Iridoviral-Associated Disease in Oscars (Astronotus ocellatus)
IAAAM Archive
Roy P.E. Yanong1; Scott P. Terrell2
1Tropical Aquaculture Laboratory, Department of Fisheries and Aquatic Sciences, University of Florida, Ruskin, FL, USA; 2Department of Pathobiology, College of Veterinary Medicine, University of Florida, Gainesville, FL, USA


An oscar breeder in Florida experienced a heater malfunction in a growout area that resulted in a temperature spike of approximately 8°F (6°C) over a two to three day period. Temperatures increased from 82 to 94°F (28 to 34°C). Prior to this event, no significant mortalities had been observed. The problem was identified and corrected, but mortalities in the area averaged approximately 20-25 percent immediately following the spike. Losses continued, reaching 90-100 percent in four to five days. Except for the temperature spike, all other water quality parameters were within normal limits.

Twelve fish were submitted to the University of Florida, Tropical Aquaculture Laboratory for evaluation. Four were dead on arrival. Fish were six to eight weeks of age, and approximately 2.5-3.5 cm TL. Clinical signs included: darkening, exophthalmia, ulcerations, hanging at the top of the water column or lying on the bottom, lethargy, isolation, and spinning. Numerous motile bacteria were observed on skin scrapes of ulcerations and very few Tetrahymena sp. on one fish only. However, no other significant findings were seen on wet mount microscopic evaluation of external or internal organs. Bacterial culture results of brain and kidney were inconsistent (most affected fish were culture negative on TSA with five percent sheep's blood).

Histopathology revealed systemic viral infection resulting in acute, diffuse, severe coagulative necrosis, with myriad intralesional intracytoplasmic viral inclusions in the kidney. Numerous cells containing large, finely granular basophilic to amphophilic intracytoplasmic inclusions were found diffusely throughout the renal parenchyma. Similar basophilic intracytoplasmic inclusions were found in other tissues, including endothelial cells in blood vessels of the gill and other organs, and in atrial tissue; and interstitial cells in the esophagus, in skeletal muscle, and in bone. Interstitial cells of parenchymal organs and cells of the reticuloendothelial system were most commonly affected. Iridoviral-like particles were seen readily in affected tissues on transmission electron microscopy.

The breeder elected to disinfect the entire system with chlorine and restart the biological filter. Temperature fluctuations have also been monitored and controlled, and no recurrence of the problem has occurred for two years since the event.

Iridoviruses and iridoviral-like particles associated with disease in fish have been described in a number of different fish species. Exact relationships between many of these viruses are not well understood. In freshwater tropical fish species, systemic disease, associated with morbidity and mortality, has been seen in blue gouramis (Trichogaster trichopterus) and angelfish (Pterophyllum scalare).3,7,10 By contrast, lymphocystis is an iridoviral disease seen in many freshwater and marine species and is usually not associated with significant mortality.11 Iridoviruses have also been isolated as an incidental finding in dwarf gouramis (Colisa lalia) with systemic amoebiasis and from clinically healthy goldfish.1,11 One paper in the scientific literature described challenge experiments carried out on food fish using an iridoviral isolate that was reported to be the same isolate from both the freshwater guppy (Poecilia reticulata) and the marine "doctorfish" (Labroides dimidiatus); however, there is some question as to actual species origin, since in that paper, Labroides dimidiatus was also incorrectly identified as a commonly traded and farmed freshwater species.4

In food and game fish, iridoviruses have been identified as a cause of disease in redfin perch (Perca fluviatilis) and rainbow trout (Oncorhynchus mykiss) in Australia, sheatfish (Silurus glanis) in Germany, white sturgeon (Acipenser transmontanus) in California, black bullhead (Ameiurus melas) in France, and tilapia (Oreochromis mossambicus) in Australia.2,5,6,9 An iridovirus has also been isolated from largemouth bass (Micropterus salmoides) (LMBV), although more research is necessary to determine the actual role of this virus in disease in the wild, because experimental transmission studies showed that the virus was not highly pathogenic.8,9

Diagnoses of iridoviral-associated disease in ornamental fish are primarily based on clinical signs, associated histopathology, and electron microscopic visualization of iridoviral particles in diseased tissues. In many instances, virus isolation is not an option. Basic pathogenicity and biology work need to be performed. Gourami iridovirus-associated disease (GIAD) has proven difficult to study in more detail because of the lack of appropriate cell lines for culture. Mortalities of gouramis in Florida were higher during summer and fall months, when water temperatures were greatest, with mortalities often as high as 70-90 percent.

The rapid temperature rise prior to onset of this disease in oscars appears to have triggered onset of this disease, similar to the role temperature appears to play in GIAD. As seen in other viral diseases, temperature is most likely an important factor in the pathogenicity of warmwater fish iridoviruses, and therefore regulation of temperature is an important management tool. Disinfection and good husbandry, including close monitoring of temperature, have been effective in preventing recurrence of this disease.


The authors wish to thank the members of the Electron Microscopy Core Laboratory, Biotechnology Program, University of Florida for their assistance.


1.  Anderson IG, HC Prior, BJ Rodwell, GO Harris. 1993. Iridovirus-like virions in imported dwarf gourami (Colisa lalia) with systemic amoebiasis. Australian Veterinary Journal 70: 66-67.

2.  Ariel E, L Owens. 1997. Epizootic mortalities in tilapia Oreochromis mossambicus. Diseases of Aquatic Organisms 29: 1-6.

3.  Fraser WA, TJ Keefe, B Bolon. 1993. Isolation of an iridovirus from farm-raised gouramis (Trichogaster trichopterus) with fatal disease. Journal of Veterinary Diagnostic Investigation 5: 250-253.

4.  Hedrick RP, TS McDowell. 1995. Properties of iridoviruses from ornamental fish. Veterinary Research 26: 423-427.

5.  Langdon JS, JD Humphrey. 1987. Epizootic haematopoietic necrosis, a new viral disease in redfin perch Perca fluviatilis. Journal of Fish Diseases 10: 289-197.

6.  Langdon JS, JD Humphrey, LM Williams. 1988. Outbreaks of EHNV-like iridovirus in cultured rainbow trout, Salmo gairdeneri Richardson, in Australia. Journal of Fish Diseases 11: 93-96.

7.  Lynch J. 1998. Preliminary investigation of gourami iridovirus infections. [Master's thesis]. Gainesville (Florida): University of Florida.

8.  Plumb JA. 1999. Health maintenance and principal microbial diseases of cultured fishes. Iowa State Press, Ames, Iowa. 328 pp.

9.  Plumb JA, JM Grizzle HE Young, AD Noyes. 1996. An iridovirus isolated from wild largemouth bass. Journal of Aquatic Animal Health 8: 265-270.

10. Schuh JCL, IG Shirley. 1990. Viral hematopoietic necrosis in an angelfish (Pterophyllum scalare). Journal of Zoo and Wildlife Medicine 21 (1): 95-98.

11. Wolf K. 1988. Fish viruses and fish viral diseases. Cornell University Press, Ithaca, New York. 476 pp.

Speaker Information
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Scott P. Terrell

Roy P.E. Yanong, BA, VMD
Tropical Aquaculture Laboratory, Department of Fisheries and Aquatic Sciences
University of Florida
Ruskin, FL, USA

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