Abstract

Histopathologic changes in gills, liver and kidney of rainbow trout fed therapeutic doses of chloramphenicol, erythromycin, nitrofurantoin, oxytetracycline and sulfamethazine have been studied. Lesions observed include: severe renal tubule vacuolization (erythromycin): dilation and degeneration of renal tubular epithelium (nitrofurantoin); severe hepatocyte vacuolization and mild renal tubule vacuolization (oxytetracycline). No lesions were seen in these select organs from fish fed diets containing either chloramphenicol or sulfamethazine.

These investigations are continuing at the electron microscope level to evaluate the toxicity of these therapeutic agents ultra structurally, and to aid in the selection of those groups of agents which demonstrate the least toxicity while maintaining the greatest efficacy.

Notes

The study of disease and preventive medicine in the field of aquaculture is quite recent. Presently, the use of antibiotics in fish is based on the knowledge gained from their use in domestic animals. The activity and toxicity of antibiotics in fish has not yet been studied.

Conditions. The fish were maintained in 10°C water. Antibiotics were administered orally, dosage being calculated per 100 pounds of fish.

Results

Chloramphenicol at a dose of 3.5 g/100 lbs. fish/day for 16 days showed no lesions. Sulfamethazine also produced no lesions (dosage not obtained).

Oxytetracycline at 3.5 g/100 lbs fish/day showed:

a). liver lesions

There was vacuolization of hepatocytes at four days after commencing treatment. The nucleus was displaced towards the sinusoidal side of the cell. This vacuolization was not random as in control fish (glycogen deposits) and was more marked and evident. The vacuolization worsened as the treatment continued.

b). kidney

The first portions of the proximal tubules were vacuolated between the nucleus and brush border of the cell. This portion of the proximal tubule is responsible for reabsorption of large molecules. Changes were mild. There were no gross effects on the fish. Blood parameters were not measured.

Nitrofurantoin at a dosage of 3.5 g/100 lbs fish/day caused kidney changes. The second portions of the proximal tubules showed foamy cells, ballooning of microvilli (early changes of nephro toxicosis) and a transition from columnar to cuboidal shape by 16 days. By 32 days the nuclei became pyknotic, cytoplasm was lost and cells began to slough leaving only basement membrane. The lesions at 32 days were histologically but not grossly evident.

Erythromycin at a dosage of 5 g/100 lbs fish/day also caused kidney changes. After one day the first portions of the proximal tubules had vacuolated epithelium. The second portions were unharmed. By 16 days however, hyaline droplets had formed in the second portions of the proximal tubules. The first portions were highly vacuolated with nuclei displaced towards the lumen side of the cell. By 32 days, the lesions seemed less marked.

Reasons for differing antibiotic effects in fish vs mammals