Delivery of DNA and Recombinant Infectious Bursal Disease Virus Vaccines in Ovo

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Infectious bursal disease virus (IBDV) remains a serious problem for commercial broiler producers throughout the world. An in ovo delivery system for plasmid DNA vaccines was evaluated by studying parameters, such as the route of delivery (air cell vs amniotic cavity), transfection reagent (IFA+DMSO vs polyethylenimine), dose of plasmid DNA (1 to 100 µg/egg), and the nature of humoral immune responses. An optimal response was detected when embryos were inoculated with 60 µg of plasmid DNA.

This system for in ovo delivery was used to determine the efficacy of a plasmid DNA vaccine against IBDV in 18-day-old embryos. The DNA vaccine expresses the polyprotein VP2-VP4-VP3 of IBDV. SPF and fertile broiler eggs with maternal antibodies were vaccinated and challenged against IBDV-STC. Two groups of birds (SPF and broilers) received a booster immunization with baculovirus expressed-proteins of IBDV. The DNA vaccine had no detrimental effect on hatchability or first week post-hatch survival. In ovo vaccination generated detectable humoral immune responses as measured by ELISA. Antibody response was significantly enhanced two weeks post the IBDV-protein boost. Broilers vaccinated with plasmid DNA or IBDV-protein boost exhibited partial protection against IBDV-STC strain, whereas, vaccinated SPF chicks were not protected and exhibited severe microscopic lesions after challenge.

A second approach in the control of IBDV used a recombinant attenuated vaccine administered in ovo to 18-day-old embryos. The vaccine was genetically tailored to protect from challenges in the field against classic and variant strains of IBDV. SPF and fertile broiler eggs were vaccinated and used to evaluate protection against IBDV-STC challenge. A full dose of the vaccine consisting of 5.6x103 pfu was administered to SPF and commercial broiler embryos. In addition, a half dose containing 2.3x103 pfu was injected in SPF embryos. The vaccine generated high antibody titers in chickens vaccinated with either dosage. All vaccinated groups were protected against mortality. The vaccine did not cause bursal damage and fully protected SPF chicks vaccinated in ovo with 2.3x103 pfu and broiler embryos that received a full dose of the recombinant vaccine. The vaccine had no effect on hatchability or first week survival in either broilers or SPF birds, even when high doses were administered.