Antibiotic sensitivity patterns in field isolates of Mycoplasma gallisepticum as a guide to chemotherapy.

Control of Mycoplasma gallisepticum (Mg) infection in commercial poultry flocks has sometimes been achieved by the intensive use of antibiotics. In some cases, a single drug, tylosin tartrate, has been used repeatedly in prophylactic and therapeutic applications. It appears that this selective pressure can lead to the emergence of strains with reduced sensitivity to tylosin. This was observed in Mg strains isolated in 1978 from turkey breeding flocks repeatedly treated with tylosin, but not in isolates from acute disease outbreaks. Moreover, in a study of four recent outbreaks of mycoplasma respiratory disease in poultry, some of the Mg strains isolated in each of the outbreaks showed reduced sensitivity to tylosin and, at the same time, to spiramycin. This suggests that the buildup of these strains in the general pool of Mg is playing an increasingly significant role in clinical outbreaks. Periodic in vitro antibiotic sensitivity testing of field isolates of mycoplasma is indicated as a means of monitoring the impact of mass medication programs and as a guide to therapeutic treatment.