Efficacy of Tylosin and Tilmicosin Against Experimental Mycoplasma gallisepticum Infection in Chickens

Mycoplasma gallisepticum, the cause of chronic respiratory disease, remains one of the most important pathogens in the poultry industry. Controlling the impact of this disease is done by eradication of positive breeder flocks or by vaccination and medication. Tylosin and tilmicosin are often used in medication programs. However, recent data on the in vivo efficacy of these macrolide antibiotics are scarce. Therefore, two dose titration studies were conducted using a recently isolated M. gallisepticum strain belonging to the wild-type population with regard to its tilmicosin and tylosin minimal inhibitory concentration. In a first trial, broilers were infected with M. gallisepticum and treated with 10 or 20 mg tilmicosin/kg body weight (BW) in the drinking water for five successive days. In a second trial, broilers were infected with M. gallisepticum and treated with 35 or 100 mg tylosin/ kg BW in the drinking water for five successive days. Clinical scoring of respiratory signs, macroscopic scoring of respiratory tract lesions, M. gallisepticum isolation from the respiratory organs, weight gain, and mortality were monitored for efficacy evaluation. All tylosin and tilmicosin treatments significantly reduced the course of clinical respiratory disease, macroscopic lesions in the respiratory organs, and M. gallisepticum numbers in the respiratory tract and obtained higher weight gains compared with the Mycoplasma-infected untreated control group. A treatment of 100 mg tylosin/kg daily for 5 days was not more clinically efficacious than the dosage of 35 mg tylosin/kg daily for 5 days. At final necropsy, in animals treated with 20 mg/kg BW tilmicosin, significantly fewer respiratory tract lesions were present than in the animals treated with 10 mg/kg BW tilmicosin. Therefore, when tilmicosin is used to treat clinical outbreaks of M. gallisepticum in broilers, a dosing scheme of 20 mg tilmicosin/kg BW for five successive days seems to be the most recommended scheme.