Fall Chinook salmon Oncorhynchus tshawytscha were fed practical diets medicated with azithromycin (30 mg kg -1 fish for 14 d) or erythromycin (100 mg kg -1 fish for 28 d) either 1, 2, or 3 times beginning 14 d after initiation of exogenous feeding (February) and ending at smoltification (June). Average tissue concentrations of azithromycin increased from 19.0 µg g -1 in fry to 44.9 µg g -1 in smolts, and persisted in the tissues > 76 d after treatment ceased. Tissue concentrations of erythromycin were comparatively low, ranging from 0.2 µg g -1 in fry to 10.4 µg g -1 in smolts. Erythromycin was not detectable 21 d post-treatment. Neither antibiotic caused histopathologically significant lesions in the trunk kidney or other organ tissues. The high tissue concentrations and prolonged retention of azithromycin in Chinook may be factors that increase the efficacy of the antibiotic against Renibacterium salmoninarum, compared with erythromycin, particularly in early life history stages before covertly infected fish show clinical signs of disease. KEY WORDS: Macrolide pharmacokinetics · Bacterial kidney disease · Juvenile · Chinook salmon Resale or republication not permitted without written consent of the publisherDis Aquat Org 64: [99][100][101][102][103][104][105][106] 2005 activity against R. salmoninarum (Fryer 1987) and has demonstrated in vivo efficacy in experimentally infected juvenile salmonids (Strom et al. 2000).The improved pharmacokinetic profile, broad spectrum of activity, and superior performance in controlling facultative intracellular pathogens make azithromycin an important antimicrobial drug to test for its ability to control Renibacterium salmoninarum. Although patterns of erythromycin accumulation and depletion in yearling Chinook salmon have been previously evaluated (Moffitt & Schreck 1988), the pharmacokinetic behavior of azithromycin in fish is unknown. The objective of this study was to compare the uptake, retention and clearance of orally administered erythromycin and azithromycin by fall Chinook salmon Oncorhynchus tshawytscha during the first feeding to smolt stage. MATERIALS AND METHODSExperimental fish and husbandry. In January 1999, unfed fall Chinook salmon fry (initial weight 0.42 g) obtained from the University of Washington research hatchery located at Big Beef Creek, Seabeck, WA, were counted into samples of 25 fish and distributed sequentially to 14 tanks until each contained 225 fish. The experiment was conducted in 1.83 m diameter (1250 l) circular polyethylene tanks. Temperature (10°C) and flow (10 l min -1 ) were held constant throughout the experiment. Natural photoperiod was simulated in the laboratory with 2 levels of incandescent lighting, and adjustments in day length were made daily.The fish were acclimated to laboratory conditions for 14 d, and fed to apparent satiation 4 to 6 times daily with a semi-moist starter diet (BioOregon starter diet, BioOregon). Thereafter, a growth model based on the delta-l method (Butterbaugh & Willoughby 1967) was used to ca...

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