The enemy of my enemy is my friend: Consumption of parasite infectious stages benefits hosts and predators depending on transmission mode.

Research Highlight: Hobart, B. K., Moss, W. E., McDevitt-Galles, T., Stewart Merrill, T. E., Johnson, P. T. J. (2021). It's a worm-eat-worm world: Consumption of parasite free-living stages protects hosts and benefits predators. Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13591 Many parasites and pathogens have infectious stages that are vulnerable to consumption by non-host organisms. This consumption can benefit both the predators that consume this unusual food resource and the hosts which are less likely to encounter infectious propagules. Yet the importance of these benefits may also depend on parasite transmission, which can influence consumer opportunities to feed upon free-living infectious stages. Hobart et al. (2021) report that freshwater snails with high densities of symbiotic oligochaetes are less likely to be parasitized by trematodes (Platyhelminthes) with an 'active' versus 'passive' (motile or stationary, respectively) mode of transmission, supporting a protective effect via oligochaete predation upon infectious propagules. However, these predators benefit from snails harbouring passively acquired infections, and likely achieve their higher abundance from easy access to prey in the form of a second infectious stage emerging from these hosts. Consumption of free-living infectious stages is thus beneficial to hosts and predators, but varies with parasite life history and ecology.