Treatment of ichthyophthiriasis after malachite green. II. Earth ponds at salmonid farms.

We tested formalin, chloramine-T-formalin and Desirox-formalin, for use against white spot disease (ichthyophthiriasis) caused by Ichthyophthirius multifiliis at 3 salmonid farms (Salmo salar and S. trutta smolt reared in earth ponds). I. multifiliis disappeared from most individuals 4 to 5 wk after the first treatment (and after the first I. multifiliis were found) with all chemicals, indicating that combinations of these chemicals, and even formalin alone, can be used to lower the parasite burden in earth ponds to such a level that no mortality occurs. This was the case when the fish were treated frequently at the beginning of the infection. Treatment can be stopped once the fish have achieved immunity to ichthyophthiriasis. The developing immunity was also revealed by the distribution of ciliates in the course of the disease. At the beginning of the infection I. multifiliis individuals were randomly distributed among the fish, but after 2 to 3 wk, when all the fish were infected, ciliates had increased in numbers and were aggregated in such a way that some fish carried quite heavy burdens. However, over 60% of the fish were free of the parasites after 4 to 5 wk, and had few or no ciliates, meaning that the distribution was even more aggregated. Sea trout had fewer parasites than salmon, and they also recovered from infection earlier even though the treatments and ponds were similar, indicating variation in resistance to I. multifiliis between fish species. It was also evident that the chemicals and their concentrations must be planned carefully to suit the conditions at each farm.

Treatment of ichthyophthiriasis after malachite green. I. Concrete tanks at salmonid farms.

Since the use of malachite green was banned in many European countries, new alternative treatments have been tested to prevent white spot disease caused by Ichthyophthirius multifiliis. We tested formalin, potassium permanganate (KMnO4), chloramine-T, hydrogen peroxide (H2O2) and Per Aqua or Desirox alone or in combinations of 2 chemicals, one of which was always formalin, in 50 m2 concrete tanks at 2 farms producing salmon Salmo salar smolt in 2001 and 2002. Both Per Aqua and Desirox are combinations of peracetic acid, acetic acid and hydrogen peroxide. The alternative chemicals or their combinations can be used successfully to lower the parasite burden to such a level that no high mortality occurs during the first 4 wk after the start of an infection. This period of time allows the fish to develop immunity against these ciliates, and treatments can be reduced and stopped in due course. I. multifiliis decreased in number 3 to 4 wk after the beginning of the infection in all the treatments. Large differences in parasite burden and mortality occurred among the replicates in all except the Desirox-formalin tanks, which means that they are not as reliable as the malachite green-formalin used previously. It was also evident that the chemicals and their concentrations must be planned carefully to suit the conditions on each farm.