engleski

Pathogenicity of Triaenophorus nodulosus (Cestoda: Bothriocephalidea) in northern pike (Esox lucius) from the Mrežnica River, Croatia

Triaenophorus nodulosus Pallas, 1781 is a well-known representative of the taxonomically complex genus Triaenophorus Rudolphi, 1793 and is widely distributed throughout the Palearctic region. The life cycle of T. nodulosus involves copepods as first intermediate hosts, many freshwater fish species of different orders (including members of the Perciformes, and Salmoniformes) as second intermediate hosts, and pikes (Esox spp.) as definitive hosts. Northern pike (Esox lucius) is an important fish for aquaculture and the fishing industry in Europe. Despite its commercial significance, there is only limited information on the health status of this species in natural waters. Here we report the results of an initial evaluation of the pathogenicity of T. nodulosus in naturally infected northern pike collected from two locations (Site 1 and Site 2) along the Mrežnica River, Croatia. In the spring and autumn of 2021, a total of 62 specimens (30/Site 1 and 32/Site 2) were captured by electrofishing and examined as part of a fish health and ecotoxicological study, conducted under the Croatian Science Foundation project, “Metal-binding biomolecules and health disturbances of freshwater organisms exposed to industrial wastes” (IP-2019-04-2636). Immediately after capture, all collected fish were screened for the presence of adult cestodes on site. The precise location of cestodes was noted for each infected fish. The intensity of infection was evaluated, and the prevalence was calculated. The weight and length of collected fish were determined, and the condition factor (CF) was calculated. Differences in CF between infected and uninfected fish were assessed for significance using the Mann-Whitney rank sum test (SigmaPlot, version 11.0). The level of significance was set at p < 0.05. For histological examination, samples of the intestine of each infected fish were removed and fixed in 10% neutral-buffered formalin, embedded in paraffin, and sliced into 5-µm sections. Sections were stained with haematoxylin and eosin (H&E) and periodic acid-Schiff (PAS). The bothriocephalidean cestode T. nodulosus was found in 14 of 62 northern pike examined, and was always located in the anterior part of the intestine. Both the prevalence and infection intensity were higher at the Site 1 (prevalence 43.3%, average intensity 1.3) than at the Site 2 (3.1%, 1.0). At the time of capture, affected fish appeared to be ‘healthy’. Importantly, CF did not vary significantly between infected and uninfected fish (p > 0.05). Microscopically, T. nodulosus causes only localized histopathological changes at/around the site of attachment.

Triaenophorus nodulosus ; northern pike ; pathogenicity

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