Effects of Sodium Percarbonate and Garlic Extract onIchthyophthirius multifiliisTheronts and Tomocysts: In Vitro Experiments

White spot disease, which is caused by infections of Ichthyophthirius multifiliis, has a severe impact on productivity in freshwater aquaculture. These infections were previously treated effectively with malachite green, a compound that is now banned on fish farms due to its carcinogenicity. However, aquaculturists need alternative substances to control the disease. Therefore, comparative laboratory tests (in vitro exposure) were performed to elucidate the effects on the infective theront stage and the tomocyst stage of (1) sodium percarbonate, a compound that releases hydrogen peroxide when dissolved in water and (2) extract of garlic Allium sativum. The efficacy of these compounds was compared with that of malachite green. It was demonstrated that at 12°C sodium percarbonate (12.5 mg/L) and garlic extract (62.5 mg/L) killed theronts within 3 and 15 h, respectively. Thus, compared with malachite green (0.1 mg/L), higher concentrations of both sodium percarbonate and garlic extract are needed. The effects of the alternative compounds on the tomocyst stage were less pronounced. As expected, malachite green (0.15 mg/L) killed the tomocysts and prevented the development of tomites. In contrast, sodium percarbonate (12.5 mg/L) did not kill the tomocysts. Garlic extract had no effect in low amounts (30 mg/L), but at high concentrations (117 and 570 mg/L) it killed the tomocysts within 24 h. It is concluded that these alternative substances are less effective than malachite green but may represent a way to reduce infections at fish farms with minimal environmental effects. It is emphasized that tolerance tests with various infected and noninfected fish species should be conducted in order to determine safety levels. Further, as the legality of these substances varies across countries, local regulatory authorities should be consulted before using these substances.