ИСТИНА

As long as cestodes may cause pathology in some farmed fish and in species important to the fishing industry it is crucial to understand the relationships which establish among the two organisms. The goal of our project was to reveal how cestodes from marine and freshwater fish response to the presence of various factors from the host blood serum. We conducted experiments with plerocercoids of Triaenophorus nodulosus infecting the livers of Perca perca, plerocercoids of Pyramicocephalus phocarum infecting the livers of Gadus morhua and plerocercoids of Ligula intestinalis residing in the Abramis brama body cavity. Parasites from experiment 1 (“Ringer”) were incubated with the physiological saline solution, which matched the host blood salt composition. Parasites from experiment 2 (“Blood serum”) were incubated with the mixture of the same saline solution and the host blood serum (up to 50%). After the incubations, experimental and control animals were fixed and prepared for the examination under the scanning and transmission electron microscope. We have found that the cestode tegument exhibits an active response to the experimental conditions. Some features of this response are unique for plerocercoids from the group “Blood serum”; other reactions are common both for the first and second experimental groups but differ in their intensity among them. Specifically, in the presence of the host blood serum: 1) the secretion of various membrane structures is intensified (fig. 1B – arrows); 2) glycocalyx is thicker and its globular particles are more prominent; 3) there are more extracellular vesicles in the space between microtriches; 4) plerocercoids of L. intestinalis produce specific membrane lamellas, which are not found in control and “Ringer” animals. Additional layer composed of vesicles, membrane structures and extracellular matrix is also formed on top of the microtrich border as a result of exposure to experimental conditions of both types. For the first time, we have evidenced that the plerocercoids of three species we studied respond to the factors from the host blood serum to a different degree. Plerocercoids of T. nodulosus exhibit extremely acute reaction which includes the complete loss of their tegument to the extent that circular muscles become the outer layer of the worm. The response of L. intestinalis is the strongest on the anterior body end, where most microtriches are lost (fig. 1A). However, some microtriches are left and others form microtrich circles (fig. 1A – arrowheads). By contrast to the two species mentioned above, we did not observe significant change in the tegument structure of P. phocarum in the presence of the host blood serum comparing to the incubation with pure saline solution. In both experimental groups so as in control animals we observed small vesicles (from 20 to 150 nm diameter) budding from the tegumental plasma membrane. We suppose that these vesicles are extracellular vesicles, which are probably important players in parasite-host cross-talk. Our result correlates well with the data on the secretion of extracellular vesicles in cestodes Taenia crassiceps, Mesocestoides corti and Echinococcus multilocularis (Ancarola et al., 2017).