White spot disease caused by the ciliate Ichthyophthirius multifiliis Fouquet, 1876 is a serious problem in freshwater aquaculture worldwide. This parasitosis is of frequent occurrence in both conventional earth pond fish farms and in fish farms using new high technology re-circulation systems. Chemicals such as formaldehyde, sodium percarbonate and sodium chloride have been applied to control infections with this parasite. However, information on the influence of concentration, length of treatment period and temperature, on the survival of theronts and tomonts, has been lacking.

The tolerance of the free-living theront stage towards formaldehyde and sodium percarbonate in solution was investigated at two temperature levels (11-12 and 21-22 °C). The theronts were exposed to the two substances in concentrations of 8, 16, 32 and 64 mg/l. The survival of theronts was examined in a four-well setup. Each well contained 5-12 theronts in 150 µl aqueous solution. Tolerance towards formaldehyde and sodium percarbonate was tested simultaneously, together with two pure water negative controls. At each concentration and temperature (11-12 and 21-22 °C) the experiment was repeated 4-9 times. The numbers of live theronts were counted every 15 minutes using a stereo-microscope (8-100× magnification). Lysed and/or immobilised theronts with no movement of cilia were considered dead. For both substances a negative relationship was seen between the survival of theronts and length of treatment, temperature and concentration of chemical. Using formaldehyde or sodium percarbonate, all theronts were dead within 15 minutes at a concentration of 64 mg/l at 21-22 °C. At the same temperature, using 8 mg/l, this was achieved within 150 minutes using formaldehyde and 135 minutes using sodium percarbonate. At the low temperature (11-12 °C) using 64 mg/l, all theronts were dead within 45 minutes using formaldehyde and 30 minutes using sodium percarbonate. At 8 mg/l all theronts died within 300 minutes when treating with formaldehyde or sodium percarbonate. Dose-response experiments on tomonts showed that this parasite stage was much more tolerant towards both chemicals. Filtration experiments on tomont containing water were conducted. Mesh sizes of 500, 300, 160 and 80 µm were used. A mesh size of 80 µm was demonstrated to filter out 100 % of the tomonts. The abiotic factors temperature, chemical and concentration all had significant influences on parasite survival. Strategic treatment using the environmentally friendly chemical sodium percarbonate in combination with continuous micro-filtering of the pond water is suggested for control and management of this parasitosis.