Growth rate, transmission mode and virulence in human pathogens

Abstract

The harm that pathogens cause to hosts during infection, termed virulence, varies across species from negligible to a high likelihood of rapid death. Classic theory for the evolution of virulence is based on a trade-off between pathogen growth, transmission and host survival, which predicts that higher within-host growth causes increased transmission and higher virulence. However, using data from 61 human pathogens, we found the opposite correlation to the expected positive correlation between pathogen growth rate and virulence. We found that (i) slower growing pathogens are significantly more virulent than faster growing pathogens, (ii) inhaled pathogens and pathogens that infect via skin wounds are significantly more virulent than pathogens that are ingested, but (iii) there is no correlation between symptoms of infection that aid transmission (such as diarrhoea and coughing) and virulence. Overall, our results emphasize how virulence can be influenced by mechanistic life-history details, especially transmission mode, that determine how parasites infect and exploit their hosts.This article is part of the themed issue ‘Opening the black box: re-examining the ecology and evolution of parasite transmission’.

Host heterogeneity affects both parasite transmission to and fitness on subsequent hosts

“…Parasite growth rate is often correlated with virulence (i.e. the damage inflicted on the host) [70], so this result implies that the host from whom an infection is acquired may affect the severity of the infection on the subsequent host. While the mechanisms behind these findings require elucidation, this study further validates recent calls for more holistic consideration of the effects of within-host processes on between-host transmission [1,2,52].…”

Lost in transmission…?

“…For example, McCallum et al [11] explicitly considers interaction of the various steps involved in successful transmission from the initial infection load in an infected 'donor' host, to the dose arriving and establishing within a 'recipient' susceptible host, and proposes a general transmission framework potentially applicable to any hostparasite system. A key point to emerge from that article, and also raised by many of the other authors [2][3][4]9,10,12], is that there may be important nonlinearities and heterogeneities acting at the different stages of the overall transmission process that can alter the magnitude and functional form of transmission. Previous work has discussed the importance of nonlinearities, with respect to the contact structure between infectious and susceptible hosts (see references in [11]).…”

Proliferation and benevolence—A framework for dissecting the mechanisms of microbial virulence and health promotion

“…A likely reason is that the evolutionary models are built around assumptions about the relationship between host harm and different components of pathogen fitness, such as the rate of transmission and the duration of infection, whilst the virulence factor concept is centred on host harm, and does not take pathogen fitness into account (Casadevall & Pirofski, 2001Casadevall, 2009). However, components of pathogen fitness are closely related to the proliferation and density of the pathogen in the host (Acevedo et al, 2019;Fraser, Hollingsworth, Chapman, de Wolf, & Hanage, 2007;Råberg, 2012), and the relationship between host harm and pathogen density is therefore key to the study of virulence evolution (Leggett, Cornwallis, Buckling, & West, 2017). An analysis of virulence factors that decomposes their effects into two components-pathogen density versus the harm done relative to this density-would therefore facilitate their incorporation into a rich body of work in evolutionary biology.…”