According to the popular press and the growers of angelfish, (Pterophyllum scalare), worldwide there is a poorly described "Mystery Disease Syndrome'' which has been reported to afflict angelfish, Discus (Symphsodon spp.) and similar cichlids. Catastrophic losses of angelfish were first reported throughout the aquarium fish industry in 1986. Unusual and inconsistent clinical signs have been associated with mortalities, including fin clamping, increased amounts of mucous secretions and a darkening of the fishes colors. To date, losses have been reported in all age groups of angelfish.

This investigation includes imported, domestically bred, wild caught and hobbyist bred angelfish. The significance of intestinal parasitism will be assessed by identification of parasites found in sick angelfish and severity of infestation. Gross findings will be correlated with pathologic changes within fixed tissues. Parasites discussed will include Hexamita, Capillaria Myxosporidians, a Coccidia-like organism, and Eustrongylus.

Introduction

Angelfish (Pterophyllum scalare) are indigenous to the Amazon River Basin in South America. This fish#, of the family cichlidae, has a distinctive laterally compressed, disc-shaped body and long, elegant fins. They have been successfully bred and raised in captivity since the 1940s (1), and today are considered one of the staples of the ornamental fish trade.

Worldwide production of angelfish has been estimated at 25 million fish per year (1) . In Florida, ornamental fish growers receive approximately 20 cents per fish for small silver angelfish, the least expensive variety, and up to several dollars each for large fish of the same variety. Prices increase substantially for the more exotic varieties such as the veiltails and blushings. An estimate of the total number of angelfish produced in Florida each year is not available. However, the fish is considered one of the most economically important species to the state's ornamental fish growers.

Reports of unusual and excessive mortality of angelfish first appeared in the popular literature in the fall of 1986. These accounts suggested that the present problem may have originated from imported fishes. Affected fishes showed signs of severe and progressive deterioration of fins, characteristic of fin rot, elevated respiratory rate, anorexia, and excessive skin mucous secretions. Abnormal behavioral patterns such as hyperactivity, loss of equilibrium, and fin clamping were also been reported (2,3). Investigations in our laboratory suggested that suboptimal water quality and severe parasitism were associated with some mortality.

The purpose of this study was to assess the significance of intestinal parasitism, grossly and histologically, as a possible etiology of the angelfish mystery syndrome.

Materials and Methods

Diseased angelfish submitted for clinical evaluation were subjected to a screening protocol which included history, physical exam, bacteriology, parasitology, and histopathology. In a few select cases tissues were collected for transmission electron microscopy which will be done at a later date.

Upon arrival the fishes were grossly evaluated for overall condition, behavioral abnormalities, external lesions, and ectoparasites. To assess internal parasitism, wet mounts were made of intestine, liver, spleen, and kidney, and were examined with a light microscope. Parasites were identified and degree of infestation (i.e. light, moderate, heavy) was noted. Next, representative fishes from each case were euthanized using an overdose of tricaine methane sulfonate (MS-222). Sections of gill, skeletal muscle and a "visceral pluck" (alimentary canal, spleen, liver, kidney) were fixed in 10% neutral-buffered formalin for histologic studies. Following processing, tissues were embedded in paraffin, cut, and stained with hematoxylin and eosin.

Wild-caught angelfish were imported from Brazil for use as control animals. In addition, healthy fish were obtained from a local hobbyist with no history of disease in his fish. Control fish were examined in an identical fashion as sick fish described above.

Results

The results reported in this study were gathered from 57 angelfish cases that were evaluated from May 28, 1987 through March 9, 1989. The intestinal parasites encountered, in descending order, were Hexamita (19.3%), Capillaria (14.0%), organisms which were presumptively identified as Myxosporidians (5.3%), Eustrongylus (1.8%), and a coccidia-like organism (1.8%). Although the angelfish in this study were found to have high levels of intestinal parasitism in many cases, it should be remembered that other factors were often present which influenced their clinical presentation.

