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Why Does My Fish Have White Spot Disease?

Updated on March 9, 2014

The White Spot Disease in tropical fish, is an infection caused by a parasite. To understand how a fish gets sick and to know which is the best moment to carry out a corrective action, it's necessary to know:

  • Which is the cause of the disease
  • Why the white spots are formed
  • Which is the biological cycle of the parasite

The parasite completes the biological cycle in some few days or in five weeks, depending on the temperature of the water.

The pathogen agent is a ciliate protozoan: Ichthyophthirius multifiliis. Several species have been identified (not only multifiliis), all of them cause the White Spot Disease. The disease is also known as Ich, for the first three letters of the scientific name.

The parasite has pear-shaped form and is surrounded of hair-like organelles called cilia, used to swim. Has a free phase (tomont), in which the parasite reproduces, and an infecting phase (trophont), in which the parasite feeds. In this stage the cysts, or white spots take place.

White Spot Disease Causes
White Spot Disease Causes

In the infecting phase, the parasite adheres to the body of the fish through their cilia and penetrates through the outer layers of the skin of the fish. It feeds from the intercellular fluids.

The epidermis of the fish reacts to the presence of the parasite, surrounding it with epidermal cells, which forms the cysts or white spots.

These cysts can house several parasites and they can measure between 0,1 and 1 mm. of diameter. It seems to be a predilection for start the parasitoses in the caudal and dorsal fin areas.

The trophont reaches its mature stage (tomont) in two to three weeks, leaves the fish crossing the epidermis and descends to the bottom of the tank to begin the multiplication by cellular division.

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The tomont coat itself with a membrane, inside the capsule divides repeatedly to produce up to 2,000 new parasites.

The multiplication phase takes 10 to 20 hours. In the free-swimming stage (theront) the parasite must find a fish host within the first 36 hours before its reserves are exhausted and die.

To know the parasite cycle of life is important at the moment of apply a treatment. The medications are effective against the theront, not against the trophont. For an effective treatment, it must be ensured that all the forms of trophont are free.

The trophont stage can be accelerated by increasing the temperature of the water so that quickly exit the host.


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