When parasites wait to transmit, they become deadlier — and not only the time within the host matter

What is it about?

Parasites don’t just vary in how harmful they are — they evolve that harm. In our new study, published in PLOS Pathogens, we explored how the timing of transmission and parasite performance outside the host shape virulence evolution, using an experimental system with Anopheles mosquitoes and their microsporidian parasite Vavraia culicis. Classic theory often frames virulence as a byproduct of how parasites replicate inside the host: the faster they grow, the more damage they cause, and the greater the risk of killing the host before they can spread. But this picture misses an important aspect of parasite and infection biology — the environment outside the host. Once parasites leave the host, they still face ecological challenges that can feed back into their evolution. We tested how this balance plays out by experimentally evolving V. culicis under two transmission regimes: one that favoured early transmission and another that favoured late transmission. After six generations, we tracked changes in parasite traits and host outcomes. In fact, we observed a trade-off: lines selected for early transmission performed better outside the host, shedding more viable spores into the environment. In contrast, late-transmission lines invested more in within-host growth but were not able to endure the outside environment as well once released. This means that parasite fitness is shaped by both within-host and environmental stages, and the evolution of virulence can depend on ecological pressures that occur after the parasite has left its host.

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Photo by National Institute of Allergy and Infectious Diseases on Unsplash

Photo by National Institute of Allergy and Infectious Diseases on Unsplash

Why is it important?

This adds a new layer to our understanding of virulence evolution — one that matters for public health. Disease control strategies and social behaviours often alter both the chances, timing and frequency of transmission. If these interventions favour parasites that linger longer in the host, they might unintentionally select for more harmful but environmentally fragile strains. Alternatively, interventions that reduce environmental transmission might push evolution in the opposite direction. Key takeaway: Virulence doesn’t evolve in a vacuum. It’s shaped by ecological trade-offs between replication, transmission, and survival outside the host. Our work highlights the importance of viewing infectious disease evolution through an ecological lens — not just examining how parasites behave within their hosts, but also considering what happens when they leave and what this means for disease spread and evolution.