Massive losses of farmed trout and native fish occurred due to Ichthyophthirius multifilis in 2012. The losses have not only had substantial impacts on commercial aquaculture operations and limited the availability of trout and native fish for the public water stocking programs of the Victorian and New South Wales governments. The timing of this work is critical, to develop and implement with key farmers new health management methods and reduce mortalities before next summer.

Ichthyophthirius multifilis has a direct lifecycle with an adult in the cells of the fish’s surface and asexual reproduction in the environment. In aquaculture systems heavy infections develop rapidly. A previous project DPI Victoria-funded project identified I. multifilis as the highest priority health issue in the Victorian trout industry and developed permits for access to new treatments for trout and other freshwater finfish aquaculture industries.

Treatment for I. multifilis centres on husbandry and strategic chemical control, but treatment dose and timing have not been optimised. Treatments include formalin, chloramine-T, or hydrogen peroxide, which are applied to the water. These products target only the environmental life cycle stages. Strategic treatment aims to kill all environmental stages, with a second dose to kill newly emerged stages after the previous treatment. The relationship between parasite lifecycle, temperature, parasite strain and water quality is currently unknown.

Growers assume that the dose is the same as the concentration applied to the system, but concentrations of these compounds are altered by water quality. This reduces efficacy and can make approved label doses ineffective. Treatments are also stressful to fish; treatments can cause substantial gill damage. Strategies for managing dose to balance efficacy and optimum fish performance need to be developed.

Better management of I. multifilis will decrease mortality, increase growth and quality and decrease husbandry costs in affected industries.