Modeling Marek’s disease virus transmission: a framework for evaluating the impact of farming practices and evolution on disease

Marek’s disease virus (MDV) is a pathogen of chickens whose control has twice been un-dermined by pathogen evolution. Disease ecology is believed to be the main driver of this evolution, yet mathematical models of MDV disease ecology have never been confronted with data to test their reliability. Here, we develop a suite of MDV models that differ in the ecological mechanisms they include. We fit these models with maximum likelihood in ‘pomp’ to data on MDV concentration in dust collected from two commercial broiler farms. Using AIC to compare the models, we find that virus dynamics are influenced by between-flock variation in host susceptibility to virus, shedding rate from infected birds, and cleanout efficiency. We also find evidence that virus is reintroduced to farms approximately once per month, but we do not find evidence that virus sanitization rates vary between flocks. Of the models that survive model selection, we find agreement between parameter estimates and previous experimental data, as well as agreement between field data and the predictions of these models, suggesting robustness of model predictions. Using the set of surviving models, we explore how changes to farming practices are predicted to influence MDV costs, should pathogen evolution undermine existing control measures. By quantitatively capturing the mechanisms of disease ecology, we have laid the groundwork to explore the future trajectory of virus evolution.