Evaluation of the Protection Efficacy of a Serotype 1 Marek's Disease Virus-Vectored Bivalent Vaccine Against Infectious Laryngotracheitis and Marek's Disease

Laryngotracheitis (LT) is a highly contagious respiratory disease of chickens that produces significant economic losses to the poultry industry. Traditionally, LT has been controlled by administration of modified live vaccines. In recent years, the use of recombinant DNA-derived vaccines using turkey herpesvirus (HVT) and fowlpox virus has expanded, as they protect not only against the vector used but also against LT. However, HVT-based vaccines confer limited protection against challenge, with emergent very virulent plus Marek's disease virus (vv+MDV). Serotype 1 vaccines have been proven to be the most efficient against vv+MDV. In particular, deletion of oncogene MEQ from the oncogenic vvMDV strain Md5 (BAC Delta MEQ) resulted in a very efficient vaccine against vv+MDV. In this work, we have developed two recombinant vaccines against MD and LT by using BAC Delta MEQ as a vector that carries either the LT virus (LTV) gene glycoprotein B (gB; BAC Delta MEQ-gB) or LTV gene glycoprotein J (gJ; BAC Delta MEQ-gJ). We have evaluated the protection that these recombinant vaccines confer against MD and LT challenge when administered alone or in combination. Our results demonstrated that both bivalent vaccines (BAC Delta MEQ-gB and BAC Delta MEQ-gJ) replicated in chickens and were safe to use in commercial meat-type chickens bearing maternal antibodies against MDV. BAC Delta MEQ-gB protected as well as a commercial recombinant (r)HVT-LT vaccine against challenge with LTV. However, BAC Delta MEQ-gJ did not protect adequately against LT challenge or increase protection conferred by BAC Delta MEQ-gB when administered in combination. On the other hand, both BAC Delta MEQ-gB and BAC Delta MEQ-gJ, administered alone or in combination, protected better against an early challenge with vv+MDV strain 648A than commercial strains of rHVT-LT or CVI988. Our results open a new avenue in the development of recombinant vaccines by using serotype 1 MDV as vectors.