Field observations on serological responses to vaccine strains of infectious bronchitis virus administered by coarse spray and via the drinking water

The agar gel precipitin (AGP) and haemagglutination inhibition (HI) responses to vaccination with infectious bronchitis virus (IBV) were investigated in three flocks. The vaccines were administered by coarse spray in one large flock of commercial layers and via the drinking water in two smaller flocks of broiler breeders. In two flocks, one vaccinated by coarse spray and one vaccinated via the drinking water, the chicks had high levels of maternal antibody and failed to respond serologically to the primary vaccination at 2 to 3 weeks of age. However, a vigorous antibody response, probably due to cycling vaccine virus, was demonstrated at 6 to 7 weeks of age after the maternal antibodies had waned. By contrast the chicks in the third flock, with low levels of maternal antibody at 2 weeks, responded promptly and vigorously to the primary vaccination. Passive immunity, mediated by maternal antibodies, was thought to have inhibited the initial AGP and HI responses to vaccine in the first two flocks. Between 10 and 20 weeks, in spite of repeated vaccination, the IBV geometric mean HI titres did not increase and the number of AGP-positive sera remained low. In the densely populated group receiving the coarse spray, the AGP response was virtually absent and in those smaller groups of birds receiving water-borne vaccine only 33% became AGP-positive. As in the young chicks with passive immunity, active immunity in the older birds, associated either with deliberate vaccination or inadvertent cycling vaccine virus, was thought to have inhibited the serological responses to the second and third exposures to vaccine. There was a marked rise in the number of AGP-positive birds (84% and 68% respectively) at 25 weeks of age in two flocks. HI test results indicated that this was due to exposure to a field strain of IBV in one flock and to a cycling vaccine strain in the other. The AGP results at this time were similar in both of these flocks and this test could not be used to differentiate the responses to the different types of IBV. Only 25% of the sera were AGP-positive at 25 and 30 weeks in the third flock and therefore in such a flock any sudden rise to above 50% in the number of AGP-positive birds might be attributed to recent exposure to field or vaccine strains of IBV.