Kathleen P. Hughes1; Stephen A. Smith1; James
The aquaculture of sturgeon species has become increasingly popular worldwide, as well as, increasingly important since new legislation has been passed by CITES to include protection for all species of sturgeon and paddlefish. As the commercial industry of sturgeon aquaculture grows, the recognition of previously unreported pathogens and disease syndromes will also expand.
A federal hatchery raising endangered freshwater pallid sturgeon (Scaphirhynchus albus), experienced high mortality rates (100% in some tanks) in their fingerling (1.5 - 2.0 grams) stock in late summer of 2000. The sturgeon had been hatched the previous spring, and were presently being reared at 17-18 ° C. Other species of fish maintained at the facility included salmonids (coho salmon, cutthroat trout, brown trout, and Shasta rainbow trout), walleye and northern pike. The hatchery water source was an open supply coming from a reservoir supplied by the Missouri River. Water quality was generally considered good except during periods of heavy siltation following high wind or reservoir turn over.
A sample of the affected population was sent to the Aquatic Medicine Laboratory of the Virginia-Maryland Regional College of Veterinary Medicine for diagnostic evaluation. Gross external examination revealed healthy looking fish with slightly pale, clumped gills. Wet mount biopsies of gill tissue revealed excessive mucus coverage, but no obvious bacteria or external parasites were observed. Bacterial cultures taken from the posterior kidney yielded moderate growth of a mixed bacterial infection. Histopathological results showed severe epithelial hyperplasia and lamellar fusion of the gill tissue. A parasitic infestation of an amoeba was also detected in histology of the gill tissue with the majority of the parasites concentrated on the gill surface in areas of severe epithelial proliferation. Specific identification of the amoebae parasite was not attempted. In addition to the amoeba infestation, fungal gill disease was also noted suggesting that the fish were also succumbing to secondary opportunistic invaders.
A single treatment of chloramine-T (15 ppm) and salt (1 ppt) was ineffective in decreasing mortality. However, fish were successfully treated with multiple formalin bath (100 ppm) treatments and sequential salt exposure (0.5 ppt). Following three treatments of the formalin and salt therapy, mortality rates approached 0%.
Natural infestations of amoebic gill disease has been previously reported in rainbow trout, Atlantic salmon, coho salmon and blue tilapia.1,2,3,4,5,6 To our knowledge, this is the first report of amoebic gill disease in sturgeon. It is possible given the history of these sturgeons, that cross-infection from salmonids in the same hatchery or transmission from wild fish populations occurred.
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6. Zilberg, Dina, Vanessa L. Findlay, Peter Girling and Barry L. Munday. 2000. Effects of treatment with levamisole and glucans on mortality rates in Atlantic salmon (Salmo salar L.) suffering from amoebic gill disease. Bulletin of the European Association of Fish Pathologists 20 (1: 23).