Efficacy of gamithromycin against Ornithobacterium rhinotracheale in turkey poults pre-infected with avian metapneumovirus

Ornithobacterium rhinotracheale is a common avian respiratory pathogen that often affects turkeys during the rearing period, resulting in important economic losses. The objective of this study was to evaluate the clinical efficacy of gamithromycin (GAM) against O. rhinotracheale in turkeys. The birds were inoculated oculonasally with 108 colony-forming units (cfu) of O. rhinotracheale, preceded by infection with avian metapneumovirus (AMPV, with 4.4 log10 50% ciliostatic dose (CD50). In addition to a negative (CONTR-, not infected, not treated) and a positive control group (CONTR+, infected, not treated), there were two treated groups administered GAM (6 mg/kg body weight) either subcutaneously (GAM SC) or orally by administration (GAM PO) as a single bolus at one day post-bacterial infection (p.b.i.). From the start of the AMPV infection until the end of the experiment, the turkeys were examined clinically and scored daily. In addition, tracheal swabs were collected at several days p.b.i. Necropsy was performed at 4, 8 and 12 days p.b.i. to evaluate the presence of gross lesions, and to collect trachea and lung tissue samples and airsac swabs for O. rhinotracheale quantification. The clinical score of the GAM SC group showed slightly lower values and birds recovered earlier than those in the GAM PO and CONTR+ groups. O. rhinotracheale cfus were significantly reduced in tracheal swabs of the SC group between 2 and 4 days p.b.i. At necropsy, CONTR+ showed higher O. rhinotracheale cfu in lung tissues compared to the treated groups. Moreover, at 8 days p.b.i. only the lung samples of CONTR+ were positive. In conclusion, the efficacy of GAM against O. rhinotracheale was demonstrated, especially in the lung tissue. However, the PO bolus administration of the commercially available product was not as efficacious as the SC bolus