Parasitic helminths have evolved strategies to evade their host's immune systems. Particularly, the early time of interactions between helminths and their hosts might be decisive for their infection success. We used the cestode Schistocephalus solidus, and its highly specific second intermediate host, the three-spined stickleback (Gasterosteus aculeatus) to investigate parasite infection and host cellular immune responses starting 1 day postexposure (dpe). We recovered live parasites from stickleback body cavities already 24 hr after exposure. Infection rates increased up to 50% and did not change from 4 dpe onwards. Thus, not all parasites had reached the body cavity at the early time points and clearance of the parasite at later time points did not occur. Stickleback head kidney leucocytes (HKLs) did not show distinct signs of activation and lymphocyte proliferation, granulocyte-to-lymphocyte ratios and respiratory burst activity of infected sticklebacks did not deviate from controls significantly. The immune system was activated only late, as indicated by an increase in the total count of HKL relative to stickleback weight (HKL per mg fish), which was significantly elevated in infected fish 32 dpe. S. solidus seems to evade leucocyte activity early during infection facilitating its establishment in the hosts' body cavity.
The cestode Schistocephalus solidus is a common parasite in freshwater threespine stickleback populations, imposing strong fitness costs on their hosts. Given this, it is surprising how little is known about the timing and development of infections in natural stickleback populations. Previous work showed that young-of-year stickleback can get infected shortly after hatching. We extended this observation by comparing infection prevalence of young-of-year stickleback from 3 Alaskan populations (Walby, Cornelius and Wolf lakes) over 2 successive cohorts (2018/19 and 2019/20). We observed strong variation between sampling years (2018 vs 2019 vs 2020), stickleback age groups (young-of-year vs 1-year-old) and sampling populations.
Helminth parasites pose a significant threat to host survival and reproductive success, imposing strong selection pressure on hosts to evolve countermessures (e.g., immune responses and behavioral changes). To gain insights into the underlying mechanisms of host-parasite co-evolution, we examined differences in gene expression in immune tissues of two Alaskan stickleback (Gasterosteus aculeatus) populations with varying susceptibility to infection by the cestodeSchistocephalus solidus. Our analyses revealed distinct patterns of immune gene expression at the population-level in response to infection. Infected fish from the high infection population displayed signs of immune manipulation by the parasite, whereas this phenomenon was absent in fish from the low infection population. Notably, we found significant differences in immune gene expression between the populations, with uninfected Rocky Lake fish showing up-regulation of innate immune genes associated with inflammation compared to uninfected Walby Lake fish. These findings highlight the divergent evolutionary paths taken by different stickleback populations in their response to the same parasite.
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