Cross-immunity and antigenic relationships among five strains of Mycoplasma gallisepticum in young Leghorn chickens.

Five strains of Mycoplasma gallisepticum (MG) were tested for their cross-protection, virulence, and antigenic relationship in young leghorn chickens. Chickens vaccinated at 1 day of age by eye drop with the F or R strain of MG had good protection against aerosol challenge with the F, R, or S-6 strain at 4 weeks postvaccination. Chickens vaccinated at 1 day of age with the 503 or 730 strain had only partial protection against challenge with R, F, or S-6; the degree of cross-protection was related to the virulence of the vaccine strain used. In the hemagglutination-inhibition test, serum from chickens with single infection (vaccination and/or challenge with only one strain) with the F, S-6, or R strain reacted strongly with antigens prepared from S-6 and A5969 and reacted weakly with 730 and 503 antigens. Chickens with a single infection with the 503 or 730 strain reacted only with antigen prepared from the homologous strain. Serum from chickens with a two-strain mixed infection had a wider spectrum of HI activities. In the serum-plate-agglutination test, serum from chicken infected with the S-6, F, or R strain reacted with all antigens. Chickens infected with the 503 or 730 strain reacted more strongly with the homologous antigen. Antigen prepared from the S-6 strain of MG detected antibodies against the 503 and 730 strains more efficiently than did antigen prepared from the A5969 strain.