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 inactive. 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 brackish water and due to temperature. The antibacterial activity of two such environmentally-protected 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 exponential 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-protected OTC may, in some cases, reach the minimal therapeutic concentration necessary to treat Vibrio spp. diseases.