Abstract

For a number of years, several species of fish in the coral reef tank at the National Aquarium in Baltimore (NAIB), have been developing dark lesions on the lateral body wall. Three species of fish were typically affected, bluestriped grunts (Haemulon sciurus), French grunts (Haemulon flavolineatum) and squirrelfish (Sargocentrun coruscum). These species periodically develop lesions, each episode lasting several months. The dark lesions progress, to large ulcers which erupt through the dermis and epidermis. Some of the fish succumb to the infection, while others survive and repair the lesions. The lesions form a dark/black scar when they heal. Cultures of blood, liver and kidney yielded Edwardsiella tarda. Samples were sent to and confirmed by the Centers for Disease Control.

Histopathology of the lesions shows ulceration of the epidermis and an intense, hemorrhagic, necrotizing, chronic dermatitis. Granulomatous lesions were present in multiple organs including heart, liver, spleen, kidney and intestines. Bacteria noted in some of the granulomata were gram negative and acid fast negative.

Introduction

Edwardsiella tarda has been considered primarily a pathogen of freshwater fish ((Austin and Austin 1987) Schaperclaus 1986). Outbreaks were described as early as 1962 (Hoshina 1962). It has been isolated in eels Hoshina 1962, Wakabayashi and Egusa 1973, Kusuda et al., 1965, PalaciosFontcuberta and Ghittino 1987), channel catfish (Meyer and Bullock 1973) and prespawining chinook salmon (Amandi et al. 1982.) There have been a few reports of Edwardsiella tarda epizootics in marine fish; mullet (Kusada et. al. 1976) and crimson sea breams (Kusada et. al. 1977). Edwardsiella tarda outbreaks are associated with organically polluted water since the natural reservoir may be the intestinal tract of multiple species, including humans (Roberts 1989).

Case Description

In the spring and summer of 1991,a high incidence of black, ulcerated skin lesions were observed in bluestriped grunts (Haemulon sciurus), French grunts (Haemulon flavolineatum) and squirrelfish (Sargocentrun coruscum) living in a 330,000 gallon artificial reef tank These species developed one or more swollen furuncles along the body wall. These lesions then ruptured exposing necrotic musculature underlying the ulcers. In surviving fish these ulcers gradually healed over a 1-2 month period. Once the lesions healed they had a dark or black appearance.

As animals became moribund or incapacitated due to musculature damage, they were removed by divers. Necropsies were performed and routine cultures were taken. Tissues were preserved in neutral buffered formalin and embedded in paraffin. Six micron sections were stained with Hematoxylin and Eosin, Grams stain and Ziehl-Neelsen (Prophet et al. 1992).

Skin lesions seen on gross necropsy examination varied from minor scale loss and areas of raised scales, to large hemorrhagic ulcerative lesions exposing soft edematous muscle. In some cases ulcerated lesions had healed forming darkened scars. Occasionally, ulcerative keratitis was noted. Internal lesions, cream to white colored nodules, were noted in many organs. The size and number of these lesions varied, but many fish had nodules in multiple organs including liver, spleen, kidney and heart.

Histopathologic examination of active skin lesions revealed an extensive necrotizing supperative chronic dermatitis. The lesions extended well into the dermal adipose tissue and often bacteria were present. Less active wounds contained areas of granulation tissue and fibrosis.

The nodules seen in the viscera during necropsy examination were large eosinophilic necrotic foci, surrounded by pale eosinophilic epitheliod macrophages. Gram negative (non acid fast) bacteria were often noted in the necrotic centers. Most of these lesions had not been encapsulated by fibroblasts, but occasionally had a pseudocapsule of compressed parenchymal cells. Lesions in the heart and other organs were frequently quite large and had destroyed a significant amount of the myocardium or the parenchyma.

Edwardsiella tarda was isolated in 9 of 17 animals examined. This organism was isolated in pure culture from the kidney in over 50% of positive animals. Identification was confirmed by the Centers of Disease Control (CDC). This strain resembles bio-group 1 because it is positive for mannitol and arabinose, but unlike this bio-group it is H2S positive and sucrose negative.

Other organisms cultured from infected animals included Vibrio sp., Pseudomonas aeruginosa, and Pseudomonas putrefaciens. These organisms were not found consistently in affected fish. In most cases only Edwardsiella tarda was found in pure culture.

Discussion

Attempts to treat the infection were made by providing oxytetracycline gel food. This appeared ineffective as the animals were observed consuming a variety of other foods intended for other species housed within the tank.

Since the spring of 1992 the incidence of ulcerated skin lesions has been minimal. Two changes in management strategies may have contributed to this. The first is that squirrelfish have not been acquired for display since the outbreak of this disease. Only six individuals remain in the tank. Second, we can now provide more effective ozonation.

Many animals can serve as reservoirs for Edwardsiella tarda. Fecal contamination from reptiles, birds and humans have been suggested in other episodes of E. tarda infection (Clarridge et al. 1980, Nagel et al. 1982). Fish that survive epizootics are known to serve as carrier. While E. tarda can survive up to 76 days in pond water and mud (Bullock and Herman, 1985) it is unclear how long it may survive in salt water.

Edwardsiella tarda has been known to infect people. In one case, a fish hook wound caused a typhoid-like illness, peritonitis with sepsis and a cellulitis (Jordan and Hadley 1969). In another case, a person developed gastroenteritis after ingesting water contaminated by a pet turtle. (Nagel et al. 1982). There is a zoonotic potential for divers and personnel in contact with water from aquaria housing fish that are infected with or are carriers of E. tarda. Appropriate hygienic procedures should be adopted to minimize the risk of infection.

References

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