Efficacy of danofloxacin and tylosin in the control of mycoplasmosis in chicks infected with tylosin-susceptible or tylosin-resistant field isolates of Mycoplasma gallisepticum

The efficacy of danofloxacin was compared with that of tylosin in the control of induced mycoplasmosis. In three experiments, disease was induced in broiler chicks by intrapulmonary injection of field isolates of Mycoplasma gallisepticum (MG) originating from Brazil, a different isolate being used for each experiment. Starting the day after inoculation, groups of chicks were medicated for 3 days via the drinking water with danofloxacin (50 ppm) or tylosin (500 ppm) or were left as unmedicated controls. Chicks were observed for 21 days. A severe mycoplasmosis was induced in unmedicated birds, characterized by mortality, depression, and respiratory signs. Danofloxacin was highly efficacious in controlling this infection in all three experiments, whereas tylosin was efficacious in only one. This difference could be related directly to the reduced in vitro susceptibility to tylosin displayed by the two isolates of MG. In the two experiments where isolates were tylosin-resistant, danofloxacin was significantly (P less than or equal to 0.05) superior to tylosin in reducing mortality, maintaining weight gain, and reducing the prevalence of air-sac lesions, isolation of MG, and seroconversion