The discus, Symphysodon sp., is a highly valued freshwater aquarium fish. In Florida, commercial production of discus is a highly specialized endeavor. The fish, particularly breeding pairs, are extremely costly. A clinical syndrome exists of some farm-raised discus which is characterized by chronic weight loss and affected fish are categorized as poor-doers. Typically, affected fish are extremely cachectic, normal coloration is lost, and a general darkening of the body is obvious. Preliminary findings suggest that chronic endoparasitism may be responsible for debilitation of the fish, and that secondary bacterial infection may cause the actual demise of the animal. Selected case reports will be presented, including pathologic findings. Several treatment regimes will be discussed, and an assessment of response to therapy will be presented when available.

The discus, Symphysodon sp., is a highly valued aquarium fish. It is a member of the family cichlidae and its native range is the Amazon River basin. There are 2 species, S. discus, and S. aequifasciata. The latter is subdivided into 3 subspecies, S. aequifasciata Aequifasciata (the green discus), S. aequifasciata Axelrodi (the brown discus), and S. aequifasciata Haraldi (the blue discus) (1). In addition, many breeders practice line­breeding which results in development of special strains which sell for premium prices.

Discus are somewhat fragile compared to many other species of ornamental fish, and are usually kept by advanced aquarists rather than beginners. They are intolerant of suboptimal water quality and require a high quality diet which is often a combination of commercially prepared diets, live foods, and homemade special recipes.

In Florida (1) commercial production of discus is a highly specialized endeavor. Breeding pairs are extremely costly and difficult to obtain. In addition to being able to spawn on a regular basis many breeders allow the parents to raise the young which emphasizes the importance of healthy breeding stock. Discus fry feed on the slime of the parents (2), and although it is possible to raise them in the absence of parental care, few breeders have achieved high levels of production without good parents.

A condition exists in discus which is characterized by weight loss, poor condition, and dark coloration. This wasting condition appears to be associated with chronic, severe endoparasitism and consequently should be treatable if diagnosis and therapy are accomplished early in the course of the disease. Because of the value of the discus, however, owners are reluctant to submit fish for clinical examination until they appear moribund. Education of discus owners, therefore, is an important aspect of case management. Clinically, affected fish are extremely thin, and appear almost black in coloration. A clinical history will often reveal the chronic progression of the syndrome, and low levels of chronic mortality in the population. In addition to direct economic loss through mortalities, debilitated animals are inadequate as breeders. Behaviorally the fish appear depressed as evidenced by a complete lack of normal activity, listlessness, and anorexia. The darkening is probably a non-specific indication of stress as hobbyists have indicated that discus maintained in suboptimal water quality will develop dark body coloration.

If fish are in good condition it is possible to do gill clippings, skin scrapings, and fecal examination without sacrificing the fish. In this situation the fish should be sedated to prevent injury during the sampling process. Sedation can be achieved by applying concentration of 20 to 50 mg/1 of neutral-buffered tricaine methanesulfonate to the gills (3). The chemical should be used to effect and fresh, oxygenated water should be available for recovery. If the fish is moribund, euthanasia is warranted so that a more thorough examination can be done and bacterial cultures can be taken.

Examination of several discus suggests that trematode infections of the gills are common and can be quite severe. Protozoans may also be present and should be treated if they are identified in large numbers. If present in small numbers removal is less of an emergency but still recommended, particularly if fish are housed in a closed system. These ectoparasites can result in malaise and contribute to poor appetite and condition.

Examination for intestinal parasites is most easily accomplished by preparing wet mounts of sections of intestine. Hexamita can be identified at 400x with a light microscope as a rapidly moving flagellate. If the degree of infestation is clinically significant massive numbers of the parasites will be present which makes diagnosis easier. Capillaria can be identified by observing the double operculated eggs in fecal material or in the intestinal tract, or the adult nematodes may be seen directly as the wet mount is scanned at low power. Cestodes can be identified with a dissecting scope and may be seen grossly if the fish is examined carefully with a good light source. Hexamita and Capillaria were frequently found in the intestinal tract, usually in high numbers. In two cases severe infestations with cestodes of the genus Bothriocephalus were found.

Bacteria have been isolated from the liver and posterior kidney of debilitated animals and may be the actual cause of death of the fish. Plesiomonas shiqelloides and Aeromonas hydrophila were isolated from the liver of a moribund discus which presented with chronic weight loss and severe endoparasitism.

Gross pathology indicated that liver and posterior kidneys were edematous, and the spleen appeared hemorrhagic and friable. The intestine was distended and fluid-filled, and the peritoneal space was filled with a clear yellow fluid which appeared to have a small amount of blood in it.

Microscopic pathology in one case was characterized by granuloma formation in the lamina propria of the intestine, the stomach, and ovarian tissue. A mixed inflammatory exudate consisting of lymphocytic and mononuclear calls surrounded the granulomas. Nematodes were present in cross-section in ovarian tissue. Multifocal areas of moderate coagulation necrosis were observed in hepatic tissue. A focal but severe mononuclear inflammatory response was observed at the periphery of the liver, and multifocal areas of inflammation were observed in centralized areas. Melanomacrophage centers were distributed in a multifocal manner throughout the parenchyma, and some were associated with areas of inflammation and necrosis. The spleen was moderately congested and many melanomacrophage centers were present. Peripheral epithelium was hyperplastic and ballooning was evident in the squamous epithelium of the capsule.

