Maternal immunity can interfere with infectious bursal disease virus (IBDV) vaccine efficacy. The effect of maternal antibody (MatAb) on the immune response and detection of two vaccine viruses, an IBDV immune complex (cx) and IBDV-2512, was investigated. Vaccines were administered in ovo at 100 mean embryo infectious dose to commerical broiler embryos derived from young (29 wk) or old (63 wk) flocks. Chickens with low MatAb were challenged with an antigenic standard IBDV strain at 21 and 35 days post in ovo vaccination (PIOV). At 28 and 42 days PIOV, birds were euthanatized and bursa weight:body weight ratios were determined. MatAb of progeny from the 29-wk-old flock was higher (P < 0.05) than that from progeny of the 63-wk-old flock. Progeny titers declined by day 21 PIOV. Birds with MatAb vaccinated with either the IBDV-2512 or IBDV-Icx vaccine did not mount an antibody response by 21 days PIOV. The IBDV-Icx vaccination protected chickens against bursal atrophy when they were challenged at 21 and 35 days PIOV (83% and 77%, respectively). Resistance to challenge in the absence of antibody implies that cellular immunity plays a role in IBDV protection. The IBDV-2512 vaccine caused atrophy of the bursae, and, therefore, we could not examine its protection by these criteria. Neither vaccine could be detected by antigen-capture chemiluminescent enzyme-linked immunosorbent assay. Therefore, total RNA was extracted and reverse transcription (RT)–polymerase chain reaction (PCR) and nested PCR were conducted to amplify the VP2 (hypervariable) region on viral RNA collected at 3, 6, 9, 15, and 21 days PIOV. Nested PCR detected the VP2 region of both IBDV vaccine viruses at day 3 PIOV. Only IBDV-2512 RNA was detected on day 6 PIOV. However, the IBDV-Icx RNA was detected on days 9 and 15 PIOV but not on day 21 PIOV. The IBDV-Icx RNA was evident in tissues and could induce protection against challenge. This work gives further insight into the mechanism of the IBDV-Icx vaccine.
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Vol. 46 • No. 4