Important Infectious Diseases of Ornamental Fish
Atlantic Coast Veterinary Conference 2001
Greg Lewbart M.S., V.M.D., Dipl. ACZM

Viral Diseases

Several viral diseases have been thoroughly described in ornamental tropical fishes. The most commonly observed viral disease of tropical fish is called lymphocystis disease. This disease is caused by an iridovirus which infects connective tissue cells of the fish. The virus induces these cells to undergo extensive hypertrophy until the cells may actually be visible to the naked eye. Affected cells can increase a thousand fold in size. The disease appears to be more common in marine and brackish water fishes. Certain species of freshwater tropical fish like the green terror (Aequidens rivulatus) are prone to the disease. Members of the genera Scatophagus, Monodactylus, and Changa are all brackish water fishes that seem predisposed to lymphocystis disease. Stress is almost certainly a factor in this disease since outbreaks are frequently observed following capture and shipping of fishes. Gross lesions appear white and granular and usually are seen on the skin and fins. Occasionally, lesions will be seen in the mouth and on the gills. There is no proven chemotherapeutic treatment. Most cases are self limiting if the fish is provided with proper water quality and nutrition. Surgery can be performed on affected fish by carefully scraping the hyperplastic fibroblasts clear of the fish with a sterile scalpel or scissors. This procedure should be performed quickly and the patient(s) should receive 5-10 days of topical antibiotic therapy following the surgery. A definitive diagnosis can be made by microscopically examining a scraping of the affected area. The enlarged connective tissue cells will appear circular and in clusters. These cells frequently emit a light orange hue under the microscope.

Bacterial Diseases

Bacterial disease is the most common infectious problem of ornamental fishes. Collectively, only water quality problems exceed bacterial diseases in the area of pet fish morbidity and mortality. The majority of bacterial infections are caused by Gram-negative organisms including the following pathogenic genera: Aeromonas, Citrobacter, Edwardsiella, Flavobacterium (Flexibacter), Mycobacterium, Pseudomonas, and Vibrio. Streptococcus, a Gram-positive genus, has been shown to cause disease in ornamental fishes. Bacterial organisms may be the primary cause of disease, or they may be secondary invaders, taking advantage of a breach in the fish's integument or compromise of its immune system. The majority of bacterial fish pathogens are natural inhabitants of the aquatic environment, whether it be freshwater or marine. Nearly every bacterial pathogen of fish is capable of living independently away from the fish host (Inglis et al., 1993). Virtually any extrinsic stress, including shipping, crowding, poor water quality, and inadequate nutrition, may predispose an ornamental fish to bacterial disease.

Bacteria are simply everywhere. One study sampled water from pet store aquariums and found a wide vareity of Gram-positive, Gram-negative, aerobic, and anaerobic organisms (Trust, 1974). In fact, every one of the eighteen samples of tropical fish water in this study contained Pseudomonas and Citrobacter while over 80% of the goldfish water samples contained Pseudomonas, Citrobacter, and Escherichia .

For a complete review of bacterial diseases of fish (primarily food fish species), the reader is encouraged to examine Bacterial Diseases of Fish, edited by Inglis, Roberts, and Bromage.

Fungal Diseases

It is common for hobbyists and pet store owners alike to refer to any grossly visible skin disease of tropical fish as "fungus." one frequently sees diseases such as lymphocystis and protozoal ectoparasitic diseases lumped into the fungus category. Fortunately, fungal disease is very easy to identify under the microscope and other disease problems can be quickly ruled out following a simple skin scraping. Fungi belonging to the genus Saprolegnia are the most commonly observed species affecting tropical fishes. Such fungi are opportunistic pathogens which typically colonize exposed damaged tissue. A typical presentation of Saprolegnia would be on the fin rays of a catfish which had been recklessly handled with a net. Under most conditions, if the fish is well supported with clean water and good food, the fungal tufts will slough off in time without the necessity of treatment. In severe cases, treatment may be warranted. These fungi are susceptible to several compounds including formaldehyde, malachite green and salt. Microscopically, the clinician will see typical fungal hyphae wound in a tight mat with the possibility of reproductive bodies being present.

Protozoal Diseases

The following list provides the correct spellings and a brief description of some important protozoal disease agents of tropical fish.

