Multilocus sequence typing reveals clonality in Saprolegnia parasitica outbreaks

Abstract

The molecular epidemiology of fish pathogen Saprolegnia parasitica is still largely unknown. We developed a multilocus sequence typing scheme based on seven housekeeping genes to characterize 77 S. parasitica strains isolated from different fish host species at different times and from different geographic areas in Switzerland between 2015 and 2017. Ten different diploid sequence types (DSTs) were identified. The majority (52%) of outbreaks in Switzerland seemed to be caused by one genotype, namely DST3, which was recovered from farm-raised and wild-caught fish in all the geographic areas and river basins included in the study. DST3 was also recovered from the rivers Bienne (eastern France) and Doubs, where the episodes of massive mortality due to saprolegniosis started in 2009. Another genotype (DST7) showed, to a lesser extent, a distribution across different river basins, while eight DSTs were unique to a defined geographic area or river basin. The occurrence of sporadic DSTs indicates a certain degree of diversity within S. parasitica in the environment. The wide distribution of DST3 suggests that a clonal population may have spread in eastern France and Switzerland across geographic barriers, with strong implications for the management of both captive and wild fish populations.

Saprolegnia molecular phylogeny among farmed teleosts in Nova Scotia, Canada

“…A partial 18S region of the nrDNA was amplified using 18sF (5′-AGGAATTGACGGAAGGGCAC-3′) and 18sR (5′-TGATCCTTCCGCAGGTT-3′) primer pairs (Spencer et al, 2002). The Cox1 and Cox2 regions were amplified using primer pairs developed by Hudspeth, Nadler, and Hudspeth (2000) and Ravasi, De Respinis, and Wahli (2018). All PCRs were prepared and run according to the protocol described by Sarowar et al (2019).…”

Novel Filoviruses, Hantavirus, and Rhabdovirus in Freshwater Fish, Switzerland, 2017

“…Clinical signs included anorexia, lethargy, skin ulcerations, multifocal hemorrhages, and eroded tail fins. Culture and PCR indicated that skin ulcerations were caused by oomycete ( Saprolegnia parasitica ) infection ( 21 ). Histopathology revealed mild to moderate gill epithelial proliferation and epithelial cell hypertrophy.…”

Tracing the oomycete pathogen Saprolegnia parasitica in aquaculture and the environment

“…Existing studies on S. parasitica mostly focus on its negative impacts in aquaculture: it is responsible for significant economic losses in salmonid farms and hatcheries worldwide 3 , 5 , and aquaculture facilities can act as pathogen pools from which S. parasitica spreads to natural environments 6 , 7 . In comparison, the potential negative impacts of S. parasitica on wild populations of salmonids and other animals remain largely unexplored, although declines in wild salmon populations caused by saprolegniosis have been reported 8 , 9 .…”

“…Humic substances, which constitute the majority of dissolved organic matter in oligotrophic freshwater ecosystems, can inhibit mycelial growth of the pathogen 16 , while Ca 2+ ions regulate processes such as adhesion, encystment and germination 11 , 17 . Despite numerous laboratory studies, knowledge about the prevalence of S. parasitica in the natural environment is very limited 9 , 18 – 20 , making it impossible to define the pathogen's distribution range and analyse ecological parameters that could influence its occurrence. Determining the relationship between the ecological and physico-chemical status of natural waters and the occurrence of S. parasitica is central to any attempt to predict the possibility of disease outbreaks in a realistic natural setting.…”