Numerous protozoan and metazoan parasites infect both freshwater and marine aquarium fishes, and many of these cause mass mortalities among fishes in tropical aquaria and oceanaria. This paper reviews the morphology, life history, pathology, and treatment of the several frequently encountered and deleterious parasites infecting aquarium fishes.

Two of these common parasites are the ciliates Ichthyophthirius multifiliis, infecting freshwater fishes, and its marine counterpart, Cryptocaryon irritans. Trophonts of both of these organisms are easily recognized by their large size, their distinct macronuclei, and their constantly rotating motion. Life cycles of these parasites are similar. Trophonts on the skin and gills mature, fall to the substratum, and encyst, becoming tomonts. After multiple unequal divisions, hundreds of tomites are liberated from each tomont. These free-swimming, infective stages penetrate into and eventually become covered by the host's epithelium, resulting in characteristic white spots or papules on the surface of the fish. Tissue irritation caused by the ciliates results in epithelial hyperplasia and excessive secretion of mucus on the body and gills. In advanced stages trophonts ulcerate and infiltrate the epithelium. Resulting petechial lesions may serve as foci for secondary infections of non-specific Pseudomonas (Nigrelli and Ruggieri, 1966). A number of chemical treatments are known to eradicate tomites, including copper sulfate, formalin, quinine hydrochloride, and malachite green.

Another lethal ciliate that parasitizes the gills of marine fishes is Brooklynella hostilis. Light infections with this Chilodenella-like parasite are characterized by a mild inflammatory response and an increase in tissue eosinophilia. Heavy infections result in desquamation, cellular proliferation, and fusion of terminal gill lamellae, and these severe lesions are responsible for the death of the hosts Mom and Nigrelli, 1970). Effective treatments include the use of copper compounds and malachite green.

Additional ectoparasites infesting the skin and gills of their fish hosts are the dinoflagellates Piscinoodinium pillulare and Amyloodinium ocellatum. Trophonts of P. pillulare occur on freshwater fishes and have an attachment disc with rhizocysts that embed in the cytoplasm of the host epidermis or gill epithelium (Lom and Schubert, 1983) and possess well-developed chloroplasts. Amyloodini ocellatum trophonts possess numerous filiform rhizoids and a stomopode which apparently aids in digesting host tissue, but they lack chloroplasts (Lom and Lawler, 1973). Life cycles of these organisms are similar, with trophonts withdrawing their penetrating processes, falling to the substratum, and covering themselves with a cellulose secretion. Cellular division produces 128 free-swimming dinospores which each divide once to form a total of 256 infective swarmers. The pathogenicity of these parasites is caused by the extensive damage to gill epithelial cells by the penetrating rhizocysts or rhizoids. The primary damage to host tissue in A. ocellatum infestations results from the injection of lytic or digestive fluids through the stomopode. Paperna (1980) has suggested that the severe histopathological changes in the gills of fishes infested with A. ocellatum might be caused by the excretion of toxic substances by the parasite, as is the case with several species of free-living dinoflagellates. Effective treatments for infested fishes include the use of copper compounds and freshwater baths.

Spironucleus elegans is a hexamitid flagellate that commonly parasitizes the intestine of the freshwater angelfish, Pterophyllum scalare, as well as several other species of cichlid fishes. Trophozoites are elongated with two sausage-shaped nuclei at the apex and have six anterior and two posterior flagella. These flagellates are believed to possess a single host life cycle with direct transmission (Becker, 1977). Cysts are released from the intestine with the feces, and hosts become infected directly by ingestion of these cysts. Schubert (1974) reported that Spironucleus infections limited to the intestine are relatively harmless; however, under certain conditions the flagellate apparently leaves the intestine and migrates to other organs, killing the host. A hexamitid flagellate infection (probably Spironucleus) in the Siamese fighting fish, Betta splendens, was reported by Ferguson and Moccia (1980). The infected fish became inappetent, had a lethargic behavior, developed a swollen abdomen, and usually died, often with a perforated stomach. The flagellates occurred in large numbers in the abdominal cavity, mesentery, liver parenchyma, spleen, and kidney, and they were closely associated with a chronic inflammatory response. Ferguson and Moccia (1980) found that nifurpirinol effectively eradicated the flagellates when administered as both a bath and a feed medication. Treatments with Flagyl® are also effective in eliminating infections.

