Concentrations of erythromycin and azithromycin in mature Chinook salmon Oncorhynchus tshawytscha after intraperitoneal injection, and in their progeny.

A single dose (40 mg kg(-1)) of erythromycin or azithromycin dihydrate was injected intraperitoneally into maturing female fall Chinook salmon 12 to 32 d before spawning to observe the distribution, retention and clearance of the drugs in plasma, kidney, coelomic fluid and egg vitellin, and their persistence in alevins derived from these fish. Salmon administered prophylactic dosages of erythromycin as subadults were also included to investigate potential interactive effects of oral and injected treatments on reproductive performance and antibiotic clearance. Erythromycin was rapidly cleared from plasma and coelomic fluid, but was detected in the kidney (3.52 to 12.40 microg g(-1)) and egg vitellin (5.32 to 8.87 microg ml(-1)) of all fish at spawning. High, stable concentrations of azithromycin were detected in plasma (14.66 to 20.33 microg ml(-1)), kidney (43.16 to 59.96 microg g(-1)), coelomic fluid (2.52 to 5.50 microg ml(-1)) and egg vitellin (12.65 to 23.51 microg ml(-1)). Oral administration of erythromycin to subadult salmon did not significantly affect tissue concentrations of either erythromycin or azithromycin administered by prespawning injection. Reductions in the percentage of eggs that yielded live embryos at the eyed stage of development occurred among eggs derived from females that had received orally administered erythromycin as subadults. Erythromycin was not detected in unfed fry derived from adults injected with the drug prespawning, but azithromycin was present for more than 2 mo after the onset of exogenous feeding.

Accumulation and clearance of orally administered erythromycin and its derivative, azithromycin, in juvenile fall chinook salmon Oncorhynchus tshawytscha.

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 microg g(-1) in fry to 44.9 microg 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 microg g(-1) in fry to 10.4 microg 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.