Abstract

Chilodonella sp. is a ciliated protozoal ectoparasite which can cause high morbidity and mortality among ornamental freshwater fish at the fish farming and wholesale levels. These parasites reproduce rapidly via binary fission and can be found in large numbers on both skin and gill surfaces. Fish mortality is highest when these ciliates feed on vulnerable gill epithelium, resulting in a severely compromised respiratory effort. In some cases mortality rates may approach 100%. Fish which are stressed by poor water quality and overcrowding are most susceptible to infection by Chilodonella.

A 6 month study of ornamental fish in a large wholesale facility found 23 affected species belonging to 20 genera representing 8 families (Belontiidae, Characinidae, Cichlidae, Cobitidae, Cyprinidae, Gymnotidae, Loricariidae and Pimelodidae). In most of the epidemics investigated, more than one species was infected; a situation resulting from numerous separate tanks sharing water in a recirculating system.

Studies showed that 30 ppm 37% formaldehyde combined with 0.12 ppm malachite green was sufficient to kill the parasites when used in a well oxygenated bath for 3.5 hours. This treatment has been used successfully in species of every family mentioned above except Pimelodidae. These scaleless fishes were sensitive to this treatment and benefitted most from preventative measures which included clean water and the removal of the fish from the recirculating system. A second successful and much safer treatment was later employed utilizing sodium chloride as an indefinite bath. Fish living in water with 0.2% (specific gravity of approximately 1.0015) sodium chloride were found to be free of Chilodonella and showed no detrimental effects from this continuous sodium chloride treatment.

Introduction

It is generally accepted that any freshwater fish is susceptible to infestation by the ciliate protozoan, Chilodonella sp. (Gratzek, 1988; Post, 1987; Reichenbach-Klinke, 1973). While the genus contains many species, the primary pathogen of North American freshwater fish is Chilodonella cyprini (Davis, 1953). These ciliated protozoans have a characteristic heart-shaped morphology, generally move in slow circular patterns when observed free from host tissue, and have numerous cilia which are organized in parallel rows. The dorsal portion of the parasite is convex and the ventral surface is flat and contains the ciliary rows. Total body length ranges from 30-70 micrometers and total width from 21-40 micrometers (Post, 1987). Reproduction is accomplished via binary fission and the organisms can survive a wide range of environmental temperatures. Chilodonella cyprini appears to live best between 5 and 10 degrees centigrade (Hoffman, 1478).

Over a period of 6 months numerous species of freshwater tropical fish were found to be infested with Chilodonella in a large wholesale tropical fish facility which contains several 20,000 to 40,000 liter recirculating systems. 23 species of fish belonging to 20 genera representing 8 families were found to be clinically affected by Chilodonella (Table 1). Clinical signs of disease included clamped fins, respiratory distress, anorexia and acute death. Recirculating aquarium water combined with crowded tank conditions were major factors related to the presence and propagation of this disease.

Chilodonella is known to be sensitive to many common parasiticides (Gratzek, 1988). Various compounds including acriflavine, methylene blue, malachite green, formaldehyde, sodium chloride, copper sulfate, potassium permanganate and acetic acid have been reported to be successful in treating Chilodonella infections (Post, 1987; Reichenbach-Klinke, 1973 and Untergasser, 1989). In the wholesale facility individual tanks were treated successfully on a routine basis using a combination of formaldehyde and malachite green. Due to the nature of the recirculating systems and the tremendous variety of fish involved, this combination possessed certain drawbacks when used as a complete system treatment. Sodium chloride (uniodized) was found to be very effective in controlling the disease as long as the system's salt concentrations were carefully monitored. Chilodonella is no longer a significant problem at the wholesale facility as a result of sodium chloride therapy.

Materials and Methods

In the formaldehyde/malachite green studies, individual 75 liter tanks containing infected fish were taken off of the system and with adequate aeration were treated with 30 ppm 37% formaldehyde combined with 0.12 ppm malachite green. Water temperature was 26 degrees centigrade, pH was maintained between 6.5 and 7.0 and total hardness was approximately 130 ppm. Fresh gill tissue whole mounts from moribund fish were examined with a compound microscope every 30 minutes to check for Chilodonella motility.

The salt treatment protocol involved the use of uniodized solar salt at a concentration of 0.2%. This concentration corresponded to a hydrometer specific gravity reading of approximately 1.0015. Despite water changes, shipment of water with fish, and evaporation, sodium chloride concentrations were carefully controlled by adding solar salt as necessary to maintain desired hydrometer specific gravity values.

