First morphological and molecular characterization of Eustrongylides excisus larvae (L4) (Nematoda: Dioctophymatidae), with description of induced oxidative-inflammatory changes and tissue injury in European perch (Perca fluviatilis) from the Volga River (Astrakhan, Russia).

Eustrongylides spp. are helminths with a multiple-host life cycle that can infect aquatic oligochaetes, various freshwater fish, fish-eating birds, and accidental mammalian hosts, including humans. This work aimed to morphologically and molecularly characterize the larval stages of Eustrongylides spp. isolated from European perch, and to outline the oxidative/nitrosative stress and histoarchitecture changes induced by active larval migration or tissue-encapsulation. Eighty Perca fluviatilis were collected from the Volga River (Astrakhan, Russia). Thirty-seven were tentatively grouped as infected based on the grossly visible Eustrongylides larvae in the oral cavity, gills, axial muscles, and internal viscera. The nematodes were examined microscopically and characterized at the molecular level by PCR amplification of nuclear small-subunit ribosomal RNA (nSSU-rRNA) gene fragments, followed by sequencing and phylogenetic analysis. Affected organs were evaluated for redox/ nitrosative status and histopathological tissue injury. Fifteen healthy fish were selected as controls. Morphological examination identified the isolated nematodes as Eustrongylides sp., which were subsequently identified as Eustrongylides excisus (E. excisus) by molecular analysis, contributing to an infection rate of 46.25% (37/80). E. excisus was identified based on 100% sequence identity with available GenBank references, although the 18S rRNA marker provides limited species-level identification. Our findings demonstrate that E. excisus infection triggers a significant shift in the host's oxidative and nitrosative burden. This oxidative-inflammatory response leads to marked pathological changes in affected organs, including blood vessel dilation, mononuclear cell infiltration, vacuolar degeneration, and necrosis. Ultimately, these larvae induce severe cellular damage in fish muscle and visceral organs, disrupting cellular integrity essential to normal physiological function.