Kinetics of Marek's disease virus (MDV) infection in broiler chickens 1: effect of varying vaccination to challenge interval on vaccinal protection and load of MDV and herpesvirus of turkey in the spleen and feather dander over time

Two experiments in commercial broiler chickens vaccinated with herpesvirus of turkeys (HVT) and challenged with Marek's disease virus (MDV) investigated the effects of the vaccination-to-challenge interval (VCI) on vaccinal protection against Marek's disease, and the kinetics of MDV and HVT load in the spleen and feather dander determined using real-time quantitative polymerase chain reaction. Experiment 1 in isolators tested VCI of 2, 4 and 7 days, while Experiment 2 in floor pens tested VCI of 0, 2, 4, 7 and 10 days. MDV challenge induced gross Marek's disease lesions in 14% to 74% of chickens by 56 days post-challenge. Vaccinal protection increased from ～40% to ～80% with increasing VCI between days 2 and 7 in both experiments, but not thereafter. MDV was detected in both the spleen and dander at 7 days post-challenge and increased rapidly to approximately 21 days post-challenge, after which levels plateaued, rose or fell gradually depending on treatment. HVT was also shed in significant amounts, 1 to 2 logs lower than for MDV, with a clear peak around 14 to 21 days post-vaccination. Vaccination significantly reduced the log₁₀MDV load in the spleen (vaccinated, 2.99±0.20/10⁶ spleen cells; unvaccinated, 4.60±0.23/10⁶ spleen cells) and dander (vaccinated, 5.28±0.13/mg; unvaccinated, 6.00±0.18/mg) from infected chickens. The MDV load had a significant negative association with the VCI and the level of vaccinal protection. Measurement of dander production in Experiment 1 and the dust content of air in Experiment 2, combined with determination of the MDV load in these, enabled estimation of total daily shedding rates of MDV per chicken and of the MDV load in air for the first time.