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.

Strict management measures including mechanical and chemical methods are generally applied and can keep the infection at an acceptable level at farms.

[7][8] In addition, novel bacterial products (surfactants from Pseudomonas) can directly kill the external stages of the parasite without harming the host.

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.

When the trophont has reached a certain size (100-1000 μm), it breaks out of the host epidermis and swim freely as a tomont (also covered by cilia).

After minutes to hours, the tomont attaches to any surface in the fishpond or fish tank and produces a thick, gelatinous cyst wall.

[3][10] Heavy infections with subsequent lesions following trophont escape leave the skin irregular, fluffy and greyish.

Due to the low host specificity of the parasite, Ich infection is known from all freshwater fish systems examined.

Rainbow trout, catfish and eels are highly susceptible fish species and uncontrolled infections lead to almost 100% mortality.

Macroscopically visible trophonts (white spots) on skin or fins is often the basis for a tentative diagnosis of I. multifiliis infection.

Malachite green was previously the drug of choice but, due to its carcinogenicity, this organic dye is now banned in some countries.

[citation needed] Formalin when applied repeatedly (30–50 mg/L) kills infective theronts and tomonts but, due to its carcinogenicity, other chemotherapeutants should be used.

Theronts, the motile and fish-infecting stage of the Ich life cycle, exit from the tomocyst at the bottom of the tank.

The parasite can be propagated in the laboratory - most successfully in the hosts (in vivo), but also cell cultures can support part of the life cycle (in vitro)[15].

[8] Surfactants (with a high parasiticidal effect) from naturally occurring bacteria, such as Pseudomonas, are being explored and prepared for marketing.

[9] Herbal extracts have been demonstrated to stimulate immune responses of fish (and thereby partly inhibit development of the trophont), such as rainbow trout.

Management procedures, based on a basic understanding of the life cycle, can reduce the infection pressure considerably.

Due to the development of aquaculture systems – affecting the life cycle and pathogenicity of the parasites - continued research is needed in order to secure control of this parasitosis also in the future.