The industrialization of farming may be driving virulence evolution

Abstract

Farming practices have changed dramatically over the years. The industrialization of farming has provided parasites with an abundance of hosts and is thought to have influenced parasite evolution. For example, the parasite that causes the highly contagious poultry disease, Marek's disease, has evolved over the past 60 years into a highly virulent pathogen. It is assumed that the industrialization of the industry and vaccination have selected for more virulent strains of the virus. Here, with the use of an impulsive differential equation model, we investigate how modern broiler farm practices could independently lead to virulence evolution. Our model suggests that longer cohort durations and more densely stocked barns both select for less virulent strains of the virus. Our model also suggests that if intensive cleaning between cohorts does not rid the barn of disease, it may drive evolution and cause the disease to become more virulent.

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

“…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. Our modeling approach is a combination of these two approaches.…”

“…This relatively high efficiency implies that current practices are good at reducing dust and virus between flocks. This efficiency, however, may be a cause for concern given that Rozins and Day (2017) found efficient cleaning between flocks can promote the evolution of increased MDV virulence by decreasing the lifespan of the pathogen and thus decreasing the cost of virulence.…”

“…Historical control measures have at least twice been undermined by virus evolution, leading to speculation that future evolution could undermine current control (Nair, 2005). The ecology of the disease appears to be the driving force behind past evolution, with explanations invoking vaccination (Witter, 1997; Atkins et al, 2013a; Read et al, 2015), rearing period duration (Atkins et al, 2013a; Rozins and Day, 2017), and virus persistence during downtime between bird flocks (Rozins and Day, 2017). Understanding the ecology of the virus is thus a key component in predicting whether and when control efforts will lose efficacy.…”

“…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.…”

Invest more and die faster: The life history of a parasite on intensive farms

“…Agri‐ and aquacultural practices have become increasingly intensive in the past decades, which represents a change in the ecology of domesticated species (Turcotte, Araki, Karp, Poveda, & Whitehead, ). In parallel, there has been growing concern that this may also cause evolutionary changes in the life histories of agricultural pests and diseases (Kennedy et al., ; Lebarbenchon, Brown, Poulin, Gauthier‐Clerc, & Thomas, ; Mennerat, Nilsen, Ebert, & Skorping, ; Rogalski, Gowler, Shaw, Hufbauer, & Duffy, ; Rozins & Day, ). High local densities of animals or plants, reduced genetic variation, and breeding of stocks for a high output usually characterize intensive agricultural systems.…”