SHORTCOMMUNICATION In vitro assessment of the chemotherapeutic action of a specific hydrogen peroxide, peracetic, acetic, and peroctanoic acid-based formulation against the free-living stages of Ichthyophthirius multifiliis (Ciliophora) Sara M. Picón-Camacho & Mar Marcos-Lopez & Alexandre Beljean & Sylvain Debeaume & Andrew P. Shinn Received: 24 November 2010 /Accepted: 27 July 2011 # Springer-Verlag 2011 Abstract Traditionally, malachite green administrated as in- bath treatment was the most effective and common strategy used in freshwater aquaculture systems to control infections of the ciliate protozoan parasite Ichthyophthirius multifiliis Fouquet, 1876. After the ban of malachite green in the USA and Europe to be used in fish for human consumption, there has been extensive research destined to find efficacious replacements. Recently, peracetic acid-based compounds have demonstrated a strong cytotoxic effect in vitro and in vivo against I. multifiliis. In the present study, we tested the efficacy of a hydrogen peroxide, peracetic, acetic and peroctanoic acid-based formulation (HPPAPA) to eliminate the free-living stages of I. multifiliis (tomonts, cysts and theronts). The results obtained showed that the administration of low doses (8, 12 or 15 mg/l) of a specific HPPAPA-based product during a short window of exposure (60 min) kills nearly all free-living stages of I. multifiliis (theronts, tomonts and cysts) within the window of treatment (∼100% mortality for all the stages; one-way ANOVA, P ≤ 0.001). Of note, even the lowest concentration of HPPAPA tested (8 mg/l) was able to disrupt normal cyst development and therefore theront release. The demonstrated in vitro efficacy of the peracetic acid-based product tested on the present study suggests its great potential to control I. multifiliis infections in commercial aquacultural systems. Introduction Ichthyophthirius multifiliis Fouquet, 1876, commonly known as “white spot” or “Ich”, is a ciliate protozoan that reputedly infects all freshwater fish species and results in significant economic losses to the food and ornamental fish industry (Ling et al. 1991; Valtonen and Koskivaara 1994; Rintamäki-Kinnunen and Valtonen 1997). There are four key developmental stages within the life cycle: the sub-epithelial, parasitic trophont (1) which, when mature, exits the host as a free-swimming tomont (2). Tomonts quickly settle on an appropriate substrate, encyst (3) and then undergo binary fission to produce tomites. Released tomites rapidly differentiate into theronts (4) which become infective (Lom and Dykova 1992; Matthews 2005). Growing concerns regarding environmental pollution and human health impacts have lead to a tightening of regulations concerning chemotherapy applications in aqua- culture. Classically, the most effective compound for use against an infection of I. multifiliis, which includes cytotoxic action against the trophonts, was malachite green (Leteux and Meyer 1972; Wahli et al. 1993; Tieman and Goodwin S. M. Picón-Camacho : M. Marcos-Lopez : A. P. Shinn Institute of Aquaculture, University of Stirling, FK9 4LA Stirling, UK A. Beljean Schering-Plough Intervet, 92 Rue Baudin, 92307 Levallois Perret Cedex, France S. Debeaume ECOLAB S.N.C., 8, Rue Rouget-de-Lisle, 92442 Issy-les-Moulineaux, France S. M. Picón-Camacho (*) Gulf Coast Research Laboratory, University of Southern Mississippi, Ocean Springs, MS 39564, USA e-mail: sarapicon@yahoo.es Parasitol Res DOI 10.1007/s00436-011-2575-1