Hexamita was present in 11 of the 57 cases (19.3%). Clinical signs included dark coloration, emaciation and unthriftiness. Severely affected fish appeared very weak as exemplified by floating with the current and lying on their sides. Internally, the livers of affected fish were pale and friable, and in some cases the spleen was black and swollen. Microscopic pathology demonstrated melanomacrophage centers and multifocal areas of moderate necrosis of the spleen. Small melanomacrophage centers and a mild to moderate mononuclear infiltrate was noted in the lamina propria of the intestine. Hyperplasia of the mucosa was also noted.

Capillaria were found in the intestine of fish from eight of the 57 cases (14.0%). These fish showed behavioral abnormalities such as loss of equilibrium in the water column and lying on the bottom of the tank. Some of these fish appeared in good condition while others appeared slightly emaciated. The gut was swollen with yellow fluid.

Identification of Myxosporidians was based on histologic findings rather than identification of the organisms from fresh material. Myxosporidians were identified in 3 cases, an incidence of 5.3%. Affected fishes appeared depressed, weak, anoretic, very dark in color and severe fin clamping was noted. Gross pathology indicated a gut dilated with fluid, some red blood cells in the gut, and an enlarged pale spleen. Associated microscopic pathology included mild, to moderate inflammation (enteritis) of the intestine, and mild multifocal hyperplasia was observed in the lamina propria and submucosa of the mucosa.

The nematode Eurystrongylus was found in 1 case (1.8%). The most obvious clinical sign noted in infected fish was a markedly distended abdomen. The problem was primarily an aesthetic one where infected fish were not suitable for sale.

A Coccidia-like organism was identified in only 1 case of 57 (1.8%). The clinical signs noted in this case were emaciation extreme curling, fin clamping and loss of equilibrium. Wet mounts showed the presence of a coccidia-like structure within the intestinal lumen. Microscopic pathology demonstrated small melanomacrophage centers located in both the lamina propria and submucosa and a mild to moderate enteritis in diffuse pattern. Of the 2 fish examined in this case, 1 showed signs of portal hepatitis. There were signs of inflammation in both the mesentery and subcutaneous tissues.

Discussion

Hexamita. Hexamita was the most commonly encountered intestinal parasite. It is a piriform shaped multi flagellated protozoan with 2 nuclei and 3 pairs of flagella at the anterior end and a single pair of flagella at its posterior end. Two axostyles give it a symmetrically bilateral appearance (4). Hexamitiasis is common among cultivated fishes but is seldom pathogenic unless the host's health is adversely affected by other factors (5).

Fishes infected with Hexamita lose their appetite, become anoretic, weak, and will swim with abnormal patterns (on their sides, curling, corkscrew fashion) (5). Post (6) also noted that rapid reproduction of Hexamita in the intestine of the host is usually accompanied by dark coloration and listlessness of affected fish. Although Hexamita has been identified with outbreaks of catarrhal enteritis it appears to be present only secondarily to a primary causative agent (5).

Hexamitiasis frequently occurs in angelfish, but infections resulting in disease are relatively uncommon. Remaining latent for long periods of time, infections suddenly appear following a change of environmental conditions such as movement from ponds to tanks, diet alteration, or a temperature fluctuation (7). Hexamita infection in aquarium fishes may result in inflammation and pathologic changes in the epithelial lining of the intestine and gallbladder (5).

Hexamitiasis responds well to oral administration of metronidazole in the food. Stoskopf (8) recommends 10.0 mg active ingredient mixed into 1.0 g of food (or 50 mg/kg body wt.) which is fed as the sole food for 5 days. Dimetridazole has also been used as an oral treatment at a dosage of 1.5 mg drug mixed into each gram of food and fed for 5 days (8). Gratzek (9) has reported satisfactory results from bath treatments of metronidazole at concentrations of 5mg/l. Ipropran has also been reported to be effective against Hexamita (9), however this product is no longer readily available.

