Status Of Attempts To Isolate And Characterize A Rickettsia-like Organism (RLO) From Arctic Char (Salvelinus alpinus)     Julie Bebak-Williams1, Alicia N. Noble1, Ana Baya2, Peter Cooke3, Frank Hetrick2, Philip E. McAllister4, Douglas McGrogan5, Caird Rexroad6, W. Bane Schill4   1Freshwater Institute, Shepherdstown, WV; 2MD Dept. of Agriculture, College Park, MD; 3 ERRC, USDA-ARS, Wyndmoor, PA; 4USGS NFHRL, Leetown, WV; 5Fish Health Management, Hamilton, Ontario; 6NCCCWA, USDA-ARS, Leetown, WV     Since 1998, Arctic char (Nauyuk and Labrador strains) cultured at the Freshwater Institute (FI) have experienced outbreaks of respiratory disease associated with infection by a putative rickettsia-like organism.  Cumulative percent mortality in 5 to 1500 gm fish has ranged from 1% to 60%. Outbreaks occur in flow-through and recirculating culture systems. Water quality parameters have always been within normal limits. Except for abnormalities on gill wet mounts and severe mucus production on skin, gross necropsy results are unremarkable. Gross microbiological examinations of fish show no evidence of significant external bacteria or parasites, nor have systemic bacteria or viruses been detected by culture. Histopathology of organs, except gills and skin, is unremarkable. The only consistent histopathologic finding has been the presence of an intracellular, coccoid organism in gill cells. Attempts to isolate and characterize this organism include culture on bacterial media and on fish cell lines, electron microscopy, immunohistochemistry and polymerase chain reaction assay, and have been unsuccessful. While horizontal transmissiom has occurred in rearing units at the FI, source of the organism and mechanisms of transmission have not been determined. A similar, possibly identical, problem occurs in cultured Arctic char in Canada.