Abstract

The mariculture of summer flounder (Paralichthys dentatus) is becoming increasingly popular due to the demand of this fish for both the recreational and commercial markets. As with intensive culture of other fish species, the summer flounder is susceptible to a multitude of bacterial, viral and parasitic infections. However, since the practice of flounder aquaculture is relatively new, there are few reports of disease incidence in this species.

A small population of commercially-reared juvenile summer flounder (28 cm, ~180 grams) were housed at the Virginia-Maryland Regional College of Veterinary Medicine Aquatic Medicine Laboratory for over a year in a recirculating saltwater (16-20 ppt) system. Historically, fish were exposed to different experimental salinity manipulations, but not intentionally exposed to any pathogens. Fish were periodically treated with formalin baths (20 ppm) to reduce a chronic external parasite infestation of Trichodina sp. Over a three to six month period all fish in the population developed severe abdominal distention. No other external clinical signs were noted. Fish gradually became anorexic and lethargic. Abdominocentesis and gastric lavages were performed and results were inconclusive.

A fish from the affected population was humanly euthanatized and a necropsy was performed. Gross examination revealed a large multilobulated retroperitoneal mass in the region of the kidney. Small multifocal yellow lesions were also noted on the liver, spleen and heart tissues. Normal renal tissue could not be identified. There was a moderate amount of pale yellow fluid in the coelom, which was later characterized as a transudate. Samples of the mass and liver were cultured on 2% NaC1 TSA agar and Middlebrook agar. Small raised yellow colonies of Gram positive acid-fast bacilli grew after ten days at 25°C.

Histopathologically, the mass was identified as renal tissue that had severe granulomatous nephritis with marked infiltration of macrophages and multinucleated giant cells which obliterated preexisting normal renal architecture. Sections of heart, brain stem, mesentery, stomach and intestine also had multifocal granulomatous inflammation. Special stains revealed Gram positive, acid-fast bacilli distributed in multiple areas or multiple tissues.

Mycobacterium spp. and Nocardia spp. are principal acid-fast bacterial pathogens isolated from cultured marine fish.² Mycobacteriosis has been reported in salmonids, striped bass and plaice while nocardiosis has been reported in Atlantic salmon, yellowtail and rainbow trout.^1,3,4

This clinical case is significant because it documents an acid-fast bacterial infection in cultured summer flounder and indicates that summer flounder are highly susceptible to this infection. The marked and disseminated granulomatous inflammation in these fish suggests that summer flounder are unable to contain the infection either because they are immuno-compromised or are poorly adapted to this bacterium.⁵

References

1.  Bransden, M.P., J. Carson, B.L. Munday, J.H. Handlinger, C.G. Carter and B.F. Nowak. 2000. Nocardiosis in tank reared Atlantic salmon, Salmo salar L. Journal of Fish Diseases 23: 83-85.

2.  Frerichs, G.N. 1989. Bacterial diseases of marine fish. Veterinary Record 125"315-318.

3.  Hedrick, P.P., T. McDowell and J. Goff. 1989. Mycobacteriosis in clutured striped bass from California USA. Journal of Wildlife Diseases 23:391-395.

4.  Timur, Gulsen, R.J. Roberts and A. McQueen. 1977. The experimental pathogenesis of focal tuberculosis in the plaice (Pleuronectes platessa L.). Journal of Comparative Pathology 87:83-87.

5.  Wolf, J.C. and S.A. Smith. 1999. Comparative severity of experimentally induced mycobacteriosis in striped bass Morone saxatilis and hybrid tilapia Oreochromis spp. Diseases of Aquatic Organisms 38:191-200.