The combination effect of N-acetyl-d-glucosamine on oral oxytetracycline treatment against Nocardia seriolae infection in the yellowtail Seriola quinqueradiata

Nocardia seriolae infection is one of the most serious bacterial diseases in Seriola species in Japanese aquaculture. In this study, the combination effect of N-acetyl-d-glucosamine (NAG) with oral oxytetracycline (OTC) treatment was evaluated with the experimental infection of artificially bred juvenile yellowtail Seriola quinqueradiata. Prior to the evaluation, the efficacy of OTC treatment against nocardiosis was assessed. The oral administration of OTC at a dose of 50 mg/kg body weight (BW) for 4 consecutive days did not show sufficient efficacy compared with that of the untreated control group, while the intraperitoneal administration of OTC, which caused no absorption loss in the gastrointestinal tract at a single dose of 50 mg/kg BW successfully suppressed mortality (P < 0.05). When OTC was orally administered with the same amount of NAG at a single dose of 50 mg/kg BW, the serum OTC concentrations significantly increased at 1 h and 6 h post-administration (P < 0.05); peak concentrations increased by 41%. In two trials evaluating the treatment of nocardiosis, the addition of NAG improved the efficacy of 5-consecutive-day oral OTC treatment by significantly reducing mortality compared with that of the untreated control group (P < 0.05). These results suggest that NAG can enhance the absorption of oral OTC and therefore improve the efficacy of nocardiosis treatment.