Brooklynella- The marine "cousin" of Chilodonella. Found on saltwater tropicals. Looks very similar to Chilodonella. This parasite is also easily treated with protozoicides.

Chilodonella- A ciliated protozoan which can cause high morbidity and mortality among freshwater tropical fishes at the wholesale and fish farming levels of the industry. Attacks skin and gills. Easily identified microscopically by its heart-shaped structure and slow circular motion when not crawling on the surface of the fish. Once diagnosed, this problem is easily treated with formaldehyde, malachite green or salt.

Cryptocaryon- The marine form of "Ich." Frequently referred to as "white spot disease." This large ciliate possesses both free-swimming and encysted stages. Many aquarists control its spread with the prophylactic use of copper sulfate in the water. A quarantine protocol for new fishes can greatly help reduce the spread of this disease.

Epistylis (Heteropolaria)- A stalked ciliate which is commonly found in freshwater containing a high organic load. Tends to colonize bottom dwelling fish such as the plecostomus catfish. Lesions appear pale and white in color and resemble a fungal disease. Microscopically, one sees a ciliated crown atop a long stalk which is prone to frequent contractions. Easily treated with formaldehyde however a clean well filtered tank is the best solution to the problem. This disease is usually not fatal in itself but may open the fish up to secondary bacterial disease.

Henneguya- A sporozoan which presents in the form of small white cysts on the fins and gills of some fish. The cysts contain infective spores. Commonly seen on the dorsal fins of imported Leporinus species. Not harmful to the fish. Careful removal by scraping with a scalpel is the best treatment since the parasite is aesthetically undesirable.

Hexamita (Spironucleus)- These flagellated protozoa may cause severe gastrointestinal disease if present in large numbers. Normal inhabitant of fish digestive tract. As an ectoparasite it is believed to be involved with "Hole in the Head Disease" (Head and Lateral Line Erosion) common to oscars and other cichlids. Treated effectively with metronidazole.

Ichthyobodo- Formerly (and still commonly) called Costia. A flagellated protozoal ectoparasite. A normal inhabitant of fish skin. Poor water quality and other stresses (especially crowding) may allow this normally mutualistic parasite to reproduce rapidly and overwhelm the host. It is responsive to treatment with formaldehyde and malachite green but tougher than most protozoa. Microscopically the protozoa are very small (5-10 microns), move rapidly, and are shaped like small sickles. They may be attached to host tissue or swimming free. Most common in freshwater species of fishes but has been reported from several marine fishes.

Ichthyophthirius- Known commonly simply as "Ich." The largest protozoal parasite of fish and one of the most commonly encountered. Trophozoites may reach 1.0 mm in diameter. Can affect skin, gills or both. Prevention is the best method of control although the parasite is susceptible to a variety of parasiticides including malachite green and formaldehyde.

Plistophora- A microsporidian sporozoan which is the causative agent for true "Neon Tetra Disease." The parasite is not specific to neon tetras and when present will attack the musculature of the affected fish. Infected muscle will contain numerous sporoblasts containing spores. Grossly infected muscle will appear white or pale. Certain bacterial skin diseases will produce similar gross lesions. Such sporozoan infections are usually unresponsive to treatment and diseased fish should be removed from the tank. High mortality is usually associated with this disease.

Tetrahymena- Commonly called "Guppy Killer Disease." A ciliated protozoan which can be free-living or parasitic. Common in crowded conditions and in water containing excessive organic debris. Unaffected by parasiticides because of its ability to burrow deeply into skin of host which ultimately protects parasites from chemotherapeutics. Best method of control is prevention through sound husbandry practices. These pear-shaped protozoa may be present in very large numbers when the infestation is severe.

Trichodina- A disc-shaped ciliate protozoan found on the skin and gills of many fish. Circular rows of denticles and a ciliary girdle give this parasite a unique radial symmetry. Probably not harmful when present in small numbers.

Uronema- The marine counterpart of Tetrahymena. This is also a significant disease causing parasite. Tissue destruction by this protozoan will allow for secondary invasion by pathogenic bacteria.

