Abstract Metacercariae of Diplostomidae are widely distributed in America and may cause diplostomiasis, an ocular disease in fishes. The aim of this study is to report the occurrence of metacercariae of Austrodiplostomum compactum in Plagioscion squamosissimus (non-native fish species) from Nova Avanhandava Reservoir, Tietê River, Brazil and an explanation for the high infection rates with this parasite in the Paraná River Basin is proposed. Eyes of 70 hosts were examined, the metacercariae were preserved and identified. The prevalence (P), mean intensity of infection (MII) ± standard deviation, mean abundance (MA) ± standard deviation, were calculated and a bibliographic review was performed. There was no difference in parasitism between male and female hosts. The values of P = 80%, MII = 21.55 ± 3.25 and MA = 17.24 ± 2.91 were high, as in most studies in areas where P. squamosissimus were introduced, while these values were low in areas of natural occurrence. This may be explained by the genetic susceptibility of the host to the parasite. The entire population of P. squamosissimus from the Upper Paraná has been founded by a few specimens, resulting in very low genetic variability. Consequently, the population may be highly susceptible to A. compactum.

Resumo Epistilíase tem sido caracterizada como uma doença emergente de grande impacto nas pisciculturas, especialmente em sistemas de produção de Oreochromis niloticus. Embora seja importante epidemiologicamente, informações sobre a dispersão desses parasitas e seus vetores mecânicos são escassos. O presente estudo relata a coocorrência de Epistylis sp. como um epibionte de Dolops carvalhoi, um crustáceo parasita de espécimes cultivados e silvestres (de escapes acidentais) de O. niloticus em área de piscicultura de tanques-rede no reservatório de Ilha Solteira, Rio Grande, SP, Brasil. A coocorrência de Epistylis sp. e D. carvalhoi, e sua relação epibionte registrada neste estudo, supõem que Epistylis sp. possa utilizar os crustáceos para dispersão e como vetores mecânicos para a disseminação de doenças em hospedeiros silvestres e cultivados. Além disso, a troca de parasitas entre hospedeiros silvestres e cultivados é possível, considerando ambos os organismos (protozoários e argulídeos). Os resultados deste estudo, também demonstram a necessidade de monitorar as áreas adjacentes às pisciculturas como medida preventiva para a dispersão de patógenos. Este é o primeiro relato de epibiose entre Epistylis sp. e parasitas argulídeos de O. niloticus cultivados e silvestres, contribuindo para o conhecimento da especificidade parasitária do hospedeiro, distribuição geográfica, dispersão de agentes etiológicos e epidemiologia na aquicultura.

Abstract Epistyliasis has been characterized as an emergent disease which has a great impact on fish farms, especially on Oreochromis niloticus production systems. Although epidemiological important, information about the dispersion of these parasites and their mechanical vectors is scarce. The present study reported the cooccurrence of Epistylis sp. as an epibiont of Dolops carvalhoi, a parasitic crustacean of cultivated/wild specimens (from accidental release) of O. niloticus from a cage fish farm area in the Ilha Solteira Reservoir, Grande River, SP, Brazil. The co-occurrence of Epistylis sp. and D. carvalhoi, and their epibiont relationship registered in this study suppose that the Epistylis may use the crustaceans for dispersion and as mechanical vectors for the dissemination of diseases in wild and cultivated hosts. Moreover, exchange of parasites between wild and cultivated hosts is possible, considering both organisms (protozoan and argulid). Furthermore, the results of the present study demonstrate the need to monitor the areas adjacent to cage fish farms as a preventive measure for the dispersion of pathogens. This is the first report of epibiosis between Epistylis sp. and argulid parasites of cultivated and wild O. niloticus, contributing to knowledge about host-parasite specificity, geographical distribution, dispersion of etiological agents and epidemiology in aquaculture.