Pathogenicity of Ichthyophonus hoferi for laboratory-reared Pacific herring Clupea pallasi and its early appearance in wild Puget Sound herring

Laboratory-reared pathogen-free Pacific herring were exposed to pure cultur es of Ichthyophonus hoferi, and reproduced the disease seen in naturally in fected fish-thus fulfilling Koch's Postulates, Pathogen-free herring used i n this study were reared from artificially spawned eggs incubated in filter ed, UV-sterilized seawater, eliminating the variables associated with multi ple infections, which are common in wild herring. Wild free-ranging herring were captured monthly from June through October by dip net from 'herring b alls' located in the northern Puget Sound. I, hoferi infections were identi fied in these fish soon after metamorphoses, about 4 mo post-hatch The prev alence increased from 5 to 6% in 0-yr fish to 24% in l-yr-old fish to 50 to 70% in fish over 2 yr old, with no associated increase in mortality. The r oute of natural transmission to wild herring was not determined, but carniv orous fish became infected and died when they were experimentally fed tissu es infected with the organism. In vitro culture of tissues was the most sen sitive method for identifying both clinical and subclinical infections.