Representatives of several groups of parasitic metazoans including turbellarians, monogeneans, larval digeneans, and crustaceans infect aquarium fishes. The most frequently encountered of these are the monogeneans belonging in the families Gyrodactylidae and Dactylogyridae. These parasites infect the skin and gills of both freshwater and marine fishes and often cause mass mortalities among aquarium fishes. The life cycle of these parasites is direct, requiring no intermediate hosts. Members of the Gyrodactylidae give birth to live young, whereas members of the Dactylogyridae lay eggs. Eggs of some species are released into the water, fall to the bottom, and after a period of development, a ciliated larva is released which is capable of finding a new host. Members of both families possess anchors which are used to attach to host tissue and cause ulcerations and hemorrhages. Heavy branchial infestations can cause mass mortalities. Respiration is impaired by mechanical damage to the epithelium and the hypersecretion of mucus. Recommended treatments include formalin baths or various formulations including Trichlorofon®.

Another common metazoan infesting marine aquarium fishes is the "tang" turbellarian, tentatively identified as Ichthyophaga. This organism is most often observed on the skin, fins, and occasionally the gills of the yellow tang, Zebrasoma flavescens, but also occurs on other members of the family Acanthuridae and has been reported from members of the Chaetodontidae (Blasiola, 1976). It or other species infest members of a few other families. The life cycle has been reported by Kent (1981) and begins with a gravid offhost worm containing up to 160 young. These occur on the tank bottom, and the actively swimming ciliated young are released from the adult and reintest additional fish. Development of the young in the off-host stage requires about 5 days at 24.5°C.

Once attached to a host, the young increase in size to about 350µm and then leave the host within approximately 6 days. Affected fishes exhibit clamped, reddened fins, turbid skin, and loss of appetite. Histologic sections of infected tangs showed worms attached by the pharynx penetrating the epithelium. Cultures of lesions from heavily infested tangs revealed a secondary bacterial infection by a Vibrio sp. (Kent, 1981). Mortalities in fishes with turbellarian infestations are probably due to a combination of the turbellarians and the secondary bacterial infection. Effective treatments include the use of formalin baths or various formulations containing Trichlorofon®.

The correct diagnosis of a parasitic organism is essential before selecting a medication for the treatment of the respective disease. All the parasites indicated above are best identified by examining fresh material from moribund fishes. This is best accomplished by examining the feces and mucus from the skin and gills under a compound microscope. If this is not possible, most of these parasites can be identified from specimens frozen or fixed in 10% formalin.

This study was conducted in cooperation with the U.S. Department of Commerce, NOAA, NMFS, under PL88-309, Project 2-422-R.

References

1.  Becker, C. D. (1977). Flagellate parasites of fish. In Parasitic Protozoa, Vol. I (ed. J. P. Kreier), pp. 357-416. New York: Academic Press.

2.  Blasiola, G. C. (1976). Ectoparasitic turbellaria. The Marine Aquarist 7, 53-58.

3.  Ferguson, H. W. and Moccia, R. D. (1980). Disseminated hexamitiasis in Siamese fighting fish. Journal of the American Veterinary Medical Association 177, 854-857.

4.  Kent, M. L. (1981). The life cycle and treatment of a turbellarian disease of marine fishes. Freshwater and Marine Aquarium Magazine 4, 11-13.

5.  Lom, J. and Lawler, A. R. (1973). An ultrastructural study on the attachment in dinoflagellates invading gills of Cyprinodontidae. Protistologica 9, 293-309.

6.  Lom, J. and Nigrelli, R. F. (1970). Brooklynella hostilis n.g., n.sp., a pathogenic cyrtophorine ciliate in marine fishes. Journal of Protozoology 17, 224-232.

7.  Lom, J. and Schubert, G. (1983). Ultrastructural study of Piscinoodinium pillulare (Schaperclaus, 1954) 8. Lom, 1981 with special emphasis on its attachment to the fish host. Journal of Fish Diseases 6, 411-428.

8.  Nigrelli, R. F. and Ruggieri, G. D. (1966). Enzootics in the New York Aquarium caused by Cryptocaryon irritans Brown, 1951 (Ichthyophthirius marinus Sikama, 1961), a histophagous ciliate in the skin, eyes and gills of marine fishes. Zoologica 51, 97-102.

9.  Paperna, I. (1980). Amyloodini ocellaturn (Brown, 1931) (Dinoflagellida) infestations in cultured marine fish at Eilat, Red Sea:epizootiology and pathology. Journal of Fish Diseases 3, 363-372.

10. Schubert, G. (1974). Cure and recognize aquarium fish diseases. New Jersey:T.F.H. Publications, Inc.