Management of these fish is relatively straight forward. Husbandry practices should be evaluated to ascertain that the aquarist is familiar with the special needs of the discus, and is in fact meeting these needs. Suggested water quality parameters include a pH range of 6.5 to 7.5, and a temperature range of 82 to 88F (4,5). Nutritional management should include a varied diet divided into 2 or 3 feedings per day. Suggested food items include beef heart, earthworms, bloodworms, brine shrimp, freeze-dried krill, and flake foods (4). Special recipes are also recommended by some breeders and these can be found in various trade publications. Disease management of discus is reviewed by Reeves (6,7).

In our laboratory we have treated sick discus in a systematic fashion. Our first priority is to eliminate life-threatening bacterial infections, and then to control parasitism which we believe is responsible for the chronic debilitation of the fish. Sensitivity tests should always be run when bacteria are isolated from ornamental fish as many antibiotics are used in the trade, most frequently in the absence of identification and sensitivity testing. The Aeromonas which was isolated from a discus affected by chronic weight loss was sensitive to oxytetracycline which is our drug of choice for opportunistic bacterial infections. Although I generally prefer to administer antibiotics in the feed these fish were not eating so a bath treatment was appropriate. Oxytetracycline can be administered as a 1 hour bath at a concentration of 50 to 100 mg/1 and repeated on a daily basis (8).

Some of the discus examined were infested with high levels of protozoans and trematodes on the gills. These parasites, when present in high numbers can contribute to poor appetite and poor condition and therefore were removed by formalin baths (7). It was important to have the fish eating as well as possible before attempting to treat the intestinal parasites.

Although Hexamita is an intestinal problem it does appear to respond reasonably well to bath treatments with metronidazole (5 mg/l) (9), as well as to oral therapy with metronidazole (50 mg/kg body weight) and dimetridazole (1.5 mg/gm food) (8,9). Good sanitation, optimal nutrition, and removal of carbon filtration are also important to successful management of Hexamita (9) in commercial hatcheries.

Capillaria can be removed with mebendazole (20 mg/kg body weight administered weekly), cambendazole (20 mg/kg body weight administered weekly), or Levamisole (100 mg in 25 gram of food administered weekly) (8). Each of these is mixed into the food for oral administration and the weekly medications are repeated for a total of 3 treatments (8). Cestodes have been treated successfully with niclosamide (200 mg/kg body weight repeated 2 weeks after initial treatment), and praziquantel (100 mg/ 20 gm food, repeated daily for 7 days). Because the discus described in this case report were severely infected with both capillaria and cestodes the decision was made to use Vercom paste (Mobay Chemical, Animal Health Division, Shawnee Mission, Kansas). Vercom contains febantel (34 mg/ g paste) as a nematocide, and praziquantel (3.4 mg/ gm paste) as a cestocide. The paste was mixed into food at a concentration of 10 grams paste into 16 ounces of food. This was fed as the sole food for 3 days (10). The producer reported that the appetite of the fish improved markedly following treatment and apparent weight gain was observed. Reproductive records were inadequate to accurately judge the impact on breeding success and hatchery production however the initial impressions were very encouraging.

Results achieved were short-lived, however, in that fish have been resubmitted to our laboratory about every 8 months with similar presentation and clinical findings. Additional work on parasite control is needed for the discus, both in safety and efficacy of veterinary drugs which may have potential for solving this problem, and in identifying husbandry problems which contribute to the parasitism and poor condition of affected animals.

References

1.  Van Ramshorst, J.D. (Ed.). The Aquarium Encyclopedia of Tropical Fish. H.P. Books, Tuscon, Az. 1981; pp.353-355.

2.  Axelrod, H.R.; Emmens, C.W.; Burgess, W.E.; Pronek, N. 1984. Exotic Tropical Fishes, expanded edition. TFH Publications, Neptune City, N.J. 1984; pp. 1132.

3.  Brown, L.A. Anesthesia in fish. In: Tropical Fish Medicine, M.K. Stoskopf (Ed.)' Vet Clin No Am: Sm An Pract. 1988; 18(2):317-330.

4.  Stark, D. Discus, king of the aquarium. North American Discus Society, Lakeland, Fla. June, 1987; 2(2):5-8.

5.  Hayes, T. Observations on the care and breeding of the turquoise discus. North American Discus Society, Lakeland, Fla. June, 1987; 2(2):1-3.

6.  Reeves, K. Disease control. North American Discus Society, Lakeland, Fla. Dec. 1986; 1(4):1-5.

7.  Reeves, K. Discus plaque. North American Discus Society, Lakeland, Fla. June 1987; 2(2):9-14.

8.  Stoskopf, M.K. Fish chemotherapeutics. In: Tropical Fish Medicine, M.K. Stoskpf (Ed.). Vet Clin No Am: Sm An Pract. 1988; 18(2):331-348.

9.  Gratzek, J.B. Parasites associated with ornamental fish. In: Tropical Fish Medicine, M.K. Stoskopf (Ed.). Vet Clin No Am: Sm An Pract. 1988; 18(2):375-399.

10. Reeves, K. Arvada, Colo., personal communication, 1987.