| Abstract | Marek’s disease is an oncogenic poultry disease affecting chickens and is estimated to cost the worldwide industry $1-2 billion annually. The causative agent of Marek’s Disease (MD), MDV, provides a well-documented example of virulence evolution occurring over the period of half a century. The reason behind this evolution is unknown, although certain untested hypotheses have been suggested. These include vaccination (with increasingly potent vaccines) and other aspects of industrialisation, including the decreased cohort time of successive generations. Disease control is mainly via vaccination, indeed MDV vaccination was the first such vaccine offering protection against an oncogenic virus. There have been pathotyping studies which serve to define ‘virulence rank’ as the percentage of vaccinated birds dying or displaying gross clinical signs. To be able to understand the cause of past virulence evolution and develop future strategies for controlling the disease, addressing this problem is of prime importance. In this study, two sections of work are undertaken. First, estimation of epidemiological parameters is tackled: virulence of MDV is quantified by looking at mortality and virus shedding rates in vaccinated and unvaccinated birds. Both of these are achieved via Maximum Likelihood Estimation and Bayesian McMC techniques. Second, viral fitness is quantified by defining a lifetime fitness function using the parameters previously estimated and fitness landscapes are calculated to understand the direction and force of selection and the optimum fitness for different cohort times and vaccination strategies. Parameter estimation results show that the time to death for an infected bird decreases and its virus shedding rate increases with virulence rank. Model results suggest that both decreasing an individual’s lifespan and increasing the potency of vaccination of the bird serve to increase the virulence corresponding to a fitness peak. This illustrates that both decreasing the broiler lifespan and introducing, and increasing the potency of, vaccination may serve to drive evolution of MDV to a higher virulence. By estimating key epidemiological parameters and using simple mathematical models to understand the virus life history, it is anticipated that a better understanding of how MDV affects poultry flocks will emerge. This work not only explains past trends, but serves as a predictive tool for the future impact of current decisions taken by the farming industry. |
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