OOS 57-3 - From eco to evo and back again: host-parasite evolution impacts the spread of experimental epidemic waves

Spreading epidemics resemble biological invasions in that eco-evolutionary dynamics can promote the rapid emergence of spatial patterns of trait variation. Similar to organisms at range expansion fronts, parasites at the front of an epidemic wave may evolve traits facilitating and accelerating their propagation. These outcomes may be also modulated by concurrent host evolution, potentially changing eco-evolutionary feedbacks and disease spread.

We investigated the interplay between spatial epidemiology and evolution in interconnected microcosms of the ciliate Paramecium caudatum and its bacterial parasite Holospota undulata. First, in long-term treatments we simulated conditions at the front and core of spatially progressing populations and epidemic waves, where infected and uninfected hosts at the front repeatedly colonise new patches through active swimming or passively in randomly picked groups.

We found that these contrasting ecological conditions affected the evolution of parasite and host traits determining epidemiological potential (parasite R0, virulence, host resistance), life history (host population growth, parasite developmental time) and dispersal capacity. In a second step, we assembled evolved hosts and parasites in different combinations and assessed their spread into linear microcosm landscapes. Evolutionary history created marked feedbacks on the spatio-temporal patterns of epidemic waves. As expected, evolved levels of dispersal and resistance were the main determinants of the epidemiological dynamics. Evolved parasites and/or hosts from front treatments produced faster spreading epidemics and more pronounced local outbreaks, whereas previously acquired resistance to the parasite slowed down epidemic spread.

Although simplified abstractions of the real world, the combined use of experimental epidemiology and evolution approaches, together with mathematical formalisation, can (i) inform on how different spatial ecological scenarios shape host and parasite trait (co)evolution during an epidemic, and (ii) help to assess the impact of such different evolutionary histories on the spatial spread of disease.