Saprolegnia species isolated from rainbow trout: molecular identification, specific PCR and drug sensitivity

Saprolegniosis is a common infectious disease caused by members of the genus Saprolegnia. This genus contains several species, some of which can cause severe infection in vulnerable fish such as rainbow trout Oncorhynchus mykiss. This study reports the isolation and identification of 4 Saprolegnia species, namely S. parasitica, S. diclina, S. aenigmatica and S. asterophora, from a rainbow trout farm. The isolates were identified by amplification and sequencing of the internal transcribed spacer region. Among the isolates, S. parasitica and S. diclina were more common. S. diclina was found only in eggs and alevins, whereas S. parasitica was observed in different life stages of rainbow trout from fry to adult. Using the isolates, a species-specific PCR for the molecular identification of S. parasitica was optimised; the PCR protocol had a limit of detection of 0.025 ng template in a 20 µl reaction. Sensitivity of S. parasitica and S. diclina to boric acid and hydrogen peroxide was also determined; this assay also included another important Saprolegnia species (S. australis isolated from rainbow trout from a different rearing facility) to evaluate the comparative response among different species. S. parasitica and S. diclina showed similar sensitivity to boric acid, whereas S. diclina showed the highest sensitivity to hydrogen peroxide followed by S. parasitica and S. australis. This highlights the importance of accurate species identification for choosing suitable control measures and the judicious use of anti-oomycete drugs.