Specific Phylotypes of <i>Saprolegnia parasitica</i> Associated with Atlantic Salmon Freshwater Aquaculture
2024
Kypher Varin Shreves | Marcia Saraiva | Tahmina Ruba | Claire Miller | E. Marian Scott | Debbie McLaggan | Pieter van West
Saprolegniosis is a major destructive disease in freshwater aquaculture. The destructive economic impact of saprolegniosis on freshwater aquaculture necessitates further study on the range of <i>Saprolegnia</i> species within Atlantic salmon fish farms. This study undertook a thorough analysis of a total of 412 oomycete and fungal isolates that were successfully cultured and sequenced from 14 aquaculture sites in Scotland across a two-year sampling period. An ITS phylogenetic analysis of all isolates was performed according to whether they were isolated from fish or water samples and during enzootic or epizootic periods. Several genera of oomycetes were isolated from sampling sites, including <i>Achlya</i>, <i>Leptolegnia</i>, <i>Phytophthora</i>, and <i>Pythium</i>, but by far the most prevalent was <i>Saprolegnia</i>, accounting for 66% of all oomycetes isolated. An analysis of the ITS region of <i>Saprolegnia parasitica</i> showed five distinct phylotypes (S2–S6); S1 was not isolated from any site. Phylotype S2 was the most common and most widely distributed phylotype, being found at 12 of the 14 sampling sites. S2 was overwhelmingly sampled from fish (93.5%) and made up 91.1% of all <i>S. parasitica</i> phylotypes sampled during epizootics, as well as 67.2% of all <i>Saprolegnia</i>. This study indicates that a single phylotype may be responsible for <i>Saprolegnia</i> outbreaks in Atlantic salmon fish farms, and that water sampling and spore counts alone may be insufficient to predict <i>Saprolegnia</i> outbreaks in freshwater aquaculture.
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