Ichthyophthirius multifiliis

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Also known as: Ich — White Spot — Ick


Ichthyophthirius multifiliis is a ciliate protozoan and infects freshwater fish to cause Ich or White Spot disease. All freshwater fish can be infected although species vary in their susceptibility to the disease.

Infections have been reported from all regions where fish are cultured and in feral fish populations in all continents.

The parasite attacks the epidermis of the skin and fins, the gill filaments and the cornea and can kill fish by interfering with their gill function and osmoregulation.

Outbreaks occur when conditions are favourable for rapid multiplication of the parasite. Temperature is a very important factor, and as it rises to 25°C parasite activity increases and the life cycle is completed in a shorter time than at lower temperatures.

Stress can also bring about an outbreak in a fish population as it decreases the immune function of the host. This may include crowding, low dissolved oxygen, pollutants and moving.

Outbreaks usually occur in the spring when the water is warming and the fish are spawning.

Fish recovered from Ichthyophthirius multifiliis infections develop protective immunity. This means that an outbreak will occur until all remaining fish are immune. With time, the level of resistance decreases and an outbreak can occur again.

As Ichthyophthirius multifiliis is an obligate parasite, it is thought that its survival between outbreaks is through low-level infection in the population. There may also be some fish that carry encysted parasites despite having developed immunity.

Life Cycle

Ichthyophthirius multifiliis settles beneath the skin epidermis where it develops in characteristic thin-walled pustules, the 'white spots'. The growing trophozoite do not reproduce on the fish, but when they reach a certain size, they leave the pustules as tomonts and encyst on plant surfaces or on the bottom of the fish tank. Within a cyst, multiplication takes place and 100-1000 small ciliated cells which become tomites are formed. These break through the cyst wall and seek out new hosts by actively swimming and penetrating the host's epidermis where a pustule is formed around them, thus completing the life cycle.

The multiplicative phase is very dependent on temperature.

Clinical Signs

Lesions on the skin, fins, gills, and cornea appear as greyish pustules. The pustules can merge and impart a turbid appearance to the affected areas. Epidermal cells slough and are replaced by mucus-producing cells.

Fish may show behaviour modifications such as congregating near water inlets, 'flashing', rubbing their body. As the disease progresses, fish become less active and stay at the bottom of tanks.

Dyspnoea and heavy infections may be apparent through rapid gill movements, lethargy and anorexia.


The white spots are suggestive.

A sample of skin mucus can be taken and examined under light microscope. This will reveal the ciliates.

Gross pathology: infected fish have enlarged spleens and kidneys and pale, mottled livers. There is peritoneal fluid exudation and gall bladder enlargement.

Treatment and Control

Several different strategies exist to treat an infection.

Chemical treatment: chemicals such as formalin, potassium permanganate, copper sulfate, salt in the tank water.

Temperature treatment: speeding up the life cycle by increasing the temperature of the tank by a few °C along with effective chemical treatment can speed up clearance of the parasite.

The parasite can also be destroyed by increasing the temperature above 32°C for 5 consecutive days. The water should be agitated to increase oxygenation as the dissolved oxygen levels will be lower as the temperature increases.

Careful cleaning of the tank can help to treat an infestation with Ichthyophthirius multifiliis.

Prevention of the disease includes an effective quarantine protocol, using separate equipment for each tank, buying tank plants which have never been in contact with fish, detecting and treating an infection promptly.

Ichthyophthirius multifiliis Learning Resources
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Leatherland, J. (2006) Protozoan and Metazoan Infections, Volume 2 CABI

Smyth, J. (1994) Introduction to animal parasitology Cambridge University Press

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