When do parasites infect hosts? And does timing differ between separate host populations? Answers to these questions are fundamental to our understanding of host-parasite interactions and co-evolution. Yet often these aspects of host-parasite interactions are understudied. We recently published a paper in Parasitology devoted to answering these questions by using a host species that has faced new environmental challenges and adapted to new parasites: the threespine stickleback (Gasterosteus aculeatus). Threespine stickleback are northern temperate fish, perhaps best known for their adaptive radiation; following the last glacial maximum, oceanic stickleback independently and repeatedly invaded freshwater habitats, encountering and adapting to a wide variety of new environmental influences. Today stickleback display a wide variety of morphological, behavioral, and physiological characteristics. On top of facing various new abiotic factors, they also had to adapt to new parasites, one of which was the cestode Schistocephalus solidus. Schistocephalus solidus is trophically transmitted and has a three-host life cycle: the first host is a copepod, the second and specific host a threespine stickleback, and the final host is typically a piscivorous bird.

Like many other parasites that require multiple hosts to complete their life cycle, S. solidus has a huge impact on the survival of the intermediate host, and therefore selection pressure to avoid infection is strong. Globally and locally, stickleback populations vary considerably in their rates of infection with S. solidus. Currently, little is known about the timing and development of infection in natural stickleback populations, limiting our understanding of this system. This is something we wanted to change with this project.

From 2018 to 2020 we collected data on infection timing and the development of infection rates over the winter, a time with limited resources for the host, in multiple Alaskan lakes. With these data we confirmed that juvenile stickleback become infected with S. solidus only few months after hatching. We also observed strong variation between sampling years, stickleback age and sampling population. Surprisingly, in some populations, infections increased over the winter, even though lakes are covered with ice.

The results of this study provide an important step in understanding how hosts adapt to new parasites and how adaptations might vary for different populations of the same host species. Ultimately, this informs us about how animals react to changes in their environment. Although I am now studying a fascinating parasitic clade of nematormophs for my postdoctoral research, there is still work to be done to fully understand the relationship between threespine sticklebacks and S. solidus. The timing of infections is likely to vary globally with climate, and future studies should focus on collecting data on the timing of infection in additional stickleback populations throughout its distribution range. We also urge researchers to consider our findings in the design of future lab infection studies. To generate data that is more applicable to natural field situations, it is important to pay attention to the natural timing of infection.

Anika Wohlleben began her research journey at the University of Münster (Germany), where she obtained her B.Sc. and M.Sc. degrees. During that time, she found her passion for studying host-parasite interactions. After completing her PhD in Biology with a specialization in eco-evolutionary parasitology at Clark University (MA, USA) in 2022 she moved back to Germany for a postdoctoral position. Currently, she is conducting her postdoctoral research at the University of Jena, where she is investigating the mechanisms underlying host manipulation by nematomorph parasites. You can find her on Twitter: @AnikaWohlleben and check out her personal homepage: https://awohlleben.wixsite.com/anika-wohlleben

The paper “The timing and development of infections in a fish–cestode host–parasite system” by Anika M. Wohlleben, Natalie C. Steinel, Néva P. Meyer, John A. Baker and Susan A. Foster, published in Parasitology, is available Open Access.