The Broad Fish Tapeworms Dibothriocephalus Spp., the Sealworm Phocanema cattani, and Other Endohelminth Parasites in the Returning Nonnative Chinook Salmon, Oncorhynchus tshawytscha, in the Valdivia River Basin, Chile

In the present study, we examined nonnative returning Chinook salmon caught between 2018 and 2023 in the Valdivia River basin, Chile, for endohelminth parasites. Zoonotic endohelminths were identified by morphological and molecular methods, based on a multiplex PCR or PCR and sequencing targeting the cytochrome c oxidase gene of diphyllobothriids or Ascaridoidea mitochondrial DNA. A total of 5,350 larvae of helminths were detected in the salmon, with 3.6% corresponding to the cestodes Dibothriocephalus latus and Dibothriocephalus dendriticus, with freshwater life cycles, and 96% to "Scolex pleuronectis," 0.2% to Hepatoxylon trichiuri, 0.1% to Nybelinia sp., and 0.1% to the nematode Phocanema cattani, all with a marine life cycle. All salmon had plerocercoids of Dibothriocephalus spp., with the highest percentage in the peritoneum and intestine. Only D. latus was identified in the muscles of 40% of salmon, with up to 7 larvae per host and a mean density of 0.1 larvae per 200 g. The present results indicate that most (63.6%) of the plerocercoids of Dibothriocephalus spp. can survive during the migration of Chinook salmon to the ocean and their return to freshwater. The sealworm P. cattani was found for the first time in Chinook salmon with a prevalence of 20% in the muscles. "Scolex pleuronectis" was abundant in the intestine and pyloric caeca, representing 96% of the identified endohelminths. One-third of the salmon had H. trichiuri in the body cavity, and Nybelinia sp. in the intestine wall. Plerocercoids of Dibothriocephalus spp. were mostly encapsulated and a few free in different organs. Histologically, the free larvae of Dibothriocephalus spp. were associated with an infiltrate of mononuclear and polymorphonuclear cells suggesting damage to their tegument. Larvae of Nybelinia sp. in the muscular layer of the intestine were surrounded by a thin capsule to which they were attached by the tegument; mononuclear and polymorphonuclear cells were observed outside the capsule and in some areas in contact with the larva. Plerocercoids of D. latus and Nybelinia sp. were associated with abundant mast cells in the intestinal wall. The presence of the zoonotic Dibothriocephalus spp. and P. cattani in returning salmon would increase their potential risk of transmission to the human population in the Valdivia River basin area. Similarly, salmon carcasses could facilitate the potential transmission of Dibothriocephalus spp. and P. cattani to wild and domestic mammals and wild birds.