multifiliis (ich, ick)


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Ichthyophthirius multifiliis
(ich, ick, fish white-spot disease)

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Taxonomic lineage

cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Hymenostomatida - Ophryoglenina - Ichthyophthirius - Ichthyophthirius multifiliis

The scientific name of the parasite is translated as "the fish louse with the many children" reflects its reproductive capacity.

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Brief facts


Ichthyophthirius multifiliis (described by Fouquet in 1876), a ciliate protozooan, causes white-spot or ich disease in fresh water fish. It has extremely low host specificity and can infect virtually any fresh water fish species. Unlike apicomplexan parasites, such as Plasmodium or Toxoplasma species I. multifiliis is not intracelllular parasite.

Concentration of the parasite in wild populations is low and massive mortalities from the infection are rare. In closed systems (ponds, fish farms, ornamental fish tanks) ich infection can lead to rapid fish death (up to 100% mortality) and devastating economic losses.


The infective stage (theronts) invades the integumentary epithelium and transformes into parasitic stage (trophonts) that becomes established in the basal layer of the epithelium just above the basal membrane. Cellular damage in low or moderate infestations remains restricted to the infested site. Prolonged infestation also induces epithelial proliferation and hemorrhagic inflammation. Heavy infestation causes widespread lysis of the basal layer of the epithelium and disintegration of the epithelial layer.


From the typical symptoms of a fish individual, it is easy to diagnose fish with ichthyophthiriasis Due to the parasitic site within the skin of a host, the infected fish exhibits some or many white spots on the fish surface in difference shapes and sizes. The foci of infection become thickened and necrotic, then ruptured and ulcerative, falling of necrotic skin and scales off the surface often occurs if fish is seriously injured. Injury in the gill is usually more severe than the skin and directly impacts respiration in fish.


The most common treatments for white-spot disease in fresh water fish are: raised temperature, raised NaCl concentration, formalin, and malachite green (banned in namy European countries). First two treatments are especially effective in combination. Fish species respond differently to different medications. It is very important to research effects of high temperature and chemicals on each fish species because some treatments can cause acute disstress and might kill fish quicker than the disease would. For example, scaleless fish cannot tolerate malachite green. Cold-water fish might suffer from high temperature.


Recovered specimens can acquire various degrees of immunity against the parasite. Fish farms may choose to immunize cultured fish (theronts injected intraperitoneally into fish elicit protection). Vaccines against I. multifiliis are under development.

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Ichthyophthirius multifiliis (ich) life cycle

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Developmental stages

Life Cycle Stages

The life cycle of ich is temperature dependent. It lasts 3 to 6 days at 25°C (77°F), and 10 days at 15°C (59°F), at 7°C as many as 40 days may be required. Outbreaks are most common at 15 to 25°C (59–77°F).

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