Parasite Portrait #4 - Ichthyophthirius multifiliis

Ichthyophthirius multifiliis (Figure 1) is a freshwater parasite able to infect all freshwater fish tested so far. It causes problems both in flow-through systems and in recirculated systems. Infection causes a disease commonly referred to as white spot disease due to the macroscopically visible trophonts in the skin and fins (Figure 2). It can survive in a temperature range from 1 to 30 °C but, as a thermophilic species, it needs temperatures above 15 °C to propagate fast and efficiently.

Figure 1: Scanning electron microscope image of Ichthyophthirius multifiliis (Photo: Ole S. Møller, University of Copenhagen)

The main species affected in European aquaculture are rainbow trout, Atlantic salmon, perch, pikeperch, European eel, common carp, and European catfish. The infection has long-been considered one of the worst parasitic diseases in these species, especially where earth pond systems are used for culture. Disease risk is growing due to the increasing use of recirculation systems in which infective parasitic stages become continuously recirculated. This can cause a major increase in the level of fish exposure to the parasite.

I. multifiliis is a protozoan parasite, meaning that it is single-celled. It is covered with numerous hair-like cilia, firmly attached to its external cell membrane and belongs to the taxonomic group termed Ciliophora (organisms carrying cilia). The beating of these cilia allow the parasite to move and swim. The parasite has a characteristic horseshoe shaped nucleus and several micronuclei. The genus comprises only one species and it has its own family, Ichthyophthiriidae, which also includes its marine counterpart Cryptocaryon irritans (which is strictly marine and needs salinities near 30 ppt).

Figure 2: Trophonts of Ichthyophthirius multifiliis (diameter 300 μm) in the epidermis of a rainbow trout tail fin (light microscopy with subillumination). The horseshoe-shaped macronucleus is visible. (Photo: Kurt Buchman, University of Copenhagen)

Many other parasites are relatively specific when they choose their host species, but I. multifiliis is not specific in its host choice and can infect all freshwater fish species tested so far. The life cycle of the parasite is direct - it can be transmitted from fish to fish. It includes a trophont stage residing in the fish surface (gill epithelia, skin and fin epidermis). This is the feeding stage, which continuously ingests cellular debris and live host cells in its epidermal location, enabling the parasite to grow rapidly over a short time, depending on temperature.

Due to the occurrence and impact of I. multifiliis in freshwater fish farm systems worldwide, considerable research efforts are being conducted in laboratories spanning Australasia, Asia, Europe and the Americas. New drugs and herbal extracts are being tested for their impact on various stages of the parasite.

In ParaFishControl, a series of control methods have been explored. The parasite can be propagated in the laboratory - most successfully in hosts (in vivo) but cell cultures can support part of the life cycle (in vitro). Experimental vaccines are being tested and have shown promises for future control. Surfactants (with a high parasiticidal effect) from naturally occurring bacteria such as Pseudomonas are being explored and prepared for market. Herbal extracts have been demonstrated to stimulate immune responses in host fish (and thereby partly inhibit development of the trophont) such as rainbow trout. Together, these approaches can be applied for integrated control of I. multifiliis infections in aquacultured fish. Due to the development of aquaculture systems, and their effect on the life cycle and pathogenicity of the parasite, continued research is needed in order to secure control of this parasitosis in the future.