Chilodonella

Photo by Dr. Thomas L. Wellborn, Jr.

by Karen Paul

Chilodonella is a leaf-shaped motile ciliated protozoan 30-70µ long and 30-50µ wide. It can be identified through a microscope at 100x magnification but, as it is slower moving, it is a little more difficult to spot than Trichodina and costia. The ventral side is flat with bands of cilia. The dorsal side is convex and has no cilia.

Life Cycle

Chilodonella survives over a wide temperature range but the optimum is 5-10°C, giving it an advantage over the fish's immune system in the early spring. Like many other parasites it can become dormant for long periods until conditions become more favourable. Reproduction is primarily by binary fission but occasionally they join together to reproduce by conjugation.

Symptoms

Chilodonella mainly affects the gills and the area between the head and dorsal fin. Infected fish may be lethargic, gasp at the surface, show increased gill movements, clamped fins, flashing and rubbing. The skin has a milky appearance due to an increased mucus secretion. In severe infestations the skin may be swollen and flake off.

These symptoms are not exclusive to Chilodonella and may be caused by other parasites or environmental conditions. It is quite common to find Chilodonella on fish already weakened by disease or other parasites such as Costia. Examination of skin and gill scrapes under a microscope is needed to make a positive diagnosis. Chilodonella leaves the host immediately after death so it is unlikely that you will find it in scrapes taken from dead a fish.

Treatment

Chilodonella can be resistant to treatment and fish weakened by severe infestation may not survive the treatment required to eradicate it.

Chemical treatments include Malachite & Formalin, salt baths or Acriflavine with salt. Salt should not be used with zeolite filters as it will release ammonia, nitrite etc back into the water. It will also kill plants.

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! Acriflavine 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.

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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