Reactivation of latent infections with migration shapes population-level disease dynamics

Annual migration is common across animal taxa and can dramatically shape
the spatial and temporal patterns of infectious disease. Although
migration can decrease infection prevalence in some contexts, these
energetically costly long-distance movements can also have
immunosuppressive effects that may interact with transmission processes in
complex ways. Here we develop a mechanistic model for the reactivation of
latent infections driven by physiological changes or energetic costs
associated with migration (i.e., “migratory relapse”) and its effects on
disease dynamics. We determine conditions under which migratory relapse
can amplify or reduce infection prevalence across pathogen and host traits
(e.g., infectious periods, virulence, overwinter survival, timing of
relapse) and transmission phenologies. We show that relapse at either the
start or end of migration can dramatically increase prevalence across the
annual cycle and may be crucial for maintaining pathogens with low
transmissibility and short infectious periods in migratory populations.
Conversely, relapse at the start of migration can reduce the prevalence of
highly virulent pathogens by amplifying culling of infected hosts during
costly migration, especially for highly transmissible pathogens and those
transmitted during migration or the breeding season. Our study provides a
mechanistic foundation for understanding the spatiotemporal patterns of
relapsing infections in migratory hosts, with implications for zoonotic
surveillance and understanding how infection patterns will respond to
shifts in migratory propensity associated with environmental change.
Further, our work suggests incorporating within-host processes into
population-level models of pathogen transmission may be crucial for
reconciling the range of migration–infection relationships observed across
migratory species.