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Virulence Evolution: Thinking Outside of the Host.
Luís M Silva1,2, Jacob C Koella1
1Institute of Biology University of Neuchâtel Neuchâtel Switzerland.
Evolutionary Applications
|September 2, 2025
Summary
Virulence evolution depends on a trade-off between host damage and transmission. This study shows that more virulent parasites survive poorly outside the host, impacting disease spread predictions.
Area of Science:
- Evolutionary biology
- Parasitology
- Disease ecology
Background:
- The evolution of virulence theory posits a trade-off between pathogen virulence and transmission rate.
- Measuring this trade-off is challenging, potentially due to a lack of data on parasite survival outside the host.
- A transmission decomposition framework highlights the importance of environmental survival for understanding virulence evolution.
Purpose of the Study:
- To investigate the relationship between within-host virulence and environmental survival in the microsporidian parasite *Vavraia culicis*.
- To explore how parasite growth and virulence impact its ability to survive outside the mosquito host, *Anopheles gambiae*.
- To assess the role of environmental survival in the broader context of parasite fitness and disease evolution.
Main Methods:
- Utilized distinct parasite lines of *Vavraia culicis* exhibiting varying virulence levels.
- Infected the mosquito host *Anopheles gambiae* with these parasite lines.
- Quantified parasite virulence within the host and survival rates in the external environment.
Main Results:
- Parasite lines with higher virulence and growth rates within *Anopheles gambiae* showed reduced environmental survival.
- This cost to environmental survival was independent of ambient temperature.
- A clear trade-off was observed between within-host fitness and ex-host survival.
Conclusions:
- Within-host virulence and environmental survival are inversely related, supporting the virulence trade-off theory.
- Understanding parasite survival outside the host is crucial for accurate predictions of infectious disease evolution and spread.
- Future studies should integrate both within-host dynamics and environmental transmission stages to fully grasp parasite fitness.
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