Antibacterial Activity of Oxytetracycline Hydrochloride, Environmentally-Protected or Not, after Feed-Pelleting and during Decay-Dispersion in Saline Water

Abstract

Bivalent and trivalent cations quelate oxytetracyline (OTC), rendering it microbiologically inac- tive. Yet shrimp diseases are often treated with OTC in pelleted feeds. Attempts have been made to chemically protect the OTC concentration in shrimp feeds against degradation in brack- ish water and due to temperature. The antibacterial activity of two such environmentally-protect- ed OTC preparations (premixes A and B) and one unprotected OTC premix (C) were tested (a) before and after pelleting the OTC premixes with the powdered feed ingredients and (b) during decay-dispersion of the OTC-medicated feed in brackish water. In all three treatments, the OTC concentration dropped by approximately half after pelleting (p<0.001). When pellets were placed in tanks containing water of 23-25°C, pH 7.6-7.8, and marine salt (40 g/l), a second order expo- nential decay of antibacterial activity followed, reaching 50% in 30 min. At 150 min, loss reached 80% in group A, 85% in group B, and 98% in group C, suggesting that only environmentally-pro- tected OTC may, in some cases, reach the minimal therapeutic concentration necessary to treat Vibrio spp. diseases.