Greater migratory propensity in hosts lowers pathogen transmission and impacts

Abstract

Animal migrations are spectacular and migratory species have been shown to transmit pathogens that pose risks to human health. Although migration is commonly assumed to enhance pathogen dispersal, empirical work indicates that migration can often have the opposite effect of lowering disease risk.Key to assessing disease threats to migratory species is the ability to predict how migratory behaviour influences pathogen invasion success and impacts on migratory hosts, thus motivating a mechanistic understanding of migratory host-pathogen interactions.Here we develop a quantitative framework to examine pathogen transmission in animals that undergo two-way directed migrations between wintering and breeding grounds annually.Using the case of a pathogen transmitted during the hosts’ breeding season, we show that a more extreme migratory strategy (defined by the time spent away from the breeding site and the total distance migrated) lowers the probability of pathogen invasion. Moreover, if migration substantially lowers the survival probability of infected animals, then populations that spend comparatively less time at the breeding site or that migrate longer distances are less vulnerable to pathogen-induced population declines.These findings provide theoretical support for two non-exclusive mechanisms proposed to explain how seasonal migration can lower infection risk: (i) escape from habitats where parasite transmission stages have accumulated, and (ii) selective removal of infected hosts during strenuous journeys. Our work further suggests that barriers to long distance movement could increase pathogen prevalence for vulnerable species, an effect already seen in some animal species undergoing anthropogenically induced migratory shifts.

Population biology of infectious diseases shared by wild and farmed fish

“…Migratory culling can lead to declines in both host population size and parasite abundance during migration. Migratory escape refers to the situation where hosts migrate to new environments where infection risk is lower because local parasite abundance has not yet accumulated (Hall et al 2014). Alternatively, a negative effect of infection on migration speed may create a dynamic that may result in a slowing or stoppage of the migration, depending on how the collective behavior of hosts in a migrating population responds to parasitism.…”

Loss of migratory behaviour increases infection risk for a butterfly host

“…However, recent work suggests that migration more typically lowers infection risk for migrants by (i) allowing animals to periodically leave behind parasite-contaminated habitats (a process termed migratory escape [20]), (ii) weeding out infected individuals during strenuous, longdistance journeys (migratory culling [21,22]) and (iii) separating vulnerable juveniles from infectious adults (migratory allopatry [23]). Support for the role of migration in lowering infection risk comes from theoretical models [24] and field studies [25,26] (reviewed in [15]). Diminished migrations could enhance pathogen transmission via the loss of migratory escape or migratory culling.…”

Behavioural influences on disease risk: implications for conservation and management

“…Populations also exhibit variation in parasite prevalence; nonmigratory populations exhibit much higher parasite prevalence than migratory populations (Altizer, Oberhauser, & Brower, 2000). These empirical observations are supported by theoretical models, which suggest that populations with more extreme migrations (in terms of time spent away from breeding sites and distance migrated) have lower rates of infection (Hall, Altizer, & Bartel, 2014). Additionally, migratory populations are expected to be less vulnerable to population declines driven by infectious disease (Hall et al, 2014).…”