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(ich, ick, fish white-spot disease)
- Ichthyophthirius multifiliis (described by Fouquet in 1876), a ciliate protozooan, causes white-spot or ich disease in fresh water fish. It has extremly 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.
- The scientific name of the parasite is translated as "the fish louse with the many children" reflects its reproductive capacity.
- 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 and devastating economic losses.
- 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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At 24-26°C the entire life cycle may take four days, but at 7°C as many as 40 days may be required.
- Free-swimming stage
- Theront At 20°C theronts lose their ability to infect the host within 24 hours after release from the reproductive cyst; theron penetrate epithelium of the host very quickly (within 5 minutes); the parasite invasion is usually restricted to epithelium of skin and gills; the theront destroys cells in the process of establishing itself in the host; in the host theronts become parasitic trophonts; developing trophonts cause localized tissue necrosis; the host dies from secondary microbial infections and osmoregulatory failure.
- Parasitic stage
- Trophont Although, imediately after invasion the trophont is surrounded by destroyed cells, intact epithelium overlies the parasite within 45 minutes of entry; trophonts grow and feed within epithelia (skin, gills, and buccal cavity) accumilating nutrients required for following non-feeding stage and can grow up to 1 mm in diameter; each "white spot" represents a maturing trophont(s) within the skin; mature trophonts rapture epithelia and exit the host
- Non-feeding stages
- free-swiming tomont Exited trophonts for a short time become free-swimming tomonts which settle on the substrate and encyst.
- Encysted and
dividing tomont Inside the cyst the tomont divides repeatedly to produce daughter cells, tomites; the number of tomites varies between 50 and a few thousand; tomites exit the cyst and differentiate into next generation of therons; the time required for the tomont division and release of theronts is temperature-dependent: from 9 days at 5°C to 18 h at 25°C.
- Ewing MS, Kocan KM. Invasion and development strategies of Ichthyophthirius multifiliis, a parasitic ciliate of fish. Parasitol Today. 1992 Jun;8(6):204-8.
- Cross ML. Localized cellular responses to Ichthyophthirius multifiliis: protection or pathogenesis? Parasitol Today. 1994 Sep;10(9):364-8.
- Buchmann K, Sigh J, Nielsen CV, Dalgaard M. Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis. Vet Parasitol. 2001 Sep 12;100(1-2):105-16.
- Scholz T. Parasites in cultured and feral fish. Vet Parasitol. 1999 Aug 1;84(3-4):317-35.