Growth, survival and histological responses of the marine shrimp, Litopenaeus vannamei, to three dosage levels of oxytetracycline

Toxicity of oxytetracycline (OTC) in prepared feed for penaeid shrimp was evaluated in a 42-day trial with Litopenaeus vannamei (initial mean weight, 10.3 g ± 0.504 S.D.). Four treatments consisted of: (1) 0.0 g/kg OTC (control feed), (2) 4.5 g/kg OTC (1× treatment, maximum recommended dose), (3) 13.5 g/kg OTC (3× treatment) and (4) 22.5 g/kg OTC (5× treatment). Each treatment consisted of 11 replicate tanks, with 8 shrimp per replicate. The test period was three times the recommended OTC label dosing period (14 days). This target animal safety study was conducted under GLP (Good Laboratory Practices) conditions, as part of submissions to the U.S. Food and Drug Administration Center for Veterinary Medicine for approval of therapeutic use of OTC in penaeid shrimp feeds. Growth rate (weight gain) was found to be OTC-dose-related. There was no significant difference in mean weight gain between control and 1× OTC treatments (9.1 g and 9.2 g, respectively), but growth was strongly depressed in the 3× and 5× treatments (1.50 g and 4.02 g, respectively). There did not appear to be a dose response in mean percent survival of test shrimp (range: 93.2% to 98.9%). Abnormal soft exoskeletons (= shell, cuticle) were observed in 66.7% to 90.9% of shrimp in the 3× and 5× treatments after 42 days, compared with 0.0% and 9.1% in control and 1× treatments, respectively (from sample N = 3 per tank = N = 33 per treatment, 37.5% of individuals stocked). The high numbers of soft exoskeletons observed could be related to a deficiency of calcium/magnesium, principal shell components, due to divalent cation-chelating properties of OTC. Some dose-related histological changes were apparent in the hepatopancreas (HP) of experimental shrimp sampled after 42 days of OTC feeds. Only slight changes were apparent in the histological presentation of the HPs of the shrimp from the 0× (untreated control) and the 1× treatment levels. In contrast, shrimp sampled from the 3× and 5× OTC treatment levels showed generally reduced levels of lipid droplet storage in the HP, and some necrosis and sloughing of the HP tubule epithelium. The virtually complete absence of HP lipids and the presence of moderate atrophy of the proximal portion of the HP tubules, indicated by markedly reduced tubule epithelial cell height, were the principal characteristics of the HP of shrimp sampled from the 5× OTC group. Mitotic activity in E-cells, a measure of the regenerative capability of the HP, was constant across