White Spot (Ichthyophthirius multifiliis)

Photo by Thomas Kaczmarczyk
For a larger view click here

by Karen Paul

White spot disease, or Ich, is caused by the parasite Ichthyophthirius multifiliis, a species of ciliate protozoan. It is a widespread and common disease that affects most fish species. The white spots are cysts containing Ichthyophthirius cells called trophozoites, or trophonts (mature trophozoites), which feed on the tissues of the host fish. Through the microscope a trophont appears as a round ciliated cell containing a horseshoe-shaped nucleus. Ichthyophthirius is the largest protozoan found on fish. In marine fish the disease is caused by Cryptocaryon irritans.

Photos by Dr. Thomas L. Wellborn, Jr.
For larger views click on the images

Life Cycle

The life cycle of Ichthyophthirius is a rather complicated one.The mature parasite (trophont) leaves the fish and settles on the bottom of the pond where it secretes a gelatinous cyst (tomont). Inside the cyst the cell reproduces by division to produce around 1000 tomites. The cyst ruptures to release the free-swimming, ciliated tomites which must find a fish host within 24 - 48 hours to survive. The tomites, also known as vagrant bodies, penetrate the skin of their hosts where they mature into trophonts. The whole cycle takes 2 - 14 days depending on water temperature.

Symptoms

Small white cysts, measuring up to 1mm, on the skin, fins and gills. In severe cases the fish looks like it has been sprinkled with salt. The skin may have a milky appearance due to an increase in mucus secretion. The fish may flash or rub itself against the sides and bottom of the pond due to the irritation. Seriously infected fish may be lethargic, show increased gill movements, stop feeding and sit on the bottom with their fins clamped. White spot is difficult to see on carp. The best method of diagnosis is to examine a skin scrape under the microscope.

Photo of white spot on a koi © Terry Williams

Treatment

Only the free-swimming tomite is susceptible to treatment. Neither the trophonts or the tomont cysts can be killed. A single treatment will only kill tomites that have emerged from cysts and not yet burrowed into a host fish. Tomites that emerge after the chemical has broken down or been flushed from the system will not be affected. Repeated treatments are therefore needed to kill new tomites as they emerge from the tomonts. Treatment will usually take about 7 - 10 days depending on water temperature and should be continued for a few days after the last white spot has been seen.

Chemical treatments available for white spot include Formalin, Malachite Green, Chloramine T and Potassium Permanganate. Any treatment method must take into account the species of fish. Do not use Formalin, Potassium Permanganate or any treatments that state not to be used with Golden Orfe (Leciscus sp.) or Rudd (Scardinius reythrophathalmus) if you have sturgeon in your pond! Chloramine T or Malachite Green can be used in a pond containing sturgeon but do not use a Malachite and Formalin mixture.

Photo of white spot on a koi © Terry Williams

There are a number of proprietary treatments available for the treatment of white spot, most of which are based on the chemicals mentioned above. Always check the labels to make sure the treatment is suitable for your fish. If in doubt consult a vet. Do not overdose. Chemical treatments are toxic not only to the parasite but also to the fish, especially those already weakened by disease. Always add extra aeration when treating and reduce feed levels.

Salt can be useful against Ichthyophthirius but it will kill plants. Salt should not be used with zeolite filters as it will release ammonia, nitrite etc back into the water. A salt bath can be effective for individual fish.

Siphoning the bottom of the pond or tank will remove some of the tomonts before they can reproduce. Water changes can help by reducing the numbers of the free-swimming tomites but be careful not to overdo it; stressing the fish will weaken their immune systems an make them more susceptible to further parasite attacks and other diseases.

As with all parasites and disease, prevention is better than cure. Poor water quality, overcrowding and poor nutrition will cause stress and weaken the immune system. Healthy fish will be able to fight off most diseases and parasites.

Before using any chemical treatments it is advisable to test the water quality. Some symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good.

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