Capillaria. Nematodes of fishes have received only limited attention because nematodiasis is considered rare in fishes (6). However, some general aspects of their life cycles and their pathogenicity on host fish can be stated. Female nematodes living in the alimentary tract of fishes, release eggs in the feces (6). Invasion of the host takes place through the intestinal tract, skin and mucous membranes (8). Infective larvae or ova are ingested in food and may develop directly in the intestinal tract. They may invade the mucosa, become encapsulated, and later emerge as immature adults which become attached to the intestinal epithelium (10). Clinical signs of nematodiasis include anemia, emaciation, unthriftiness and reduced vitality when excessive numbers of nematodes are found in the alimentary canal (6). Stoskopf (8) reviews use of drugs for nematode infestations in tropical fish. In addition to medicating infected fish it is important to eliminate intermediate hosts, particularly if infected fish are being reared in ponds. Drying or disinfecting ponds is recommended as a means of eliminating crustaceans and oligochetes which often serve as intermediate hosts.

Myxosporidians. Myxosporidians are characterized by a spore covered by 2 valves united in a distinct sutural plane and contains one to four polar capsules, each with a coiled extensible filament. Each spore contains a protoplasmic body known as the sporoplasm (11). According to Olsen (11) only the basic aspects of the Myxosporidian life cycle are known. Fishes are infected when free spores are ingested and, upon reaching the intestine of the host the spores rupture to release the sporoplasm. Lesions seen with Myxosporidian were formations of gross or microscopic opaque masses in various tissues of the body including liver, intestine, muscle, kidney of spleen (6). Fishes with Myxosporidian infestations usually appear normal although subtle signs of debilitation may become more apparent as the disease progresses (6). There are no chemotherapeutic agents available to eliminate Myxosporidian from either the aquarium or cultured fishes.

Eurystrongylus. Adult worms live in the proventriculus of fish-eating birds. Eggs are passed in feces and annelid worms which live in fish ponds serve as the first intermediate host. Fish that eat the worms develop cysts in the intestine, and the larvae then migrate into the coelom. Treatments involve eliminating the intermediate hosts (i.e. covering ponds from bird activity, or draining and drying ponds to eliminate annelid worms). In our laboratory, attempts to treat infected angel fish with injectable ivermectin and ivermectin baths have not been rewarding.

Coccidia. Coccidia has been reported in the intestine, feces and other organs of Rost fishes, although the organism has not been associated with disease except in a few cases (6). Diagnosis of coccidiosis depends upon the demonstration of either characteristic sporulated oocysts in the feces of the infected fish or in tissue sections (12). Gross clinical signs of coccidia include emaciation, lethargy and general poor health. Internally, coccidia results in white blisters on the intestinal wall, the intestine is swollen with fluid and the feces are of a light color and made up of many oocysts. Currently, there are no therapeutic procedures known for Coccidiosis.

In conclusion, it is apparent that sick angelfish are frequently infected with intestinal parasites. A mild to severe enteritis has been a common finding in suspect fish submitted to our laboratory. In some cases parasitism was probably of sufficient degree to cause the clinical signs of anorexia, wasting and listlessness. However, in several cases signs of systemic disease, including an obvious enteritis visible at the microscopic level, were noted in the absence of internal parasitism. In a recent disease outbreak, veterinarians at the Florida Department of Agriculture and Consumer Services Diagnostic Laboratory located in Kissimmee, Florida, were able to photograph virus-like particles in the intestine of sick angel fish with transmission electron microscopy. The possibility of a viral etiology is being vigorously pursued at present.

Funding for this project was provided by the Division of Marketing, Florida Department of Agriculture and Consumer Services, Doyle Conner, Commissioner, under the Aquaculture Market Development Aid Program.

References

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10. Belding DL. Textbook Of Parasitology. 3rd ed. New York; Appleton-Century-Crofts, 1965; 346-371.

11. Olsen OW. Animal Parasites-Their Life Cycles And Ecology. 3rd ed. Baltimore: University Park Press, 1974; 62-64, 178-184.

12. Georgi JR. Parasitology For Veterinarians. 3rd ed. Philadelphia: WB Saunders Co, 1980; 97-132.