The roles of environmental variation and parasite survival in virulence–transmission relationships

Abstract

Disease outbreaks are a consequence of interactions among the three components of a host–parasite system: the infectious agent, the host and the environment. While virulence and transmission are widely investigated, most studies of parasite life-history trade-offs are conducted with theoretical models or tractable experimental systems where transmission is standardized and the environment controlled. Yet, biotic and abiotic environmental factors can strongly affect disease dynamics, and ultimately, host–parasite coevolution. Here, we review research on how environmental context alters virulence–transmission relationships, focusing on the off-host portion of the parasite life cycle, and how variation in parasite survival affects the evolution of virulence and transmission. We review three inter-related ‘approaches’ that have dominated the study of the evolution of virulence and transmission for different host–parasite systems: (i) evolutionary trade-off theory, (ii) parasite local adaptation and (iii) parasite phylodynamics. These approaches consider the role of the environment in virulence and transmission evolution from different angles, which entail different advantages and potential biases. We suggest improvements to how to investigate virulence–transmission relationships, through conceptual and methodological developments and taking environmental context into consideration. By combining developments in life-history evolution, phylogenetics, adaptive dynamics and comparative genomics, we can improve our understanding of virulence–transmission relationships across a diversity of host–parasite systems that have eluded experimental study of parasite life history.

Environmental drivers of biseasonal anthrax outbreak dynamics in two multihost savanna systems

“…In this study, we analyzed patterns of anthrax mortalities in ENP and KNP. Leveraging two systems with different disease dynamics, yet many of the same host species, will help us to better understand the underlying environmental mechanisms that drive outbreaks (Turner et al, 2021). We first examined outbreak dynamics and biseasonal patterns in anthrax cases in both systems.…”

Immunological Evidence of Variation in Exposure and Immune Response to Bacillus anthracis in Herbivores of Kruger and Etosha National Parks

“…The variation in anthrax ecology worldwide has served as an impediment for the blending of knowledge and outbreak forecasting ( 45 ) and therefore, identifying the variables that play a role in disease dynamics warrants substantial attention. Comparing two natural systems allows us to study the differences, patterns and pathways that may be unnoticed under the limited lens of a single system ( 46 ). In addition, comparing systems that differ in disease dynamics, but share the same potential host species, allows us to “control” for the large differences in ecology, behavior and immunity between different species, while exploring how exposure and immune response vary among populations of the same species.…”

Survival of the sickest: selective predation differentially modulates ecological and evolutionary disease dynamics

“…2a). While the link between parasite virulence and disease transmission remains unclear (Williams et al 2014, Ben‐Ami 2017, Ben‐Shachar and Koelle 2018), there is strong evidence for a positive relationship between within host replication and virulence, as well as between replication and transmission (Acevedo et al 2019, Turner et al 2021). Reduced infectious periods, as a result of selective predation on infected individuals, is thus expected to increase the replication rate of parasites, amplifying the strength of selection between parasites and their hosts.…”