Age related differences in the immune response to vaccination and infection with Mycoplasma gallisepticum

Several studies have suggested that there are age related differences in the responses of chickens to vaccination and infection with Mycoplasma gallisepticum, but there have not been any systematic comparisons of the responses of young birds to vaccination with those of birds the same age to infection. The aim of the studies described here was to examine the immune responses of chickens between 1 and 6 weeks of age to vaccination and to infection with M. gallisepticum. Birds under 4 weeks of age developed significantly more severe clinical signs after infection with virulent M. gallisepticum than birds that were 4 or 6-weeks old at the time of infection, with greater concentrations of M. gallisepticum in the trachea. Chickens younger than 4 weeks old at the age of vaccination were protected from clinical signs of disease after challenge and from development of more severe lesions in the trachea and air sacs, although this protective immunity was significantly less effective in controlling the replication of M. gallisepticum in the trachea. Examination of the subsets of lymphocytes infiltrating the tracheal mucosa did not detect any age related differences. Thus the ts-11 vaccine has been shown to be safe in birds between 1 and 4 weeks of age and to be effective in preventing development of severe disease in them after challenge, even though birds of the same age were more susceptible to development of disease when infected with virulent M. gallisepticum.