Trematode Diseases

Both monogenean and digenean trematodes parasitize tropical fishes. Monogenean parasites including Dactylogyrus and Gyrodactylus are ectoparasitic and can cause considerable damage to the host when present in high numbers. These parasites possess a multiple hooked attachment organ called an opisthaptor which disrupts the integrity of the host's skin and mucus membranes. These monogeneans can complete their entire life cycle on a single host and in some species the cycle may be as short as 60 hours if all environmental conditions are optimal. Crowding and other stress factors predispose tropical fish to monogenean trematode problems. These parasites are generally resistant to low doses of formaldehyde and even some organophosphates. Most freshwater monogeneans can be killed quickly with a 3 to 5 minute saltwater bath (30-35 parts per thousand). Glacial acetic acid or hydrogen peroxide dips will also kill these parasites. Dosage information is given in the provided articles and references. Praziquantel baths have also proved to be effective in killing some monogenean worms. While expensive, this is a relatively safe treatment when used at a concentration of 10 parts per million for 3 to 6 hours.

The majority of digenean fluke problems appear to be primarily aesthetic in nature among tropical fish. Fish commonly serve as an intermediate host for these parasites which frequently have a complex life cycle. Invertebrates may be the first host and a bird or mammal the primary host. Encysted digeneans are commonly observed as metacercaria in the skin and underlying tissues of tropical fish. Occasionally these metacercaria are found in the coelomic cavity of tropical fish. Imported silver dollar fish species from South America are commonly infected with metacercaria belonging to the genus Neascus. Some fish may have only one or two metacercaria while others may harbor hundreds. This disease will not harm the fish and will not progress unless the fish is consumed by an appropriate primary host animal. Fish which are affected are sometimes said to have "Salt and Pepper" disease since the cysts become pigmented and the uplifted scales appear especially white or shiny. Another common digenean parasite is Clinostomum which is called the "Grub" by fish farmers in Florida. Excysted worms may be more than 5 millimeters long and are easily visible to the naked eye. If the metacercaria are not too numerous, they can be removed safely with a clean scalpel.

Occasionally, larvae belonging to the genus Diplostomum have been found associated with the lens in the eyes of tropical fish. In such cases the lens will become opaque and the fish may be blinded. There is no reported treatment for this disease.

Cestode Diseases

Tapeworms are found inhabiting the digestive tract of wild tropical fishes. Diagnosis can be made by fecal examination observing proglottids exiting the vent of a fish, or during necropsy. Recently, work has been published using praziquantel to treat infected fishes and it appears that certain tapeworms are susceptible to a dose as low as 2 parts per million in the water. Infected fish can be bathed in this solution for 3 hours with adequate aeration.

Tropical fish commonly act as an intermediate host in a cestode's life cycle and encysted tapeworm larvae called procercoids can be found in the coelomic cavity of tropical fishes.

Nematode Diseases

Nematodes are common parasites of fish and can be especially abundant in wild species. In some cases the tropical fish is the definitive host and the nematodes will be found in the gastrointestinal tract. In other instances the fish is an intermediate host and the larval nematodes will be seen encysted beneath the skin, in the musculature or in the coelomic cavity. Medical treatment of the larval forms is very difficult because these nematodes are encysted and well protected. Some species of Eustrongyloides form large cysts just under the skin of tropical fish and can be removed surgically, especially if the fish is relatively large. As is the case with other encysted larval helminth parasites, the disease will usually not progress unless the fish is eaten by the definitive host.

Gastrointestinal nematodes can be observed on necropsy and ova are readily seen on examination of the feces. While the presence of these parasites may not cause a problem in nature, the stresses of captivity and shipping may exacerbate any parasitic problem. Nematodicides such as fenbendazole and piperazine may be incorporated into food in order to successfully treat these problems. The attached articles contain dosage and treatment information.

Crustacean Diseases

There are several important crustacean parasites of tropical fish. Laernea, known commonly as "Anchorworm," is a modified copepod parasite which infects large scaled freshwater tropical and temperate species of fish. This parasite possesses a life cycle that includes microscopic pelagic larval stages that molt and grow several times before attacking the fish host. On the host the female anchorworm matures and produces two large egg sacs containing hundreds of Laernea eggs. This parasite is easily visible to the naked eye and may be more than 2 centimeters in length. They get their name from the attachment organ which is a highly modified structure which resembles the anchor on a ship. This structure is buried in the host's musculature and allows for the invasion of pathogenic bacteria. Plucking the parasites from the fish is warranted and usually results in inflamed areas which heal quickly. Organophosphates and glacial acetic acid dips are successful in treating the problem. The disease is especially common in imported and domestic goldfish.

