Introduction:
Parasites are ubiquitous in aquaculture and can have devastating effect with serious socioeconomic, ecological, and welfare consequences. Of these, monogenean parasites are of particular problem as the conventional treatment for this infection, application of organophosphates is no longer permitted due to its neurotoxic effect, leaving no treatment option for this disease, which induces high morbidity and mortality due to skin and gill lesions associated with the attachment of the parasite to the fish.
In the past years there is a trend in aquaculture of moving away from chemical to natural treatments, i.e. alternative strategies for disease control and natural treatments effective in eradicating fish parasites and safer to public health and the environment.
Our previous research had demonstrated that an extract produced from the diatom Phaeodactylum tricornutum was effective against fish monogenean parasites. P. tricornutum is a potent photosynthetic marine diatom, which is commercially produced for high value nutraceuticals, such as LC-PUFA and fucoxanthin. Its extracts also have been shown to have antibacterial properties, mainly associated with fatty acids liberated from glycerolipids during cell treatment and extraction.
To further develop a sustainable and cost-effective treatment for aquaculture, the use of residue material from commercial P. tricornutum production was explored.
Methods:
Gyrodactylus turnbulli, a monogenean parasite affecting guppies (Poecilia reticulate) was used. As infection is high on the guppies’ tail fin which is transparent, direct observation of live fish and tail clips is feasible. Residue material of P. tricornutum, which was produced as a source of fucoxanthin, was obtained as dry powder from a commercial producer. Several extracts and preparations were tested, including Ethanolic residue extract (RE), free fatty acids (FFA) produced from this residue extract (RE) and fatty acid ethyl esters (FAEEs), produced directly from the P. tricornutum residue powder. These preparations were initially tested in situ, using infected tail clips, followed by in vivo testing.
To characterize the active ingredients, the extracts and preparations were analyzed by thin- layer chromatography (TLC), GC, followed by direct examination of pure compounds with potential anti-parasitic toxicity that were identified by these analyses.
Results:
All extracts and preparations were effective against the parasite, but FAEE fraction was the most effective in situ and in vivo (Fig 1). FAEE can be easily produced by a single step, direct transethylation of the algal residue powder. The active ingredients included FAEE of EPA 16:0, 16:1 and 14:0.
The research presents the potential utilization of P. tricornutum residue as a source for natural treatment against G. turnbulli and potentially other monogenean parasites.