Two outbreaks of scuticociliatosis affecting farmed turbot Scophthalmus maximus in Galicia are described. Moribund fish showed cutaneous ulcers, darkened skin, swimming behaviour alterations, exophthalmos, and/or abdominal distension as a result of accumulation of ascitic fluid in the body cavity. Ciliates were detected in fresh mounts of practically all organs and tissues, including the blood and ascitic fluid. Histopathological studies revealed severe encephalitis and meningitis (associated with different degrees of softening or liquefaction of the brain), necrosis of the hepatic parenchyme, severe oedema of the intestinal wall, degeneration of muscle fibres, hyperplasia of the branchial epithelium, and/or vascular and perivascular inflammation. In some cases, parasites are surrounded by abundant monocytic and lymphocytic infiltrate. We report the morphological and biometric characteristics of this ciliate, which allow identification as Philasterides dicentrarchi. We discuss possible routes of entry into the host, and environmental factors possibly facilitating infection.

Philasterides dicentrarchi is a histophagous ciliate causing systemic scuticociliatosis in cultured turbot. This study demonstrates that turbot which survive this disease have serum antibodies that recognize ciliary antigens of this ciliate in ELISA and immobilize/agglutinate the ciliate in vitro. Mouse sera raised against ciliary antigens and integral membrane proteins are likewise capable of immobilizing/agglutinating the ciliates, indicating that P. dicentrarchi, like other ciliates, expresses surface immobilization antigens. Furthermore, the antigen agglutinating reaction induces the parasite to shed its surface antigens rapidly, replacing them with others with different specific serology. This antigen shedding and variation response is similar to that detected in other protozoan parasites. Immunization of turbot with ciliate lysate plus adjuvant or with formalin-fixed ciliates induced synthesis of agglutinating antibodies and conferred a degree of protection against challenge infection, suggesting that the response to surface antigens may play an important role in defence against this pathogen, SDS-PAGE and immunoblotting studies indicated the existence of a predominant polypeptide of about 38 kDa in the ciliary antigen and membrane protein fractions, and this may be the principal surface antigen of P. dicentrarchi.

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The histophagous ciliate Philasterides dicentrarchi causes fatal scuticociliatosis in farmed turbot Scophthalmus maximus and sea bass Dicentrarchus labrax. The present study screened 52 candidate antiprotozoals for activity against this pathogen in vitro. Of these compounds, 14 were effective (i.e. killed all ciliates within a 24 h assay period). In descending order of efficacy (minimum lethal concentration 100 to 0.8 ppm), these were niclosamide, oxyclozanide, bithionol sulfoxide, toltrazuril, N-(2'-hydroxy-5'-chloro-benzoyl) 2-chloro-4-nitroaniline, furaltadone, doxycycline hyclate, formalin, albendazole, carnidazole, pyrimethamine, quinacrine hydrochloride and quinine sulfate. Administration in filtered seawater rather than phosphate-buffered saline inactivated doxycycline hyclate and albendazole, and markedly reduced that of bithionol sulfoxide and toltrazuril, suggesting that these compounds may not be effective in bath administration. In view of these findings, we discuss the potential utility of chemotherapy as a strategy for the control of scuticociliatosis in farmed turbot and sea bass. KEY WORDS: Turbot · Scuticociliatosis · Philasterides dicentrarchi · Chemotherapy · In vitro assayResale or republication not permitted without written consent of the publisher Dis Aquat Org 49: 191-197, 2002 192 pensions of powder compounds (tablets were previously powdered in a mortar) were prepared in distilled water or dimethylsulfoxide (DMSO; Sigma Chemical). The resulting stocks, or commercially purchased solutions, were diluted in physiological phosphatebuffered saline (PBS; pH 7.2) or 0.2 µm-filtered seawater (salinity 28 ‰) to the final concentrations used in the screening (see following subsection). Tests performed in filtered seawater allow automatic exclusion of the possibility of partial or total inactivation of the test substance under marine conditions. A substance found to be effective in vitro by this procedure can thus be expected to be effective in bath administration to infected fish. Determination of lethal activity. Ciliates in the late exponential phase or early plateau phase of culture were concentrated by centrifugation at 650 × g for 5 min and then resuspended in PBS or filtered seawater. After counting in a haemocytometer, 10 µl of ciliate suspension containing 10 4 ciliates were added to each well of 96-well microtitre polystyrene plates containing 90µl well -1 of the candidate antiprotozoal at the required dose in PBS or filtered seawater. Final doses tested were: (1) 250, 125, 62, 31, 16, 8.4, and 2 ppm for formalin, and (2) 100, 50, 25, 12.5, 6.2, 3.1, 1.5 and 0.8 ppm for the remaining test substances. Each determination was performed in duplicate. Wells containing ciliates in assay solution (PBS or filtered seawater) without chemicals were also assayed as negative controls. To rule out possible effects of the solvent in DMSO-dissolved compounds, duplicate wells with PBS or filtered seawater containing the highest concentration of DMSO used (up to 2.5%) were also included. Pl...

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