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Ich (Ichtyophthirius multifillis), also known as white spot, is a very common protozoan parasite that most koi keepers will encounter at some time. Ich encysts under the epidermis of the fish and in this form causes small white spots all over the fish’s body, fins and gills. In the encyst form it feeds on blood and skin tissue by continuously moving in a circular manner. Once the parasite reaches maturity, it will fall from the host and attach itself to plants, rocks or against the side and bottom of the pond where it will develop into a cyst. This is the reproductive stage where the parasite multiplies by division (splitting) and many hundreds of “swarmers” will form. These swarmers or tomites will then break free of the cyst and actively look for a new host to start the cycle over again. The life cycle is temperature dependant and may take 20 days or more during colder months. In warmer water, the life cycle may be as short as 4 days. If the free swimming parasite cannot find a host within 24 hours, it will perish. This explains why overstocking of a pond is so dangerous.

It has been reported that some fish may be carriers of this parasite without showing the telltale white spots on the body. This may be true because in tropical fish tanks, a perfectly healthy population may be infected when a temperature drop occurs for whatever reason.

It is also said that once a fish survived a white spot attack, it may become immune to the parasite. In some fish that survived a white spot attack and became immune, the parasite may burrow into the fish and burrow out again to look for another host. Recent studies have indicated that a certain mechanism of immunity can develop in a fish that has survived a non lethal attack by the ich parasite, but rather than being killed on the host, the parasites are forced to exit fish prematurely in response to antibody binding. Scientists believe that the forced exit is triggered by some kind of signal that leads to one of the following responses by the ich parasite:

1) an avoidance response,
2) a change in developmental state, or
3) a short-circuiting of the normally metabolic pathway used by parasites to exit the host.

 In some cases the parasite may encyst inside the fish and remain inactive until some factor triggers the life cycle to continue. That fish will therefore remains a carrier. Ich may cause mass mortalities in fry and small tropical fish.
                                                        The parasite under a microscope.


Symptoms associated with Ich
  1. Initially there are no external signs of the parasite except for a change in the behaviour of the fish.
  2. Fish starts to display clamped fins.
  3. Becomes lethargic.
  4. Occasional flashing/scratching/flicking.
  5. In later stages a few white spots will appear on the fish and in time will be noticeable all over the body.
  6. As the spots increase in number, the fish will refuse food and the frequency of flashing will increase.
  7. Hangs in the water with fins clamped.
  8. Fish spends more time near the air stones or water returns.
  9. Increased mucus production
  10. Numerous lesions will appear
  11. Bacterial infections set in, followed by fungal infections.

Prevention of Ich
  1. Ich will take advantage of stressful solutions when the immune system of fish is compromised.
  2. Good water quality is essential.
  3. Avoid frequent temperature changes of the pond water as this will trigger an outbreak. Heating a pond in spring and autumn will help.
  4. Equalise water temperatures when releasing new fish. Disinfect new plants, live food and equipment.
  5. Maintaining a fish free system for seven days at a temperature above 21 degrees Celsius may eradicate the parasite as the newly hatched tomites in the absence if a host. The problem will be that Ich may be re-introduced to the pond when the fish are returned.

Treatment of Ich
  1. White spot are protected against the effects of medication when encysted between the layers of the skin as well as when encysted during the dividing stage on the pond bottom.
  2. Treatment should be aimed at the free swimming tomites when the parasites are vulnerable to chemical treatment.
  3. Malachite Green will be effective.
  4. Potassium Permanganate.
  5. Treatment should be repeated at intervals depending on water temperature.
  6. Treatment for bacterial and fungal infections will be necessary.
  7. Another successful treatment is to increase the water temperature to more than 28 degrees Celsius. This temperature should be maintained for more than a week to terminate the lifecycle of the parasite.

Last Updated on Thursday, 22 October 2009 12:35