Ichthyophthirius Lifecycle and Treatment

Ichthyophthirius multifiliis, the ciliate parasite behind white spot disease (ich), is one of the most common parasites of freshwater fish and one every fish farmer will encounter. The adult trophont burrows under the skin and epithelium, feeding on host tissue and forming the visible white nodules; it then leaves the fish, encysts on surfaces or vegetation as a tomont, and divides into hundreds of infective theronts that must find a host within roughly two days or die. This temperature-driven cycle, taking about a week at 24°C but a month below 10°C, defines treatment strategy: theronts and tomonts are vulnerable, the embedded trophont largely is not, so therapies must span multiple cycles. Treatment options include raising temperature to speed the cycle, formalin, malachite green where permitted, copper or salt at appropriate concentrations, and modern agents such as quinine derivatives. For producers the key operational points are early detection on a few fish, treating the whole system because theronts are waterborne, removing activated carbon during medication, and watching dissolved oxygen since dying parasites and chemicals stress gills. Quarantine of new stock is the cheapest control, as ich is routinely introduced with apparently healthy carriers.