Identification and genetic characterization of Saprolegnia parasitica, isolated from farmed and wild fish in Finland

Abstract

Oomycete infections in farmed fish are one of the most significant disease issues in salmonid aquaculture worldwide. In the present study, Saprolegnia spp. in different farmed fish species in Finland were identified, and the molecular epidemiology of especially Saprolegnia parasitica was examined. We analysed tissue samples from suspected oomycete‐infected salmonids of different life stages from a number of fish farms, as well as three wild salmonids. From collected oomycete isolates, the ITS1, 5.8S and ITS2 genomic regions were amplified, analysed phylogenetically and compared with corresponding sequences deposited in GenBank. Of the sequenced isolates, 91% were identified as S. parasitica. Isolates of yolk sac fry were identified as different Saprolegnia spp. Among the isolates from rainbow trout eggs Saprolegnia diclina dominated. In order to determine potential dominating clones among the S. parasitica, isolates were analysed using Multi Locus Sequence Typing (MLST). The results showed that one main clone contained the majority of the isolates. The MLST analysis showed four main sequence types (ST1–ST4) and 13 unique STs. This suggests that the Saprolegnia infections in farmed fish in Finland are not caused by different strains originating in the farm environment. Instead, one main clone of S. parasitica is present in Finnish fish farms.

Description of Saprolegnia velencensis sp. n. (Oomycota), a novel water mold species from Lake Velence, Hungary

“…australis and S . ferax are often found in water samples [ 7 , 20 , 34 – 36 ], thus the new species could be still considered potential pathogen. Nevertheless, further in vivo experiments are required to understand if S .…”

“…n. was isolated from water sample, thus we cannot draw any conclusions regarding its pathogenicity yet. The most common fish pathogen species, S. parasitica, or the known saprotrophs damaging fish eggs, such as S. australis and S. ferax are often found in water samples [7,20,[34][35][36], thus the new species could be still considered potential pathogen. Nevertheless, further in vivo experiments are required to understand if S. velencensis sp.…”

Diversity of fungus‐like stramenopilous organisms (Oomycota) in Lithuanian freshwater aquaculture: Morphological and molecular analysis, risk to fish health

“…(2011, 2015) in Norway isolated S. diclina and S. parasitica predominantly from Salmo salar eggs, while S. ferax and S. hypogyna were usually isolated from adult Salmo salar fish. Recent studies in Finland have showed that, as in Lithuania, about 90% of affected salmonid fish in Finnish fish farms were infected with S. parasitica , especially at the post‐hatching stage, whereas S. diclina was usually favoured eggs (Engblom et al., 2023). Other fish pathogens from aquaculture in Finland included S. australis , S. diclina , S. torulosa , Saprolegnia sp.…”

“…ferax and S. hypogyna were usually isolated from adult Salmo salar fish. Recent studies in Finland have showed that, as in Lithuania, about 90% of affected salmonid fish in Finnish fish farms were infected with S. parasitica, especially at the post-hatching stage, whereas S. diclina was usually favoured eggs (Engblom et al, 2023).…”

“…and Leptolegnia sp. (Engblom et al, 2018(Engblom et al, , 2023. In the USA, Songe et al (2016) commonly isolated S. diclina also from the eggs of Salmo salar, while in Chile, as in Lithuania, S. ferax and S. parasitica were more common in aquaculture, especially in adult Salmo salar fish, while the S. diclina was strongly associated with embryonic stages (Sandoval-Sierra, Latif-Eugenin, et al, 2014).…”