Influence of maternal antibodies on vaccination of chicks of different ages against infectious bronchitis

Abstract

In chicks from immune hens levels of antibody to infectious bronchitis (IBV) measured by the neutralisation test (NI values) decreased linearly with age. In 1-day-old chicks NI values were high, whereas they were zero in 30-days-old birds. Vaccination of 1-day-old chicks with high NI values by the conjunctival and intranasal routes with H 120 vaccine virus resulted 4 weeks later in an immunity that was as good as that obtained by vaccination of 20- and 15-days-old birds with lower levels of maternal antibody. In these age groups successful vaccination coincided with an obvious stimulation of the Harderian gland, resulting in increased numbers of plasma cells and lymphocytes. Two weeks after vaccination of 6 and 10-days-old birds an extensive degeneration of plasma cells was observed in the Harderian gland. Following challenge, these birds exhibited respiratory symptoms and histopathological changes in the trachea indicating an incomplete immunity. These results suggest a significant role of the Harderian gland in the immune response to IBV. After vaccination NI values did not always correlate with protection; they have only a limited value as a criterion for immunity. Very low or zero NI values in vaccinated birds over 4 weeks of age indicate that vaccination may not have been effective. The significance of maternal antibodies at the time of vaccination is discussed.