WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2022 35 for their potential against Ich and other fish parasites. In these studies, a molecule extracted from a bacterium of the Pseudomonas genus (Liu et al. 2015) turned out to be very efficient in rapidly killing the Ich parasite. The molecule is a cyclic lipopeptide (CLP) that, due to its strong surfactant properties, disturbs and destroys certain naked cellular membranes. In a follow-up study (Al-Jubury et al. 2018), the molecule, now named “Biokos”, killed all three free-living life stages of Ich, most within 5-20 min at low milligrams/L concentrations. A challenge study with Ich-infected rainbow trout fry demonstrated full protection against Ich infection by Biokos at 10 mg/L (Li et al. 2022). Matthiesen et al. (2021) tested the propensity of Biokos and a range of currently used Ich biocides (including formalin, hydrogen peroxide and peracetic acid) to raise molecular inflammation markers. Of all the molecules and extracts tested by ParaFishControl, Biokos had, by far, the lowest inducing activity and peracetic acid the highest. At effective concentrations Biokos is non-toxic to fish (juvenile and embryonic zebrafish), crustacean zooplankton Daphnia magna, freshwater cyanobacteria Phormidium autumnale and green algae Raphidocelis subcapitata (Korbut et al. 2022). These results strongly suggest that Biokos could become a convenient and attractive alternative to current remedies used against Ich. In 2019, our company, Sundew ApS, acquired a license to the underlying intellectual property and set out to prove the feasibility of using Biokos to combat Ich disease in commercial fish culture. Here we report on three studies undertaken: to further prove in vivo efficacy of Biokos in a goldfish challenge experiment; to exemplify its lack of toxicity to both fish and beneficial biofilter bacteria; and to assess in detail the propensity of “natural water” to degrade Biokos. Goldfish Challenge Study To confirm the published data on efficacy of Biokos to counter white-spot disease on rainbow trout (Li et al. 2022) we set out to do The parasite Ichthyophthirius multifiliis, often referred to simply as “Ich”, causes “white spot disease” in many finfish species in the ornamental fish industry as well as in foodfish aquaculture. Ich is a ciliate protozoan, for which fish is an obligate host. The Ich parasite has four life stages (Fig. 1): 1) the trophont stage embeds in and feeds on fish skin or gill epidermis, 2) the liberated and free-swimming tomont, which eventually encysts, to dwell at the bottom, 3) the proliferating tomocyst, containing up to 1000 daughter cells and 4) daughter cells that are eventually liberated to seek out new hosts, named theronts. The length of the parasite’s life cycle is dependent on water temperature, ranging from days to weeks. Depending on the fish species, a strong, large and mature fish may survive a mild infection of Ich, but prolonged disease will always be detrimental and fry and small fish will most often succumb. Being a long-known and widespread disease, a number of remedies have historically been used to counter Ich. The most efficient is malachite green, which is the only one affecting the embedded trophont. Unfortunately, malachite green may be a carcinogen and is persistent in nature and so its use is mostly banned. The stages of the parasite not sitting on the fish host may be targeted by several agents: sodium chloride, copper sulphate, oxidizing agents such as hydrogen peroxide, sodium percarbonate, peracetic acid, potassium permanganate and formalin. Sodium chloride and copper sulphate are impractical to use because the fish needs to be moved to a separate treatment tank. Oxidizing agents work relatively well but need to be applied repeatedly at rather short intervals. Formalin works well but the therapeutic window is narrow, meaning that the concentration needed to kill the Ich parasites is close to concentrations that will harm beneficial water organisms or fish. This potential toxicity means that formalin is banned in many places and only used under an exemption granted by regulatory agencies and, if used, much care needs to be taken. Some years ago, researchers in the European Union research program ParaFishControl tested many extracts and molecules Remedy for Combating Ich in Finfish Aquaculture M.J. Hansen, F. Mehrdana, E.H. Hansen, G. Salerno and J. Hansen ( C O N T I N U E D O N P A G E 3 6 ) FIGURE 1. The life cycle of Ichthyophthirius multifilis. A molecule extracted from a bacterium of the Pseudomonas genus turned out to be very efficient in rapidly killing the Ich parasite. The molecule is a cyclic lipopeptide (CLP) that, due to its strong surfactant properties, disturbs and destroys certain naked cellular membranes.
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