Immunity following vaccination with the H120 strain of infectious bronchitis virus via the drinking water.

Administration of the H120 vaccine strain of infectious bronchitis virus (IBV) via the drinking water caused mild respiratory symptoms in six out of 57 susceptible 3-week-old chickens and 45 became positive to the agar gel precipitin (AGP) test for IB. This was associated with complete protection against subsequent intratracheal challenge with the H52 strain of IBV at 11 weeks of age. The vigorous AGP response to H120 recorded here in susceptible birds contrasts with previous reports of failure of precipitins to appear following administration of water-borne IB vaccines to 1-day-old and 2-week-old chicks carrying maternally derived antibodies. We have previously shown that artificial passive immunity suppresses the AGP response to intravenous challenge with the H52 strain of IBV. Natural passive immunity, mediated by maternal antibodies, may similarly have been responsible for the failure of precipitins to appear following vaccination under field conditions.