Migratory host vectors can maintain the high-dose refuge effect in a structured host-parasite system: the case of sea lice and salmon Evolutionary Applications

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  • Migration can reduce parasite burdens in migratory hosts, but it connects populations and can drive disease dynamics in domestic species. Farmed salmon are infested by sea louse parasites, often carried by migratory wild salmonids, resulting in
    a costly problem for industry and risk to wild populations when farms amplify louse
    numbers. Chemical treatment can control lice, but resistance has evolved in many
    salmon-farming regions. Resistance has, however, been slow to evolve in the northeast Pacific Ocean, where large wild-salmon populations harbour large sea louse
    populations. Using a mathematical model of host–macroparasite dynamics, we explored the roles of domestic, wild oceanic and connective migratory host populations
    in maintaining treatment susceptibility in associated sea lice. Our results show that a
    large wild salmon population, unexposed to direct infestation by lice from farms; high
    levels of on-farm treatment; and a healthy migratory host population are all critical to
    slowing or stopping the evolution of treatment resistance. Our results reproduce the
    “high-dose/refuge effect,” from the agricultural literature, with the added requirement of a migratory host population to maintain treatment susceptibility. This work
    highlights the role that migratory hosts may play in shared wildlife/livestock disease,
    where evolution can occur in ecological time.

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    Article (Published)
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    Attribution-NonCommercial 4.0 International