Results

The formaldehyde/malachite green combined therapy killed Chilodonella after 3.5 hours of treatment and was not harmful to the fish when used at the concentration listed above. As a safety measure, fish were usually treated for 4-5 hours and then retreated in 48 or 72 hours if necessary. Tanks of fish which were returned to the recirculating system after the initial treatment were found to commonly become re-infected within a 72 hour period if they did not receive a second treatment. Fish which were showing signs of respiratory difficulty frequently died despite treatment. Those fish which were sub-clinical but in tanks with moribund fish generally responded well to the parasiticide therapy.

When salt was used at a specific gravity of 1.0015, Chilodonella was effectively eliminated from the aquatic systems. Many necropsies (over 50) were performed on moribund fish which were living in the salt treated systems and in none of these fish was there any evidence of disease caused by Chilodonella.

Discussion

Chilodonella weakens and kills fish by feeding on skin and gill epithelial tissues. Once infected, fish become susceptible to secondary invasion by other pathogens such as bacteria and fungi which contribute to the disease problem. At the wholesale level of the tropical fish industry, fish are stressed considerably through overcrowding, excessive handling and poor nutrition. Ciliated protozoal parasites like Chilodonella which reproduce rapidly via binary fission take advantage of these stressed fish and can cause high morbidity and mortality.

Formaldehyde and malachite green proved to be very effective in the control of Chilodonella, especially when treatments involved individual tanks containing one species of fish. When this drug combination was used to treat an entire 40,000 liter system, certain significant problems arose. The scaleless catfishes (Pimelodidae) did not tolerate the formaldehyde in the treatment solution. Secondly, both formaldehyde and malachite green are toxic to humans and dangerous to handle in concentrated forms. Finally, it was difficult to control and measure the concentrations of formaldehyde and malachite green in the system a day or two after treatment. With the sodium chloride there was control, safety and efficacy.

The amount of sodium chloride dissolved in aquarium water can be accurately evaluated using a good hydrometer. Hydrometers measure relative density which is also known as specific gravity. The relative density is measured as grams/cm3 and is dependent on three factors: pressure, salinity and temperature. Salinity is the most important factor controlling density, and temperature, while significant, plays a lesser role. Pressure has very little effect on density under normal environmental conditions and need not be considered here. Relative density is defined as the ratio of a substance's density to the density of water at 0 degrees centigrade, which is 1.000. At 26 degrees centigrade pure fresh water has a density of slightly less than 1.000 and sea water (3.4%) has a density of 1.022. Considering these factors, at 26 degrees centigrade a specific gravity measurement of 1.002 represents approximately 0.3% sodium chloride in the water and a reading of 1.001 would indicate approximately 0.15% sodium chloride. In this study Chilodonella was not found in water with a specific gravity of 1.0015 and even when the specific gravity dropped to 1.001, Chilodonella outbreaks were not observed, indicating that this ciliate is very sensitive to salt even when the salinity is barely measurable.

Nearly all of the species of tropical fish present in the wholesale facility studied were tolerant of a 1.0015 salinity. The livebearers and brackish species of fish were always kept in water ranging from 1.002 to 1.003 and none of these fish ever experienced a Chilodonella problem. Some species of freshwater plants may be adversely affected by salt in the water (Untergasser, 1989) but at the wholesale level this is usually not a consideration since plants are not normally kept with the fish.

Table 1. Species Found to be Infected with Chilodonella sp.

Acknowledgements

I am grateful to David O'Beirne and Herman Neffle of O'Beirne Tropical Fish for providing the fish and access to the aquatic systems. I also thank Dr. John Gratzek and Dr. Marvin Lewbart for their advice and technical input.

References

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2.  Gratzek, J.B. 1988. Parasites Associated with Ornamental Fish. In M.K. Stoskopf, ed. Tropical Fish Medicine. The Veterinary Clinics of North America, W.B. Saunders Co., Philadelphia. 18(2):378-385.

3.  Hoffman, G.L. 1978. "Ciliates of freshwater fishes," In J.P. Kreier, ed. Parasitic Protozoa: Intestinal Flagellates, Histomonads, Trichomonads, Amoeba, Opalinids and Ciliates. Academic Press, New 'York City. 2:533-632.

4.  Post, G. 1987. Textbook of Fish Health. T.F.H. Publications, Neptune City, NJ. 288 pp.

5.  Reichenbach-Klinke, H.H. 1973. Fish Pathology. T.F.H. Publications, Neptune, NJ. 512 pp.

6.  Untergasser, D. 1989. Handbook of Fish Diseases. T.F.H. Publications Neptune, N.J. 160 pp.