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The demand for ornamental fish has led to a steep rise in aquaculture for the hobbyist trade, promoting the emergence, persistence and spread of various infectious diseases. Complete control of disease outbreaks with antibiotics and chemical-based medicines is rare, but plant compounds may herald potential alternatives effective against a range of pathogens. Melafix® and Pimafix® are formulated with the essential oils cajuput (Melaleuca cajuputi) and West Indian bay (Pimenta racemosa) and are marketed against bacterial and fungal infections, respectively. Previous experiments showed high efficacy of emulsified cajuput oil against gyrodactylids; the current study tested Melafix® and Pimafix® and their individual compounds against Gyrodactylus turnbulli infecting the guppies Poecilia reticulata. In particular, a combination treatment of Melafix® and Pimafix® was highly effective at reducing in vitro survival of parasites from 15 to 2 h and eradicating 95% of gyrodactylids in vivo. The unexpected high efficacy of this combination treatment is likely explained by the high content of terpenes and phenol propanoids in the cajuput and West Indian bay oils, as well as the anti-helminthic properties of the emulsifier Crovol PK 70. Hence, Melafix® and Pimafix® effectively reduce gyrodactylid burdens on fish, increasing the chances of efficient disease control in ornamental fish.

Spironucleus vortens is a protozoan fish parasite of veterinary and economic importance in the ornamental aquaculture industry. Despite this, key aspects of the life cycle of this organism, including its mode of transmission, have not been fully elucidated. We developed a non-invasive method for quantifying S. vortens in freshwater angelfish, which was then used to investigate parasite transmission and aggregation within host populations. As previously observed for S. meleagridis and S. salmonis, motile S. vortens trophozoites were detected in host faeces using light microscopy. Species-level identification of these flagellates was confirmed using 16S rDNA PCR. Faecal trophozoite counts were significantly correlated with trophozoite counts from the posterior intestine, the primary habitat of the parasite. This novel finding allowed effective prediction of intestinal parasite load from faecal counts. Overall, faecal count data revealed that 20% of hosts harbour 83% of parasites, conforming to the Pareto Principle (80/20 rule) of parasite aggregation with implications for parasite transmission. Trophozoites survived for ≥36 d outside the host within faeces and remained motile at low pH (comparable with that of angelfish stomach). No putative S. vortens cysts were observed in cultures or faecal samples. This calls into question the commonly accepted hypothesis that a protective cyst is required in the life cycle of S. vortens to facilitate transmission to a new host.

Since Norway experienced the devastating Gyrodactylus salaris (Monogenea) epidemics in Atlantic salmon Salmo salar, there has been heightened interest in how to treat gyrodactylosis in fish. Here we summarize chemical treatments previously used against gyrodactylids and discuss the main problems associated with these control measures including efficacy, host toxicity, human health concerns and application of treatments. Unfortunately, for these reasons and because of the different methodologies and different parasite and host species used in previous studies, it is difficult to recommend effective chemotherapeutic treatments. However, we suggest a method for manual removal of gyrodactylids from the host suitable for use in small-scale research facilities.