Behavioral effects and inflammatory markers in the brain and periphery after repeated social defeat stress burdened by Opisthorchis felineus infection in mice

•Repeated social defeat stress (RSDS) was combined with O. felineus infection.•The combination induced hepatic, spleen, and adrenal-gland hypertrophy.•Blood interleukin 6 levels and hippocampal cathepsin B and L activities increased.•Infection reduced the blood corticosterone level and liver cathepsin B activity.•RSDS causes depressive whereas O. felineus causes anxious behavior in mice. The combination of 4-week repeated social defeat stress (RSDS) and Opisthorchis felineus infection was modeled in C57BL/6 mice. Various parameters were compared between three experimental groups of male mice (SS: mice subjected to RSDS, OF: mice infected with O. felineus, and OF + SS: mice subjected to both adverse factors) and behavior-tested and intact (INT) controls. The combination caused liver hypertrophy and increased the blood level of proinflammatory cytokine interleukin 6 and proteolytic activity of cathepsin B in the hippocampus. Meanwhile, hypertrophy of the spleen and of adrenal glands was noticeable. Anxious behavior in the elevated plus-maze test was predominantly due to the infection, with synergistic effects of an interaction of the two adverse factors on multiple parameters in OF + SS mice. Depression-like behavior in the forced swimming test was caused only by RSDS and was equally pronounced in SS mice and OF + SS mice. Helminths attenuated the activities of cathepsin B in the liver and hypothalamus (which were high in SS mice) and increased cathepsin L activity in the liver. The highest blood level of corticosterone was seen in SS mice but was decreased to control levels by the trematode infection. OF mice had the lowest level of corticosterone, comparable to that in INT mice. Thus, the first data were obtained on the ability of O. felineus helminths—even at the immature stage—to modulate the effects of RSDS, thereby affecting functional connections of the host, namely “helminths → liver↔brain axis.”