Helminths of freshwater fishes in the reservoir of the Hydroelectric Power Station of Itaipu, Paraná, Brazil

: This study presents results from several expeditions in 1985, 1991-1995 and 2003 to the Medium Paraná River in the section that begins below the Itaipu Dam and ends at the trinational border of Brazil, Argentina and Paraguay, in the lotic and lentic zones of the reservoir of the Hydroelectric Power Station of “Itaipu Binacional” (localities Foz do Iguaçu, Santa Helena and Guaira). Ninety-eight species of freshwater fishes belonging to 22 families were examined for helminths. A host-parasite list based on Acanthocephala, Cestoda, Digenea, Monogenea and Nematoda collected from the region in question is provided. New host records are presented for Digenea and Nematoda. The Monogenea and Acanthocephala are being studied and will be published in a later paper, but are referred in the host-parasite list, in order to demonstrate the parasitism in the fishes of the reservoir. The results are compared with those presented by other authors from the Upper Paraná River.

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The floodplain of the Upper Paraná River was considered the last free stretch of the Paraná River. However, it has undergone severe changes in its system of flood and drought, since hydroelectric plants were built upstream and now control the water level of the river. Considering these changes, all the fauna, including the parasites of fishes, may be affected. Oscillations in the hydrologic flow, such as occur in floodplains, may influence the occurrence and size of fish parasite infrapopulations (Dogiel 1970). All these observed impacts on the floodplain can directly and indirectly affect the parasitic fauna of fish. Endoparasites, which typically have a complex life cycle, can be affected by changes in environments where the intermediate hosts live. Some species of the organisms that can serve as intermediate hosts may be favored and others may even be eliminated from the environment. Ectoparasites, those which are in direct contact with the environment, suffer directly from the changes caused by these impacts.
Immature forms of nematodes of the family Anisakidae (Contracaecum sp., Hysterothylacium sp. and Anisakidae gen. sp.) were found in 24 fish species examined in the localities studied on the Medium Paraná River. In the floodplain of the Upper Paraná River, 17 hosts were reported to be parasitized by Contracaecum and/or Hystherothylacium larvae. Species of the Anisakidae deserve special attention; they parasitize fish as larvae, using them as intermediate or paratenic hosts and are known to be agents of parasitoses in humans. However, to date, no reports of such zoonotic diseases have been made in the region. This is probably because the parasites are large and mainly parasitize the mesentery, which is not used as food by people.
Some species of Digenea also exhibit a low degree of host specificity. The metacercariae of Austrodiplostomum compactum, parasitic in the eyes of fish, were recorded for the first time in Plagioscion squamosissimus from the reservoir of the Hydroelectric Power Station of Itaipu by Kohn et al. (1995). In the floodplain, this larva was reported in the same host species by Pavanelli et al. (1997), as well as in some other host species: Hoplias aff. malabaricus, Satanoperca pappaterra, Crenicichla britskii, Cichla kelberi (= Cichla monoculus), Cichlasoma paranaense (Machado et al. 2005). Yamada et al. (2008) also reported it as parasitising Hypostomus regani, Schizodon borellii, Serrasalmus marginatus and Auchenipterus osteomystax. This parasite is very common in the "corvina" P. squamosissimus. Machado et al. (2005) reported a prevalence of 95% and recorded 397 parasites in one fish. Due to this high prevalence, this trematode species was probably introduced together with its definitive host.
The prevalence was also relatively high in S. pappaterra (71.9%) and in C. kelberi (65%). This parasite is ecologically important since it lives in the eyes of fish, damaging their vision and making them susceptible to predators. Thus, the parasite can complete its life cycle.
Among the hosts examined, 47 helminth species were common in both sampling sites. However, fishes from the Upper Paraná River floodplain showed a greater species diversity of helminth parasites. This difference probably occurred because, despite all the above-mentioned changes that are occurring in the floodplain, all animals necessary for completing the life cycles of the respective helminths are present in this environment. In the Itaipu reservoir, the impact was greater following the impoundment and many organisms may have disappeared. Some of them may act as intermediate hosts of helminth parasites.
The differences in the taxonomic diversification of the parasite assemblages of different fish species were mainly related to the environment, trophic level and temperature (Luque and Poulin 2008). Therefore, the Upper Paraná River floodplain, characterized by the presence of a wide variety of habitats and species, favors the occurrence of a greater diversity of fish parasites.