Ichthyophthirius multifiliis

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Ichthyophthirius multifiliis
Cichlid showing the white spots characteristic of Ich
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Alveolata
Phylum:	Ciliophora
Class:	Oligohymenophorea
Order:	Hymenostomatida
Family:	Ichthyophthiriidae
Genus:	Ichthyophthirius
Species:	I. multifiliis
Binomial name
Ichthyophthirius multifiliis Fouquet, 1876

Ichthyophthirius multifiliis, often termed "Ich",^[1][2] is a parasitic ciliate described by the French parasitologist Fouquet^[who?] 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.^[3]

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.^[4][5]

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.^[6]

Research within the Horizon2020 project ParaFishControl pointed to a range of new approaches for control. For example, the fish immune system has an ability to combat invading parasites and a vaccine may be developed in the future.^[7][8] In addition, novel bacterial products (surfactants from Pseudomonas) can directly kill the external stages of the parasite without harming the host.^[9]

Ichthyophthirius multifiliis inflicts considerable damage to gills and skin in two ways. Firstly, the theronts penetrate the host epithelia and, when the number of parasites is high in relation to the fish size, the penetration may directly kill the fish by destroying the integrity of the fish surface. Secondly, if the invasion is successful, the invading theronts transform into the trophont stage in the fish epidermis where they develop and expand their volume manifold.^[10][11] When the trophonts burst out from their epidermal residence, severe ulceration follows, leading to high host mortality. The osmoregulation of the fish is challenged both by penetration and by trophont escape. Damage to the host's gills also reduces the respiratory efficiency of the fish, reducing its oxygen intake from the water.