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Abstract
Philasterides dicentrarchi is a scuticociliate that causes high mortalities in farmed fish. Although vaccination is an effective method to prevent scuticociliatosis caused by the homologous serotype, a universal vaccine has not been developed yet. Many compounds have been shown to be toxic to this ciliate species; moreover, most of them are toxic to aquatic life and cannot be used to prevent the disease. We have evaluated the toxicity to P. dicentrarchi of several compounds of natural origin to be used to reduce parasite levels in the seawater. Ciliates were exposed to several compound concentrations, and the mortality was determined at several incubation times. Tomatine, plumbagin and 2′,4′-dihydroxychalcone displayed the highest anticiliate activity, with a dose-dependent response. The effects of these compounds on the EPC cell line were also evaluated, finding that 2′,4′-dihydroxychalcone displayed the lowest toxicity to fish cells. At 7.54 μM, 2′,4′-dihydroxychalcone inhibited 50% parasite growth but only killed about 10% of EPC cells after 24 h incubation. Finally, we evaluated the toxicity of Pseudomonas H6 surfactant (PS) to P. dicentrarchi, finding that PS was toxic to the ciliate but showed lower toxicity to EPC cells. At a concentration of 7.8 μg/mL (LC50 for the ciliate after 3 h incubation), PS killed 14.9% of EPC cells. We conclude that 2′,4′-dihydroxychalcone, and PS could be used to reduce parasite levels in seawater, thus decreasing the risk of scuticociliatosis infection in cultured fish.
Highlights
The histiophagous scuticociliate Philasterides dicentrarchi (Ciliophora, Scuticociliatia) is the main aetiological agent of scuticociliatosis in cultured turbot [1,2,3] and is a serious threat to the aquaculture industry, causing high mortalities and significant economic losses [1,4,5]
P. dicentrarchi is an amphizoic ciliate that causes scuticociliatosis in many cultured fish species, especially flatfish, which can result in high mortalities and economic losses in fish farm systems
The use of methods to reduce the risk of scuticociliatosis by decreasing the number of ciliates in water or the development of treatments to reduce mortalities in infected fish may be very important in order to minimise economic losses
Summary
The histiophagous scuticociliate Philasterides dicentrarchi (Ciliophora, Scuticociliatia) is the main aetiological agent of scuticociliatosis in cultured turbot [1,2,3] and is a serious threat to the aquaculture industry, causing high mortalities and significant economic losses [1,4,5]. P. dicentrarchi can induce severe systemic infections in turbot because of its ability to penetrate and spread throughout fish organs [6]. Vaccination is the most effective method of preventing disease outbreaks [7,8,9]. Experiments with different serotypes of this scuticociliate did not reveal any cross protection [10,11]. Until a universal vaccine is obtained, other methods must be developed to reduce the fish mortality associated with scuticociliatosis
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