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Disentangling the causality between parasite infections and poor host conditions in the wild population
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  • Ryota Hasegawa,
  • Yasuhiko Otsuki,
  • Yohsuke Uemura,
  • Chiharu Furusawa,
  • Masahiro Naka,
  • Itsuro Koizumi
Ryota Hasegawa
Hokkaido University

Corresponding Author:ryotahase344922@gmail.com

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Yasuhiko Otsuki
Hokkaido University
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Yohsuke Uemura
Hokkaido University
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Chiharu Furusawa
Hokkaido University
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Masahiro Naka
Hokkaido University
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Itsuro Koizumi
Hokkaido University
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Abstract

Although parasites reduce host health, parasite infections also occur as a consequence of compromised host health. Both causalities could induce positive feedback, in which infected hosts with poor body conditions may suffer further infection, but it has rarely been demonstrated in the wild, possibly due to methodological difficulties. We used a mark-recapture survey combined with structural equation modelling (SEM) to examine whether both causalities and positive feedback occurred in stream salmonid and parasitic copepod systems. We found that parasitic copepods reduced host conditions and hosts with poor conditions were likely to be infected, suggesting that positive feedback can occur in the wild. Importantly, heavily infected hosts with poor body conditions showed lower apparent survival rates. Our findings provide robust evidence showing host condition–parasite infection dynamics, offering novel insights into how positive feedback could strongly undermine the wild host population via reduction of host survival.