The other major crustacean parasite is Argulus. This branchiuran crustacean is commonly called the "Fish Louse." Fish lice are flattened creatures with a very distinctive shape and appearance. They have a pair of eyespots and are about 5-10 millimeters in length. They move about the skin of a fish very effectively and camouflage themselves well on the host. They suck bodily fluids from the fish via a sharp stiletto that actually injects a small amount of toxin into the fish. These parasites are especially harmful to small fish. Argulus also possesses a life cycle with pelagic larval stages so the entire aquarium system may have to be treated with organophosphates to control the disease. Depending on temperature, the total life cycle takes between 6 and 20 weeks.

A less commonly seen group of crustacean parasites are the isopods. While most isopods are free living, members of the genus Livoneca can be parasitic. Terrestrial pill bugs commonly seen under rocks and logs are isopods and the aquatic parasitic forms resemble their land dwelling relatives. While Argulus is dorsoventrally compressed, isopods like Livoneca are laterally compressed and appear segmented.


The following list contains references which will aid the interested veterinary student or clinician in diagnosing and treating infectious disease problems of ornamental fish.

1.  Brown L: Aquaculture for Veterinarians. Pergamon Press, Oxford, England, 1993, 447 pp.

2.  Butcher RL: Manual of Ornamental Fish. British Small Animal Veterinary Association. Gloucestershire, England, 1992, 200 pp.

3.  deGuzman E. and E.B. Shotts. Bacterial Culture and Evaluation of Diseases of Fish. Vet Clinics of North America Small Animal Pract., 1988, 18(2): 365-374.

4.  Gratzek JB: Control and therapy of fish diseases. Adv Vet Sci Comp Med 27:297-324, 1983.

5.  Gratzek JB: An overview of ornamental fish diseases and therapy. i Small Anim Pract 22:345-366, 1981.

6.  Gratzek JB: Aquariology: the Science of fish Health Management, Tetra Press, 1992.

7.  ¬†Herwig N: Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Springfield IL, Charles C Thomas, 272 pp., 1979.

8.  Hoffman GL and FP Meyer: Parasites of Freshwater Fish. TFH Publications, Neptune City, NJ, 1974.

9.  Johnson, EJ: Koi Health and Disease. Johnson Veterinary Services, 3805 Robinson Rd., Marietta, GA 30068, 1997, 141 pp.

10. Lewbart, GA: Self-Assessment Color Review of Ornamental Fish. Iowa State University Press, 1998, 192 pp.

11. Lewbart, GA: Emergency pet fish medicine. In Bonagura JD and Kirk RW (eds.): Current Veterinary Therapy XII: Small Animal Practice. Philadelphia, WB Saunders Co, 1995, 1369-1374.

12. Lewbart, G., Vaden, S., Deen, J., Whitt, D., Manaugh, C., Doi, A., Smith, T., and K., Flammer. Pharmacokinetics of enrofloxacin in the red pacu (Colossoma brachypomum). Journal of Veterinary Pharmacology and Therapeutics, 20:124-128, 1997.

13. Noga, EJ: Fish Disease: Diagnosis and Treatment. ISU Press, 2000, 367 pp.

14. Stoskopf, MK: Fish Medicine. Philadelphia: W.B. Saunders Company, 1993, 882 pp.

15. Tocidlowksi, ME, Lewbart, GA., and Stoskopf, MK. 1997. Hematologic study of red pacu (Colossoma brachypomum). Veterinary Clinical Pathology, 26(3):119-125.

16. Untergasser D: Handbook of Fish Diseases. TFH Publications, Neptune, NJ, pp.160, 1989.

17. The Veterinary Clinics of North America: Exotica Animal Practice. Various volumes, WB Saunders.

18. Inglis V, Roberts RJ, Bromage NR (eds): Bacterial Diseases of Fish, New York, NY, Halsted Press, 1993

19. Trust TJ, Bartlett KH: Occurrence of potential pathogens in water containing ornamental fishes. Applied Microbiology 28(1):35-40, 1974

20. Frerichs GN: Isolation and identification of fish bacterial pathogens, in Inglis V, Roberts RJ, Bromage NR (eds): Bacterial Diseases of Fish, New York, NY, Halsted Press, 1993, pp 257-283

Speaker Information
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Greg Lewbart