Ichthyophthirius multifiliis

From Fish
Typical white spots characteristic of ich on gold sevrums. Courtesy of Dr. Ruth Francis-Floyd
White spot cause by Ichthyophthirius multifiliis on an aquarium fish
The gill of Japanese eel infected with Ichthyophthirius spp

Ichthyophthirius multifiliis, or 'Ich' as it is referred to colloquially, is a ciliated protozoan parasite that infects freshwater fish and causes 'white spot disease'.. This protozoan infects the subepithelial tissues of the skin, fin and gill.

Life cycle

Parasitic stage in the skin or gills of fish is the trophont. It is oval to round (0.5-1.0 mm in diameter) and contains a horseshoe-shaped macronucleus. The trophont escapes from the host in about 1 week and encysts on the bottom of water. Within its cyst, the tomont divides to produce the small tomite, which breaks through the cyst wall to become the 'theront'. The theront penetrates into a host[1].

Clinical signs

White spots appear on the entire body. Heavily infected fish exhibit mucous secretion and abnormal swimming. Rotating parasites are observed in the epithelial tissues of the skin, fin and gill under a stereomicroscope.

Mucous cells proliferate in the surface tissue and inflammatory infiltration occurs in heavily infected fish. The epithelial tissues of the skin and gills disintegrate associated with the detachment of the trophont, leading the fish to failure of the osmoregulation and respiration.

Since this parasite is not infectious to human, it is harmless in food hygiene.


Diagnosis is confirmed by detection of cilia and a horseshoe-shaped macronucleus of the parasite by microscopy.

This disease is well-studied because it is one of the most important parasitic diseases in cultured, ornamental and aquarium fishes.


Some of the best treatments historically have been formalin or malachite green, or a combination of the two. Copper, methylene blue, and baths of potassium permanganate, quinine hydrochloride, and sodium chloride have also been used but do not appear to offer an advantage over the more readily available formalin and malachite green products.

Salt can also be used to control this parasite in small volumes of water. Fish can be dipped in a 3% (30,000 mg/L) solution for thirty seconds to several minutes, or they can be treated in a prolonged bath at a lower concentration (0.05% = 500 mg/L). Salt at low concentrations (0.01 to 0.05% solution) is an excellent means of controlling this parasite in recirculating systems without harming the biofilter. An ultraviolet filter is recommended as an aid in preventing the spread of the parasite in a recirculating system.


  1. Lom, J and I. Dykova (1992) Protozoan Parasites of Fishes, Developments in Aquaculture and Fisheries Science, 26, Elsevier, pp:315.

Ogawa, K. (2004): Protozoan diseases. Infectious and parasitic diseases of fish and shellfish. (ed. by Wakabayashi, H. and K. Muroga), Koseisha koseikaku, pp.285-338. (In Japanese)