Ichthyophthirius multifiliis (White Spot or Ich)

Ichthyophthirius multifiliis is an opportunistic or ectocommensal parasite of skin and gills of freshwater fish. It is a common problem in farmed fish as well as aquarium species and in all water temperatures. It is a holotrichous ciliate, class Oligohymenophora, subclass Hymenostomata, order Hymenostomatida, suborder Ophryoglenina, family Ichthyophthiridae.

Among the various factors considered favourable to the parasite are poor water quality, high levels of decomposing organic matter, high stocking density, stress in handling, inadequate transport and low nutritional condition of the fish.

This disease is usually called “White Spot Disease”, since the parasite can be seen with the naked eye as one mm diameter white spots distributed over the body and fins when it is in the trophozoite stage, the feeding phase of the parasite. It should be noted that, despite appearances, the parasites actually sit within the epidermis, and not on the surface. This makes treatment more complicated, as a topical approach will not easily work.

Structurally, the entire surface of the organism is covered by motile cilia that are responsible for its motility in water, as well as its movement within the host tissue.

These parasites have a complicated life cycle that includes stages within the host as well as in the environment. The time taken for development within the fish is temperature-dependent; for example it requires 3–4 days at 22°C, up to 11 days at 15°C, and nearly 30 days at 10°C.

As soon as the theronts (infectious stage) begin to infect their host, the irritated fish can be seen shaking and rubbing their opercula and flanks against surfaces. The host cellular response is usually minimal but heavy infections of the gills can result in lamellar fusion leading to reduced respiratory function and electrolyte imbalance.  

In histological sections of skin and gills, parasites are observed within the epithelium, usually sitting on the basement membrane. Structures from the parasite like the horseshoe-shaped macronucleus, cytoplasmic food vacuoles and superficial cilia are usually visible. When infections are intense, there may be hyperplasia and inflammatory cell infiltration, as well as necrosis and vacuolar degeneration of epithelial cells that can often be severe enough to cause clinical disease. This is especially true if the host is partially immune i.e. has had prior exposure. Once mature, the parasites erupt through the epithelium and encyst within the substrate to continue their life cycle. In heavy infections, large numbers of parasites erupting through the epidermis at the same time can have a severe impact on the host.

Diagram 1. Life cycle and parasitic stages of Ichthyophthirius multifiliis. All stages of the organism are ciliated. The free-swimming theront penetrates through the mucus and invades the epithelium of the skin and gills. It then transforms into the trophont, which feeds and grows up to 800-1000 μm in diameter. The trophont actively moves within the epithelium. The parasite exits the fish as the mature tomont, which secretes a protective cyst and, maturing within a substrate, internally divides to form 500-1000 daughter cells (tomites), which then emerge, ready to reinfect.


Figure 1. Rainbow trout with easily-seen white spots on the skin, gills and fins. Irritation may lead to increased mucus production, seen as foam on the water surface. Clinically, fish may try to scrape off the parasites on any available substrate, leading to “flashing”.
Figure 2. Rainbow trout with variable sized parasites on the fins.   
Figure 3. Rainbow trout with parasites on the gills. A simple gill scrape and examination in a microscope will easily confirm the presumptive diagnosis.  

Figure 4. Histopathology in S. salar. Identification of the macronucleus (red arrow), feeding vacuoles (blue arrow) and cilia (orange arrow) in Ichthyophthirius multifiliis.

Figure 5. Histopathology of Ichthyophthirius multifiliis in S. salar showing trophont stage encapsulated by fused lamellae together with limited inflammatory response, mostly neutrophils.
Figure 6. Histopathology of Ich infection in trout, showing the characteristic horseshoe-shaped macronucleus.
Figure 7. Histopathology of Ich infections in trout fry. The parasites here are almost bigger than the gills! When these erupt through the tissue, the damage is proportionately large.
Figure 8. Eye. HIch infection in trout, showing the parasite sitting on the basement membrane of the cornea. Note the lack of host response.
Figure 9. Histopathology of skin from partially-immune rainbow trout, showing invading parasites within the epidermis and on the basement membrane (black arrows). Note that there are pockets of cellular debris (green arrow) within the epidermis; such damage tends not to be seen in immunologically naïve fish. Infection of such partially immune fish has a more obvious clinical impact.
Figure 12. S. salar. Numerous Ichthyophthirius multifiliis trophonts located on the basement membrane of the gill. Eruption of large numbers of parasites at the same time causes obvious clinical distress.


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By: Hugh Ferguson

Dr Ferguson earned his veterinary degree from the Royal (Dick) School of Veterinary Studies, Edinburgh, Scotland and held a Wellcome Research Fellowship at the Institute of Aquaculture, Stirling University where he obtained his PhD. He then worked for 4 years as a diagnostic pathologist at the Veterinary Research Laboratories, Belfast, Northern Ireland, prior to moving to Canada. He left Ontario Veterinary College after 19 years as a full professor of veterinary pathology, to return to Scotland to become head of diagnostic pathology in Stirling. During all this time he became board-certified in the American college of veterinary pathology (ACVP), and a Fellow of the Royal College of Pathologists (FRCPath, London). After Scotland he moved to become chair of veterinary pathology, and Senior Research Fellow in Windward Islands Research and Education Foundation (WINDREF), St George’s University (SGU), Grenada, West Indies. He has published more than 230 papers in refereed journals.