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

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Abstract
In chicks from immune hens levels of antibody to infectious bronchitis virus (IBV) measured by the neutralization test (NI values) decreased linearly with age. In 1 day old chicks NI values were high, whereas they were 0 in 30 day 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 wk later in an immunity that was as good as that obtained by vaccination of 20 and 15 day 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 day 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. A significant role of the Harderian gland is suggested for 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 0 NI values in vaccinated birds over 4 wk of age indicate that vaccination may not have been effective. The significance of maternal antibodies at the time of vaccination is discussed.