Infection burdens and virulence under heat stress: ecological and evolutionary considerations

Abstract

As a result of global change, hosts and parasites (including pathogens) are experiencing shifts in their thermal environment. Despite the importance of heat stress tolerance for host population persistence, infection by parasites can impair a host's ability to cope with heat. Host–parasite eco-evolutionary dynamics will be affected if infection reduces host performance during heating. Theory predicts that within-host parasite burden (replication rate or number of infecting parasites per host), a key component of parasite fitness, should correlate positively with virulence—the harm caused to hosts during infection. Surprisingly, however, the relationship between within-host parasite burden and virulence during heating is often weak. Here, we describe the current evidence for the link between within-host parasite burden and host heat stress tolerance. We consider the biology of host–parasite systems that may explain the weak or absent link between these two important host and parasite traits during hot conditions. The processes that mediate the relationship between parasite burden and host fitness will be fundamental in ecological and evolutionary responses of host and parasites in a warming world. This article is part of the theme issue ‘Infectious disease ecology and evolution in a changing world’.

Into the danger zone: How the within‐host distribution of parasites controls virulence

“…While disease mitigation efforts frequently look to modify the host, by enhancing host resistance or reducing density, these results emphasize the parallel importance of parasite source population in driving disease outcomes. This variation may even be climate-driven, in that the higher virulence parasites we observed occur in lower and warmer environments, consistent with other disease systems in which temperature has been positively linked to virulence (Hector et al, 2023;Kirk et al, 2018;Marcogliese, 2008;Pulkkinen et al, 2010). Looking forward, we underscore the value of studies that identify the mechanistic basis of differential virulence and evaluate how virulence outcomes change with alterations to hosts, parasites or their environments (e.g.…”

Infectious disease ecology and evolution in a changing world

“…The final two papers in this theme address the evolutionary potential of hosts [ 23 ] and pathogens [ 24 ] in the face of rising temperatures. The geographical ranges of hosts, and the pathogens they carry, are expanding owing to changing climates [ 25 ].…”

“…In addition to driving vector evolution, will climate change select for more virulent pathogens? In the light of the classic virulence-transmission trade-off [26], Hector et al [24] discuss what happens to pathogen virulence, burdens, and replication in host populations suffering heat stress. The authors find predicting the evolution of pathogen virulence amidst climate change might require a better understanding of transmission strategies and covariation among pathogen traits.…”

Dual stressors of infection and warming can destabilize host microbiomes

“…Shifting global temperatures is also changing the geographical distribution of infectious diseases [2,3]. As temperatures increase, hosts and parasites are experiencing shifts in their thermal environment, driving variation in disease outcomes [4,5]. Thus, projections of species persistence in the changing world will need to account for threats posed by both warming and infection, as well as the interaction of these dual stressors [6,7].…”