The effectiveness of mass vaccination on Marek's disease virus (MDV) outbreaks and detection within a broiler barn: A modeling study

Atkins, Katherine E.; Read, Andrew F.; Walkden‐Brown, Stephen W.; Savill, Nicholas J.; Woolhouse, Mark

doi:10.1016/j.epidem.2013.10.001

Cited by 24 publications

(29 citation statements)

References 28 publications

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“…The first set by Atkins et al (2013a,b) uses an individual-based approach that captures many of the biological details particular to MDV dynamics. The second set by Rozins and Day (2016, 2017) uses an impulsive-differential-equation based approach that allows the models to be completely communicated through a set of equations.…”

Section: Discussionmentioning

confidence: 99%

“…Several of our model parameters are already known from previous experiments. These include the rate of dust production as a function of bird age d ( τ ) (Atkins et al, 2013a), the incubation period of the virus (Islam and Walkden-Brown, 2007; Read et al, 2015), which is determined by β , and the mean transmission rate of the virus μ α (Atkins et al, 2011, 2013b). Placement dates, load out dates, and flock sizes were fixed at known values from the respective datasets being modeled.…”

Section: Methodsmentioning

confidence: 99%

“…Such models quantitatively relate changes in ecology to changes in disease dynamics, which is particularly useful when experimental manipulation is unethical or, as with commercial-scale chicken rearing, financially costly. Models provide cheap and safe opportunities to explore the impact of system manipulation on pathogen control, and this approach has been applied to MDV (Atkins et al, 2013a,b; Rozins and Day, 2016, 2017). The reliability of a model, however, can only be assessed by challenging it with data, and this has never been done for MDV.…”

Section: Introductionmentioning

confidence: 99%

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

2018

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Marek’s disease virus (MDV) is a pathogen of chickens whose control has twice been undermined 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 using iterated filtering in ‘pomp’ to data on MDV concentration in dust collected from two commercial broiler farms. We find that virus dynamics are influenced by between-flock variation in host susceptibility to virus, shedding rate from infectious 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. Using the set of surviving models, we explore how changes to farming practices are predicted to influence MDV-associated condemnation risk (production losses at slaughter). By quantitatively capturing the mechanisms of disease ecology, we have laid the groundwork to explore the future trajectory of virus evolution.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2018

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“…Even so, improvements to vaccination, shortened rearing periods, increased genetic resistance, and many other factors have presumably decoupled the link between the prevalence of MDV infection and leukosis condemnation rates. What is needed, therefore, is to combine surveillance and quantification of the virus on farms (Walkden-Brown et al 2013) with mechanistic mathematical models (Atkins et al 2013a,b) to learn about the mechanisms that govern viral dynamics. Nevertheless, analyses of leukosis condemnation data can be useful for generating hypotheses about historical and current drivers of MD dynamics.…”

Section: Discussionmentioning

confidence: 99%

An observational study of the temporal and spatial patterns of Marek's-disease-associated leukosis condemnation of young chickens in the United States of America

Kennedy

Dunn

et al. 2015

Preventive Veterinary Medicine

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Marek’s disease, a disease primarily affecting immature chickens, is a worldwide problem that has on at least three occasions threatened the poultry industry in the United States. A rich dataset to study the epidemiology of this disease is available because the United States Department of Agriculture has required mandatory inspections of all commercially sold poultry of significant scale since the mid-20th century with over 99% of all chickens inspected. This dataset includes monthly totals aggregated by state since 1961 of the number of “young chickens” inspected and the number with “leukosis”, a condemnation category that is almost always associated with Marek’s disease in this category of birds. The objective of this study was to analyze temporal and spatial patterns in this condemnation data to gain insight into the ecology and epidemiology of the causative virus. We extracted visual patterns in the data using seasonal trend decomposition, and we tested for statistical significance using extended linear modeling techniques. The analysis confirmed previous findings that there are differences in leukosis condemnation rates between states, across years, and within years. The analysis also revealed several patterns not previously highlighted, including spatial and temporal autocorrelations in leukosis condemnation, changes to the amplitude of seasonality over time, and increasing within-year variation in condemnation rate over time. These patterns suggest that locally shared farm practices, virus transmission between farms, or viral persistence may be important to understanding the dynamics of the disease. We also discuss the plausibility of other potential explanations for these patterns.

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“…These findings support theoretical predictions (Gandon et al., ; Smith & Mideo, ) that the use of vaccines to suppress but not block pathogen replication or transmission (so called antidisease, imperfect, or leaky vaccines) results in the evolution of viruses with increased replication (i.e., fitness) or transmission and therefore virulence. Meanwhile, previous studies addressing the industrialization of farming have discussed how MDV virulence can increase independently of vaccine use (Atkins, Read, Walkden‐Brown et al., ; Atkins, Read, Savill et al., ; Rozins & Day, ). In these studies, denser flocks and longer durations for rearing in combination with shorter intercohort intervals and limited virus elimination by cleaning and disinfection can lead to reduced MDV virulence.…”

Section: Introductionmentioning

confidence: 99%

A phylogenomic analysis of Marek's disease virus reveals independent paths to virulence in Eurasia and North America

Trimpert

Groenke

Jenckel

et al. 2017

Evolutionary Applications

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Virulence determines the impact a pathogen has on the fitness of its host, yet current understanding of the evolutionary origins and causes of virulence of many pathogens is surprisingly incomplete. Here, we explore the evolution of Marek's disease virus (MDV), a herpesvirus commonly afflicting chickens and rarely other avian species. The history of MDV in the 20th century represents an important case study in the evolution of virulence. The severity of MDV infection in chickens has been rising steadily since the adoption of intensive farming techniques and vaccination programs in the 1950s and 1970s, respectively. It has remained uncertain, however, which of these factors is causally more responsible for the observed increase in virulence of circulating viruses. We conducted a phylogenomic study to understand the evolution of MDV in the context of dramatic changes to poultry farming and disease control. Our analysis reveals evidence of geographical structuring of MDV strains, with reconstructions supporting the emergence of virulent viruses independently in North America and Eurasia. Of note, the emergence of virulent viruses appears to coincide approximately with the introduction of comprehensive vaccination on both continents. The time‐dated phylogeny also indicated that MDV has a mean evolutionary rate of ~1.6 × 10−5 substitutions per site per year. An examination of gene‐linked mutations did not identify a strong association between mutational variation and virulence phenotypes, indicating that MDV may evolve readily and rapidly under strong selective pressures and that multiple genotypic pathways may underlie virulence adaptation in MDV.

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The effectiveness of mass vaccination on Marek's disease virus (MDV) outbreaks and detection within a broiler barn: A modeling study

Cited by 24 publications

References 28 publications

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

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

An observational study of the temporal and spatial patterns of Marek's-disease-associated leukosis condemnation of young chickens in the United States of America

A phylogenomic analysis of Marek's disease virus reveals independent paths to virulence in Eurasia and North America

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