Immunity to avian infectious bronchitis.

Avian infectious bronchitis is recognized clinically as a respiratory disease in its only natural host,the chicken, but the virus is disseminated throughout other systems by a viraemia with localization especially in the kidney and oviduct. The sensitivity or instability of the antigenic and immunogenic properties of the virus under laboratory stress or natural influences complicates the selection of seed virus for vaccines. Modified or attenuated active virus vaccines induce greater protection against subsequent infection than do inactive virus vaccines. Maternal antibody is effective in providing passive immunity for about two weeks after the chick is hatched. Immunoglobulins induced by primary infection or by vaccination are IgG effective for neutralization of virus by circulating antibody, and presumably secretory IgA in the respiratory tract, with the latter being more effective as evidenced by protection of the trachea against reinfection. The level of humoral antibody is not necessarily correlative with immunity based on chicken protection tests against challenge with virulent virus. A true carrier status apparently does not result from primary infection but it has been suggested that the chicken may possibly become an immune carrier.