Early stages of infection of three‐spined stickleback (Gasterosteus aculeatus) with the cestode Schistocephalus solidus

Abstract

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 timing and development of infections in a fish–cestode host–parasite system

“…Cyclopoid copepods lower their metabolism at low temperatures (Maier, 1990 ), and unfortunately the interaction between S. solidus and its first intermediate host is extremely understudied, so it is unclear how long the copepod host can stay infected and successfully transmit S. solidus . We are aware that early after infection, S. solidus larvae are still relatively small (~100 μ m), and detection is difficult but can be accurate if the parasites are still alive (Wohlleben et al ., 2018 ). For this reason, the infection rate was assessed right after euthanizing young-of-year stickleback.…”

“…Each stickleback was either screened 3 times by 2 different people or until S. solidus parasites were found. This method was found to be reliable for detecting parasites as early as 24 h after infection (Wohlleben et al ., 2018 ).…”

“…In August, young-of-year stickleback should be between 2 and 3 months old, depending on the time of hatching during the spawning season (Heins et al, 1999). On the day of collection, we euthanized the stickleback with an overdose of MS-222 (Tricaine-S, Syndel, Ferndale, USA), and determined the infection status based on a protocol previously established by Wohlleben et al (2018). In short, following dissection, we screened the body cavity and organs for S. solidus under a dissection microscope [Trinocular Zoom Stereo Microscope, EMZ-TR (0.7×-4.5×), Meiji, Tokyo, Japan].…”

Tapeworm infection affects sleep behavior in three-spined stickleback

“…We then opened the body cavity of the fish on the ventral side with sterile scissors from the urogenital pore to the gills and screened the interior for a life tapeworm. The parasite was easy to recognize at 32 dpe, but at 4 dpe, it was still very small (approximately 100 µm, Wohlleben et al, 2018). Therefore, we incubated the body and organs of the fish at room temperature in saline solution (PBS) to detach the parasite from the fish tissue.…”

Exploring Gene Expression Patterns and Evolutionary Responses in Host-Parasite inteactions: Insights from theSchistocephalus solidus– Threespine stickleback System

“…Both the stickleback innate and adaptive immunity are manipulated by S. solidus upon infection (Scharsack et al, 2004a(Scharsack et al, , 2007Wohlleben et al, 2018). Differences in gene expression related to ROS production and MHC have been observed between populations with high and low infection prevalence (Lohman et al, 2017b).…”