Ichthyophthirius multifiliis

Ichthyophthirius multifiliis, often termed "Ich", is a parasitic ciliate described by the French parasitologist Fouquet in 1876. Only one species is found in the genus which also gave name to the family. The name literally translates as "the fish louse with many children". The parasite can infect most freshwater fish species and, in contrast to many other parasites, shows low host specificity. It penetrates gill epithelia, skin and fins of the fish host and resides as a feeding stage (the trophont) inside the epidermis. It is visible as a white spot on the surface of the fish but, due to its internal microhabitat, it is a true endoparasite and not an ectoparasite. It causes a disease commonly referred to as white spot disease due to the macroscopically visible trophonts (up to 1 mm in diameter) in the skin and fins. The trophont, continuously rotating, is surrounded by host cells (epidermal cells and leukocytes), producing a minute elevation of the skin. These light-reflecting nodules are recognized as white spots. If strict bio-security rules are violated, the parasite may be introduced into a fish rearing unit by transfer of fish or equipment from infected systems. When the organism gets into a large fish culture facility, it is difficult to control due to its fast-reproductive cycle. If not controlled, the infection may lead to 100% mortality in the tank. Strict management measures including mechanical and chemical methods are generally applied and can keep the infection at an acceptable level at farms. However, these measures are costly in terms of labour, chemicals and lost fish.