Acute LD 50 and kidney histopathology following injection of erythromycin (Erythro‐200) and its carrier in spring chinook salmon, Oncorhynchus tshawytscha (Walbaum)

We evaluated the toxicity of commercial injectable erythromycin (Erythro‐200) and the injectable drug carrier alone (PEG‐400, ethyl alcohol and ethyl acetate) in sub‐adult spring chinook salmon, Oncorhynchus tshawytscha (Walbaum). Both compounds were toxic to salmon when administered in high dosages. The 50% lethal dose (LD 50 ) of the erythromycin within its carrier (Erythro‐200) at 12 °C was estimated at 429 mg erythromycin kg –1 or 2.145 mL kg –1 when measured by volume. The LD 50 of the drug carrier alone was 2.95 mL kg –1 by volume or equivalent to 3.21 g kg –1 of the carrier. We evaluated and scored the histopathology in kidney sections removed from fish 96 h after injection with Erythro‐200, the drug carrier alone, or sterile phosphate buffered saline (PBS) that served as a reference control. Kidneys from fish injected with saline had lowest average pathology scores; those injected with the carrier had moderate scores; and the pathology score was highest in tissues from erythromycin‐injected fish. In tests at 12 °C, vacuoles were more prominent in proximal and distal tubules of fish injected with higher dosages of 403 and 545 mg erythromycin kg –1 than in tissues from fish injected with 299 mg kg –1 .