Protozoa: Ichthyophthirius multifiliis

Volume 3, Issue 4
April 2001
Updated November 2002

Ichthyophthirius multifiliis (Greek Ichthyo, fish; phthir, a louse; multi, many; fil, a thread). Scientific names sometimes provide a wealth of descriptive information about an organism.


Fig. 2. Whitespot caused by Ichthyophthirius multifiliis on young of the year yellow perch. Protozoan unstained. Scale (across top) = 1 millimeter.

This organism commonly known as “Ich” or “Whitespot,” can be found on the skin of many, probably most, freshwater fishes in the world’s temperate zones. Grossly the skin of an infected fish displays raised white nodules (50 um to 1 mm). These nodules contain individual Ich trophonts. Ich’s large size and horseshoe-shaped macronucleus are it’s best identifying features. These growing parasites feed on the fish’s epithelium for between 4 and 40 days depending upon water temperature. When they mature, they release from the fish, drop to the lake or river substrate and metamorphose into a cyst. Each cyst may contain 1,000 new ciliated theronts ready to infect another fish.

Ichthyophthirius multifiliis

Fig. 1. Ichthyophthirius multifiliis. Protozoan unstained. Scale (across top) = 1 millimeter.

When fish are severely infected with growing trophonts, they may have their epithelium eaten to the extent that the skin necrosis and sloughs off. These fish rapidly die. Because Ich can reproduce so quickly and kill fish so rapidly, it may be the most important fish parasite in the world. New hosts are continually being discovered and currently include (at least): salmonids, perch, bass, and ornamental fishes. Ich has been found on fish in North America, Africa, South America, Australia, Asia, and Europe. The first reported case of Ich occurred in 1876 and was reported by Fouquet.

The Ich lifecycle is greatly influenced by temperature. Epizootics occur with relative predictability. For example, in Maine, Ich outbreaks occur in waters immediately after ice melts. In other parts of the United States, it has been reported in early spring when water temperatures reach between 16°C and 19°C.

Treatment of Ich in the wild is not practical. Treatment of Ich in aquaculture facilities can be difficult and successful treatment requires early detection and treatment. Ich has been implicated as a vector in spreading other fish diseases such as Myxidium. Myxosporidean parasites cause disease in many fishes and many fish tissues.

Special points of interest:

Fish with Ich cannot transmit the disease to people.

Ich is a protozoa.

Ich infects many species of fish and is highly contagious.

For more information read: G. Hoffman 1998. Parasites of North American Freshwater Fishes.

Images were made possible by a grant from the Maine Outdoor Heritage Fund.