Viral Disease in Angelfish

The freshwater angelfish, Pterophyllum scalare, is one of the most popular aquarium fish in the world.  It is loved for its beautiful coloration, elaborate finnage, and peaceful disposition.  This ornamental fish is commonly bred at home by hobbiyits, and is also produced in large quantities both in the United Sates and in foreign countries.  All of this fame is not without a price.  Many of these fish enter our petstores with a multitude of disease.  The most prevelant instance of disease in my research has been that of a virus. Either Lymphosistis virus, or an  Iridovirus.

Systemic irodovirus infctions have been reported with increased frequency in a wide variety of fish species and are now recognized as a cause of a serious systemic disease (Hedrick & McDowell 1995).  Species commonly affected include Turbot, Scophthalamus maximus (Block and Larsen 1993), Chromide cichlids , Etroplus maculatus,(Armstrong and Ferguson 1989)  catfish, Ictalurus melas (Rafinesque 1192).  From a pathological comparison, the nature of infection in the aofrementioned species is similar to what was observed in the angelfish.

Most commonly iridovirus shows itself in the form of increased mortalities at the wholesaler, or at the lcoal fish store.  Mortalities began 2 days after fish were acquired and moribund fish exhibited unusual circling behavior prior to death.  Once a stock of fish was infected, the mortality rate exceeded 70% within 10 days.

Upon clinical examanation of 22 fish, I identified some common characteristics of the disease.  Abdominal distentsion, exophthalmia, pale gills, but no toehr external abnormalities.  Postmortem examanations revealed the abdominal swelling was caused by ascities, with evidence of interperiteneal bleeding, and all other internal organs appeared pale.  Microscopic gill examanation showed clean and healthy gills in most specimens, and in 5% some gill flukes.  The flukes were determined ot significant because they exisited on only some of the deceased fish, and on some of the healthy fish. Examanation of the scales and skin showed no ectoparasites.

In order to confirm iridovirus, I prepared some moribund fish for microscopy detail.  Whole fish were fixed in buffered formalin solution, routinely processed, and wax embeded.  These specimens were then examined by light microscopy . Abnormal spleen tissue and renal tissue was observed.  The heomopoietic renal tissue exhibited numerous necrotic cells and cytomegaly.  The enlarged cells were had basophilic granular cytoplasm and were similar to cells seen in the pharangyl region where inflammation and necrosis also ocurred.  The same celluar anomoly was seen in the vasculature system, and epicarditis was noted.  Localized inflammation in the retrobulbar area of the eyes was also observed.  The stomach and intestines had marked necrosis of the epithelium and lamina propria.  Within the cranial cavity, evidence of congestion in the blood vessels and capillaries.

Additional specimens were preapred for electron micrscopy and samples from the spleen and kidneys were viewed.Analysis of the micrograph showed that the enlarged cells from the spleen showed numerous virions inlarge groups in the cell cytoplasm. These 6 sided paticles ranged from 148 to 167 n point to point. The histopathology and electron micrsopy findings suggest that the disease is viral in nature.  Based on the relative size of the virions, it would suggest they are uin the Iridoviridae family.  In order to positively identify the viral component, cultivation and transmission studies would need to be conducted.

 Lymphocystis, an iridoviral infection of over 30 marine species, also presents with cytomegaly of infected cells.  However, and infection of Lymphocystis is extreme when compared to the more moderate cell hypertrophy seen in this case. It has also been documented that Lymphocystis is more commonly associated with dermal legions, although it is known to infect visceral bodies (Colorni & Diamont 1995).

Froma management standpoint, the control of iridovirus is critical.  As this virus effects both ornamentals and commercial fishes, it is a public health risk to not act on preventing mortality in farmed fish species, or in the ornamental fish populations.  Future regulations of the industry should take into consideration the increasing possibilty of ssuch an infection.  We need to look into animal health guidelines for the hobby and hpw we manage fish disease.

As aquarists, we frequently encounter fish with diseased conditions. Its commonplace for the fish to be ‘flushed’ or tossed out with the trash.  Its  entirely possible that the pathogenicity of the moribund fish could be a public health threat to farm fishes, or natural commercial fisheries.  It is my recommendation that he ornamental hobby look into a set of voluntary regulations to curb this potential problem.