Immunogenicity of a temperature sensitive mutant Mycoplasma gaffisepticum vaccine

SUMMARY The immunogenicity of the ts‐11 vaccine strain of Mycoplasma gallisepticum was assessed following eye drop or coarse aerosol administration in chickens of various ages. Protection was evaluatted following intra‐abdominal (IA) or fine droplet aerosol administration of virulent M. gallisepticum , usually the Ap3AS strain and was measured mainly by the scoring of gross air sac lesions or by egg production. Vaccination of chickens with ts‐11 did not elicit a substantial serum antibody response as measured by rapid serum agglutination test, or ELISA. Protection was never demonstrated when no M. gallisepticum serum antibody response was detected in a vaccinated group of chickens. Failure to protect occurred usually, although not invariably, following aerosol administration of the vaccine. Vaccination by eye drop usually, although not invariably provided protection against challenge. In one experiment, chickens vaccinated by eye drop at 8‐weeks were as susceptible as non vaccinated controls when challenged by IA inoculation at 13‐weeks‐of‐age. Yet other birds from the same vaccinated group were resistant when challenged in an identical way at 23‐weeks. No measurable increase in M. gallisepticum specific serum antibody concentrations occurred in the intervening period. Equally surprising was the response of another group of birds in the same experiment that had been vaccinated with a higher dose of ts‐11. An antibody response was detected in this group, but they were susceptible to challenge at 23‐weeks. Interestingly, a drop in egg production commenced 4 weeks after challenge, 2 weeks later than that observed in a non vaccinated group challenged at the same time. This suggested that there was a degree of protective immunity which subsequently broke down.