Superspreading and the evolution of virulence
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Superspreading, where a small proportion of a population can cause a high proportion of infection transmission, is well known to be important to the epidemiology of a wide range of pathogens, including SARS-CoV-2. However, despite its ubiquity in important human and animal pathogens, the impact of superspreading on the evolution of pathogen virulence is not well understood. Using theory and both deterministic and stochastic simulations we examine the evolution of pathogen virulence under a range of different distributions of infection transmission for the host. Importantly, for many pathogens, superpreader events may be associated with increased tolerance to infection or asymptomatic infection and when we account for this super-spreading selects for higher virulence. In contrast, in animal populations where highly connected individuals, that are linked to superspreader events, also have fitness benefits, superspreading may select for milder pathogens. In isolation, the transmission distribution of the host does not impact selection for pathogen virulence. However, superspreading reduces the rate of pathogen evolution and generates considerable variation in pathogen virulence. Therefore, the adaptation of an emerging infectious disease, that exhibits superspreading, is likely to be slowed and characterised by the maintenance of maladaptive variants. Taken as a whole, our results show that superspreading can have important impacts on the evolution of pathogens.
Author Summary
The impact of infectious disease can vary from individual to individual. Superspreader events, where a few individuals cause a high proportion of infections, are critical to the spread and outbreak size of a wide range of important infectious diseases of humans and animals. Superspreading events and pathogen evolution, were important features of the Covid-19 pandemic, highlighted by the succession of dominant strains during the pandemic. There is a clear need to understand how superspreader events will affect the evolution of pathogens, in particular how the level of virulence (the additional mortality due to the infection) will evolve. In this study we use mathematical models to show that superspreading reduces the rate of pathogen evolution and generates considerable variation in pathogen virulence. Therefore, the adaptation of an emerging infectious disease, that exhibits superspreading, is likely to be slowed and characterised by the maintenance of maladaptive variants. Importantly, for many pathogens, superpreader events may be associated with increased tolerance to infection or asymptomatic infection, and when we account for this, superspreading selects for higher virulence. Our results show that superspreading can have important impacts on the evolution of pathogens.
