Vaccination against Marek's disease and infectious bursal disease. I. Development of a bivalent live vaccine by co-cultivating turkey herpesvirus and infectious bursal disease vaccine viruses in chicken embryo fibroblast monolayers.

Vaccine viruses of Marek's disease (MD), the FC-126 strain of the herpesvirus of turkeys (HVT), and infectious bursal disease (IBD), the Bursa-Vac-M strain of IBD virus (IBDV), were propagated in the same chicken embryo fibroblast (CEF) monolayers by superinfection. Co-infection of the two viruses in the same CEF culture or in a single cell can be demonstrated by staining with acridine orange and by electron microscopy.l This study was conducted with a superinfected live bivalent vaccine (HVT/IBDV) in one-day-old specific pathogen-free (SPF) and conventional White Leghorn (CWL) chicks. Chickens vaccinated with HVT/IBDV produced persistent HVT viremia for at least 6 weeks postvaccination; significantly higher IBD virus neutralizing antibody levels were observed for the entire 8-week experimental period when compared to unvaccinated controls. Differences between virus neutralizing antibody titers stimulated by HVT/IBDV and IBDV alone were not statistically significant in SPF chickens; however, due to maternal antibody there were some significant differences in CWL chickens. Both the HVT/IBDV bivalent vaccine and the monovalent IBDV vaccine protected the chickens against challenge with virulent IBDV. Based on microscopic lesions and bursa: body weight indexes, challenge of vaccinated chickens with virulent IBDV caused no atrophy in the bursa of Fabricius, but severe tissue destruction was observed in unvaccinated challenged controls. Chickens vaccinated with HVT/IBDV vaccine or with HVT alone were effectively protected against MD when compared to the controls.