Costia (Ichthyobodo)

Photo by Dr. Thomas L. Wellborn, Jr.
For a larger view click here

by Karen Paul

Ichthyobodo is a microscopic flagellate protozoan measuring 10 - 20µm long and 5 - 10µm wide with one pair of flagella. Under a microscope you will need 200x - 400x magnification to see Ichthyobodo clearly but it can be identified at 100x magnification by it's erratic movements. Ichthyobodo is fast moving and can be seen flitting across the field of vision in jerky movements as it propels it's kidney-shaped body with the flagella.

Ichthyobodo is occasionally found in small numbers on the skin and gills of healthy fish without apparently causing any harm and lives on cellular debris. The natural defences of the host appear to keep the population reduced to an acceptable level. Ichthyobodo is a serious threat when it becomes established in large numbers and reverts to a parasitic existence. This normally happens when the host's defences are overwhelmed by other factors such as overcrowding, low oxygen, poor water quality or other parasitic infestation. Because it attacks the gills Ichthyobodo can kill quickly. Being a cold-water parasite it is particularly dangerous in the spring when the immune system is weaker after the winter.

Life Cycle

Reproduction is by division and is temperature dependent. It is a cold-water parasite that reproduces rapidly at temperatures between 10°C - 25°C but does not apparently survive above 30°C. The life cycle is completed in 10 - 12 hours at 25°C. At temperatures below 8°C Ichthyobodo encysts, reverting to trophozoites when conditions become more favourable. The trophozoites must then find a host within a short time to survive.

Symptoms

Seriously infected fish may be lethargic, show increased gill movements, clamped fins, flashing and rubbing. The skin has a milky appearance due to an increase in mucus secretion and there may be white spider-web lesions. At an advanced stage fish sometimes isolate themselves, often gasping at the surface. These symptoms are not exclusive to Ichthyobodo though and may be caused by other parasites or environmental conditions. Examination of skin and gill scrapes under a microscope is needed to make a positive diagnosis. Ichthyobodo is commonly known as "blue slime disease" among catfish farmers.

Photo by Dr. Thomas L. Wellborn, Jr.
For a larger view click here

Treatment

Chemical treatments include Malachite & Formalin, 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 can be used with sturgeon.

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.

For fish that can tolerate it Potassium Permanganate can be used as a bath but we don't recommend adding to the pond because it will kill the filter bacteria.

Salt can be useful against Ichthyobodo but some strains have become salt resistant. Salt will kill plants and 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.

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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