A historical account of the diagnosis and characterization of strains of Mycoplasma gallisepticum of low virulence.

Numerous chicken flocks were studied beginning in 1970 because of questionable results on their serologic tests for Mycoplasma gallisepticum (MG). Typically a low number of hens in the flocks were positive reactors to the rapid serum plate test and rarely had hemagglutination-inhibition (HI) titers over 1:80. Usually no clinical signs were observed. Isolates of MG eventually were cultured from most of the flocks that exhibited that type of marginal serologic pattern. In the laboratory, the MG isolates were frequently less virulent and less pathogenic than the typical field isolates recovered in previous years. Most isolates produced airsacculitis of varying severity when broilers were exposed to the MG cultures as aerosols following exposure to infectious bronchitis virus. They became positive on the rapid serum plate test and developed moderate to high HI titers. Egg-transmission appeared to be the most likely means of transmission, even though the infected progeny rarely showed clinical signs of disease.