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

Abstract

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 approximately 40% to approximately 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 log(10)MDV load in the spleen (vaccinated, 2.99+/-0.20/10(6) spleen cells; unvaccinated, 4.60+/-0.23/10(6) 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.

Modelling Marek's Disease Virus (MDV) infection: parameter estimates for mortality rate and infectiousness

“…Our estimate of the primary latent period of between 2 and 6 days is much shorter than earlier estimates of around 13 days [31] but only slightly shorter than more recent estimates based on new PCR techniques which have detected significant quantities of virus in feather tips [32] and feather dander [27,28] at 7 days post infection. This discrepancy may be due to a number of factors: detectability of virus at low quantities, the sampling time of the experiments and heterogeneities between the experiments.…”

“…The estimates here also have confidence intervals between 0-6 days which suggest the data are not able to give a great deal of certainty on the exact time of onset of viral shedding. Further analysis might take into account biological knowledge through prior distributions to get a more exact value, and past experimental studies would suggest shedding begins towards the end of the estimated interval [27,28]. …”

“…However, the relationship between viral load (or replication) and virulence has not been convincingly tested [16]. PCR testing methods have been developed in order to quantify viral loads [20-26], which have enabled the viral loads in shed dust to be directly measured [27,28]. The infectious period is defined as the total time in which an infected bird sheds virus [29].…”

Modeling Marek's disease virus transmission: A framework for evaluating the impact of farming practices and evolution

“…These differences would likely alter the ecology of the virus, and in turn, our parameter estimates. For example, vaccination is known to suppress virus shedding rates (Islam et al, 2008; Atkins et al, 2011; Read et al, 2015). But of course, there are numerous differences between poultry rearing in the United States and other places in the world, suggesting that many potential factors in addition to vaccination, may lead to differences in disease dynamics.…”

“…Although vaccinated birds can still be infected with MDV and can still shed MDV (Witter et al, 1971; Islam et al, 2008; Ralapanawe et al, 2016), vaccination greatly reduces clinical signs of disease (Witter et al, 1971). This, along with other measures to ensure bird health, means that total mortality from hatch to processing is typically minimal (≈3% and ≈8% in the two farms used for model inference below – in line with the national average of 4.8%, National Chicken Council, 2016).…”