Antiprotozoals effective in vitro against the scuticociliate fish pathogen Philasterides dicentrarchi

Abstract

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...

Pseudocohnilembus persalinus (Ciliophora: Scuticociitida) is an additional species causing scuticociliatosis in olive flounder Paralichthys olivaceus

“…Although several attempts have been made to treat scuticociliatosis by various chemotherapeutics (Iglesias et al 2002, Quintela et al 2003, there is at present no effective in vivo chemotherapeutic treatment, especially for internal infections. Therefore, development of an effective vaccine may be the best way to control scuticociliatosis.…”

Scuticociliate infection and pathology in cultured turbot Scophthalmus maximus from the north of Portugal

“…Formalin baths can eliminate the parasite while it remains outside the host, but are ineffective once the ciliate is located inside the fish body (Iglesias et al 2002).…”

In vitro efficacy of new antiprotozoals against Philasterides dicentrarchi (Ciliophora, Scuticociliatida)

“…No effective control measures are currently available. This parasite may be readily and rapidly eliminated while it remains outside the host body by formalin baths (Iglesias et al 2002); however, once it has penetrated the body and initiated endoparasitic infection, this treatment is no longer effective.…”