body clearance was relatively slow (45.2, 82.4 and 58.0 ml/h.kg, respectively). A suitable dosage in terval for the i.v. administration of amikacin sulphate to horses, pon ies and donkeys, at a dose rate of 6 mg/kg, would be every 8 h in hors es, and every 6 h in ponies and donkeys. Following i.v. administration there were no marked alterations in caecal liquor pH, the number of v iable bacteria isolated, or the short chain fatty acid (SCFA) concentr ations in caecal liquor and faeces. Amikacin was not detected (< 0.02 mug/ml) in plasma following administration by nasogastric tube to poni es with cannulated caecal ristulae; however, there were high concentra tions of amikacin measured in caecal liquor (maximum 16.2-99.4 mug/ml) . Despite the high drug concentrations in caecal liquor, there were on ly slight alterations in the number of viable bacteria isolated. Howev er, there was a reduction in caecal liquor pH to < 6.6, but few change s in caecal liquor SCFA concentrations. Faecal SCFA concentrations, dr y matter content and consistency did not alter markedly.