This study reports for the first time the presence and molecular characterization of Cryptosporidium in farmed rainbow trout (Oncorhynchus mykiss Walbaum, 1792). A total of 360 fish, with no apparent clinical signs of disease, were collected and classified into groups according to their size. Cryptosporidium oocysts were detected by immunofluorescence microscopy in 33 specimens (9.2%), which were located in pyloric caeca samples (42.4%), intestinal scrapings (39.4%), or at both locations (18.2%). In the smallest (youngest) fish group, a higher percentage of positive samples were detected in the pyloric caeca relative to the intestinal location (58.8 vs. 17.6%; P = 0.01), including a cluster with more than 10 oocysts observed in the pyloric caeca of one specimen. PCR amplification and sequencing of fragments of SSU-rDNA and hsp70 genes identified a novel Cryptosporidium piscine genotype (genotype 9) in two specimens and Cryptosporidium parvum in seven fish, including the specimen in which the oocyst cluster was observed. Moreover, Cryptosporidium oocysts were detected in farm water samples (41.7 and 16.7% from influent and effluent, respectively). Although Giardia was not found in gastrointestinal samples, Giardia cysts were observed in 50.0 and 33.3% of the influent and effluent water samples, respectively. The results support the existence of natural infections by C. parvum in freshwater cultured fish, suggesting that the rainbow trout could shed infectious oocysts in aquatic environments and it may be a potential source of human infection when this edible fish is handled.

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Species of the genus Cryptosporidium (phylum Apicomplexa) infect the epithelium of the gastrointestinal tract of several vertebrate hosts, including humans and domestic and wild animals. In the past 20 years, several studies have focused on Cryptosporidium in fish. To date, a total of four piscine-host-specific species (Cryptosporidium molnari, Cryptosporidium huwi, Cryptosporidium bollandi and Cryptosporidium abrahamseni), nine piscine genotypes and more than 29 unnamed genotypes have been described in fish hosts. In addition, Cryptosporidium species and genotypes typical of other groups of vertebrates have also been identified. This review summarizes the history, biology, pathology and clinical manifestations, as well as the transmission, prevalence and molecular epidemiology of Cryptosporidium in wild, cultured and ornamental fish from both marine and freshwater environments. Finally, the potential role of piscine hosts as a reservoir of zoonotic Cryptosporidium species is also discussed.

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