Effects of co-infections on Marek's disease in poultry and development of novel recombinant Marek's disease virus vector vaccines

Marek's disease (MD) is a contagious fatal endemic disease of poultry caused by serotype 1 strains of MD virus (MDV-1). MDV serotype 2 (MDV-2) is also widespread in UK poultry, in both presence and absence of MDV-1, and with or without pathology. Prevalence of MDV-2 infection and its effect on flock productivity and disease will be investigated by longitudinal sampling of blood and dust from MDV-2-positive and negative broiler-breeder and layer poultry farms and real-time qPCR to determine kinetics of replication and shedding of MDV-2 and frequency of co-circulation with MDV-1 field strains and vaccine viruses. Mathematical modelling will be used to investigate transmission dynamics of MDV-2. Flock health and production data will enable analysis of effects of MDV-2 co-infection on these parameters. MDV-2 will be isolated from four positive farms by culturing splenocytes. The isolates will be sequenced and a phylogenetic tree constructed to determine relatedness to known MDV-2 vaccine and field strains. Two strains will be chosen for controlled trials in SPF birds to examine replication kinetics, persistence, clinical signs and horizontal transmission to sentinel birds, in comparison with MDV-2 vaccine strain SB-1. One strain will be chosen to develop a novel MDV-2 recombinant vaccine using CRISPR/Cas9 gene editing to insert infectious bursal disease virus VP2 gene and Newcastle disease virus F gene. The resulting virus, rMDV2-IBD-ND, will be tested in a vaccine trial in SPF birds to investigate protection against MD following challenge with MDV-1, and generation of antibodies against the IBDV VP2 and NDV F proteins. The data will give an understanding of the effect of MDV-2 infection on poultry health and productivity, while development of a novel rMDV-2 vaccine could be a useful addition to the set of live vaccines used to control MD and other poultry diseases, with advantages of transmissibility and no interference with HVT-based recombinant vaccines.