Field studies on the detection, persistence and spread of the Rispens CVI988 vaccine virus and the extent of co- infection with Marek’s disease virus S Ralapanawe* a b , KG Renz a , SK Burgess a and SW Walkden-Brown a Objective: Use specific real-time quantitative polymerase chain reaction (qPCR) tests to determine: a) vaccination success determined by viral load of Rispens CVI988 (Rispens) in feathers or dust b) persistence of Rispens infection in vaccinated layer chickens; c) extent of co-infection with wild-type MDV (MDV) in vaccinated layers; and d) presence of Rispens virus in unvaccinated broiler flocks. Methods: Feather and dust samples for qPCR to detect MDV and Rispens, and serum samples to detect anti-MDV antibody using ELISA were collected from birds aged three days to 91 weeks from three layer farms. DNA extracted from MDV-positive dust samples from 100 broiler flocks, was tested for the presence of Rispens using the qPCR. Results: Overall 66% and 93% of feather and dust samples respectively from Rispens- vaccinated layers were Rispens–positive. Viral load in these samples varied between farms during early life reaching readily detectable levels at 2-3 weeks of age. Vaccinated chickens maintained high Rispens load in feathers and dust and MDV antibody levels until 91 weeks of age. MDV infection was detected in 6.7% of feather samples from vaccinated chickens. Rispens virus was detected in 7% of samples from unvaccinated broiler flocks. Conclusion: Vaccine take can be measured effectively by Rispens-specific qPCR of feathers or dust from around three weeks post vaccination. Infection with Rispens is persistent with lifelong shedding and serological response. Detectable infection rate of vaccinated chickens with MDV is low and there is preliminary evidence of escape of Rispens virus to unvaccinated flocks. a Animal Science, School of Environmental and Rural Science, University of New England, Armidale, NSW, Australia 2351 b Veterinary Research Institute, B365, Peradeniya, Sri Lanka 20400; sralapan@myune.edu.au Key words Rispens vaccine; Marek’s Disease Virus; imperfect vaccine; Viral load; Feather; Dust; Serology; Diagnostics Abbreviations avian encephalomyelitis, AE; egg drop syndrome, EDS; enzyme linked immunosorbent assay, ELISA; fowl pox, FP; Gallid herpesvirus 2, GaHV-2; Gallid herpesvirus 3, GaHV-3; Herpesvirus of Turkeys, HVT; infectious bronchitis ,IB; infectious laryngotracheitis, ILT; Marek’s Disease Virus, MDV; Marek’s Disease, MD; Meleagrid Herpesvirus 1, MeHV-1; New Castle disease, ND; qPCR, quantitative real-time PCR; Rispens CVI988 vaccine, Rispens; viral copy numbers, VCN